Science Final Part1
11.1 Distance and Displacement
Choosing a Frame of Reference
The motion of an object looks different to observers in different frames of reference.
EX: A passenger in the rear seat of a car moving at a steady speed is at rest relative to the front seat of the car. But is not at rest relative to: the side of the road, a pedestrian on the corner ahead, or even the rotating wheels of the car.
EX: Examine the figure. If you were standing under the tree, which object would appear to be moving?
Measuring Distance
The SI unit for measuring distance is the meter.
EX: One kilometer equals 1000 meters, because the prefix kilo- means 1000.
EX: Know what instruments & units you would use to measure different distances.
Q: Which distance can be most accurately measured with a ruler?
a. the length of a river
b. the width of a book
c. the distance between two cities
d. the size of an object under a microscope
Measuring Displacements
The direction and length of a straight line from the starting point to the ending point of an object’s motion is displacement.
Displacement and velocity are examples of vectors because they have both magnitude and direction.
EX: A person walks 1 mile every day for exercise, leaving her front porch at 9:00 A.M. and returning to her front porch at 9:25 A.M. What is the total displacement of her daily walk?
EX: A person drives north 3 blocks, then turns east and drives 3 blocks. The driver then turns south and drives 3 blocks. How could the driver have made the distance shorter while maintaining the same displacement?
Combining Displacements
The sum of two or more vectors is called the resultant vector.
EX: A ball is rolled uphill a distance of 3 meters before it slows, stops, and begins to roll back. The ball rolls downhill 6 meters before coming to rest against a tree. What is the magnitude of the ball’s displacement?
EX: Displacement vectors of 1 km south, 3 km north, 6 km south, and 2 km north combine to a total displacement of _____.
EX: A girl walks from her home to a friend’s home 3 blocks north. She then walks east 2 blocks to the post office, 1 block north to the library, and 1 block east to the park. From the park, she walks 2 blocks west to the movie theater. After the movie, she walks 4 blocks south to the pet store. What is the girl’s displacement from her starting point to the pet store? Where is the location of the pet store in relation to her home? Calculate the distance she walked in blocks.
11.2 Speed and Velocity
Speed
Speed is the ratio of the distance an object moves to the amount of time needed to travel the distance.
Speed is measured in base units of meters per second or m/s.
Speed can also be measured in other units of distance and time.
EX: What is the most appropriate SI unit to express the speed of a cyclist in the last leg of a 10-km race?
V=d/t is the equation that defines average speed.
EX: A car traveled 88 km in 1 hour, 90 km in the next 2 hours, and then 76 km in 1 hour before reaching its destination. What was the car’s average speed?
Instantaneous speed is measured at a particular instant.
EX: A car’s speedometer measures instantaneous speed.
Graphing Motion
The slope of a line on a distance-time graph is speed.
A constant slope on a distance-time graph indicates constant speed.
A horizontal line on a distance-time graph means the object is at rest.
EX: A distance-time graph indicates an object moves 20 km in 2 h. The average speed of the object is 10 km/h.
EX: What is the speed of a bobsled whose distance-time graph indicates that it traveled 100 m in 25 s?
EX: A distance-time graph indicates that an object moves 100 m in 4 s and then remains at rest for 1 s. What is the average speed of the object?
Velocity
Velocity is a description of both speed and direction of motion. Velocity is a vector because it has both magnitude and direction.
The difference between speed and velocity is that velocity indicates the direction of motion and speed does not.
Combining Velocities
Two or more velocities add by vector addition.
Vector addition is used when motion involves more than one speed in more than one direction, or more than one velocity.
EX: A river current has a velocity of 5 km/h relative to the shore, and a boat moves in the same direction as the current at 5 km/h relative to the river. How can the velocity of the boat relative to the shore be calculated?
Because its direction is always changing, an object moving in a circular path experiences a constant change in velocity.
11.3 Acceleration
What is Acceleration
The rate at which velocity changes is called acceleration.
A moving object does not accelerate if its velocity remains constant.
An object that is accelerating may be gaining speed, slowing down, or changing direction.
EX: a ball moving at a constant speed around a circular track is an example of acceleration because the direction of motion is changing.
Objects in free fall near the surface of the Earth experience constant acceleration.
Freely falling objects accelerate at 9.8 m/s2 because the force of gravity acts on them.
The velocity of an object moving in a straight line changes at a constant rate when the object is experiencing constant acceleration.
Calculating Acceleration
The acceleration of a moving object is calculated by dividing the change in velocity by the time over which the change occurs.
EX: A car that increases its speed from 20 km/h to 100 km/h undergoes positive acceleration.
EX: A train approaching a crossing changes speed from 25 m/s to 10 m/s in 240 s. How can the train’s acceleration be described?
EX: Suppose you increase your walking speed from 1 m/s to 3 m/s in a period of 2 s. What is your acceleration?
EX: An object moving at 30 m/s takes 5 s to come to a stop. What is the object’s acceleration?
Graphs of Accelerated Motion
A curved line on a distance-time graph represents accelerated motion.
The slope of a speed-time graph indicates acceleration.
EX: A speed-time graph shows that a car moves at 10 m/s for 10 s. The car’s speed then steadily decreases until it comes to a stop at 30 s. What would describe the slope of the speed-time graph from 10 s to 30 s?
Instantaneous Acceleration
Instantaneous Acceleration is how fast a velocity is changing at a specific instant.
An object at rest has an instantaneous acceleration of zero.
12.1 Forces
What is a Force?
A push or pull is an example of a force.
The SI unit of force is the newton.
1 N = 1 kgm/s2
The type of force measured by a grocery store spring scale is weight.
Combining Forces
The sum of all the forces acting on an object is called the net force.
If the forces acting on an object produce a net force of zero, the forces are called balanced forces.
EX: When a pair of balanced forces acts on an object, the net force that results is equal to zero.
When an unbalanced force acts on an object, the object accelerates.
Friction
The force that opposes the motion of objects that touch as they move past each other is called friction.
EX: As you push a cereal box across a tabletop, the sliding friction acting on the cereal box acts in the direction opposite of motion.
It usually takes more force to start an object sliding than it does to keep an object sliding because static friction is usually greater than sliding friction.
Fluid friction occurs as a fish swims through water.
Gravity
The two forces acting on a falling object are gravity and drag.
EX: The forces acting on a falling leaf are gravity and air resistance.
The drag force acting on a falling skydiver is also known as air resistance.
An open parachute increases air resistance of a falling skydiver by increasing surface area.
When a falling object reaches terminal velocity, the net force acting on it is zero.
Projectile Motion
The path of motion of a thrown javelin is an example of projectile motion.
The downward force of gravity and an initial forward velocity causes projectile motion.
The figure above shows the motion of three balls. The curved paths followed by balls B and C are examples of projectile motion.
12.2 Newton’s First and Second Laws of Motion.
Aristotle, Galileo, and Newton
Newton’s First law of Motion
According to Newton’s first law of motion, the state of motion of an object does not change as long as the net force acting on the object is zero.
The property of matter that resists changes in motion is called inertia.
EX: During a head-on auto collision, inertia causes a passenger in the front seat to continue moving forward.
EX: An orange might roll off your cafeteria tray when you stop suddenly because of the orange’s inertia.
Newton’s Second Law of Motion
According to Newton’s second law of motion, the acceleration of an object equals the net force acting on the object divided by the object’s mass.
a = F/m
EX: If a force of 10 N is applied to an object with a mass of 1 kg, the object will accelerate at ___.
EX: A crane exerts a net force of 900 N upward on a 750-kilogram car as the crane starts to lift the car from the deck of a cargo ship. What is the acceleration of the car during this time?
EX: A small engine causes a 0.20-kg model airplane to accelerate at a rate of 12 m/s2. What is the net force on the model airplane?
Weight and Mass
The force of gravity acting on an object is the object’s weight.
EX: Your weight equals your mass times the acceleration due to gravity.
The acceleration due to gravity at Earth’s surface is 9.8 m/s2
EX: The acceleration due to gravity on the surface of Mars is about one-third the acceleration due to gravity on Earth’s surface. The weight of a space probe on the surface of Mars is about one-third its weight on Earth’s surface.
EX: The mass of a newborn baby is 4.2 kilograms. What is the baby’s weight?
12.3 Newton’s Third law of motion and momentum.
Newton’s Third Law
Newton’s third law of motion describes action and reaction forces.
EX: When you push on a wall, the ____________________ pushes back on you.
EX: action reaction forces
When a tennis racket strikes a tennis ball
When stepping from a curb
When rowing a boat
Momentum
The product of an object’s mass and velocity is its momentum.
= mv
EX: If a golf ball and bowling ball are rolling at the same speed, the bowling ball has greater momentum.
EX: What is the momentum of a 50-kilogram ice skater gliding across the ice at a speed of 2 m/s?
EX: A small 32-kilogram canoe broke free of its dock and is now floating downriver at a speed of 2.5 m/s. What is the canoe’s momentum?
Conservation of Momentum
Momentum is conserved when two objects collide in a closed system.
In a closed system, the loss of momentum of one object equals the gain in momentum of another object.
12.4 Universal Forces
Electromagnetic Forces
The electric force is responsible for the repulsion between two positively charged particles.
The observation that a charged object can attract or repel other charged objects led scientists to conclude that there are two types of charges.
The magnetic force is the reason that when opposite poles of two magnets are brought together, the poles attract each other.
Nuclear Forces
The strong nuclear force acts only on the protons and neutrons in a nucleus.
The weak nuclear force is associated with nuclear decay.
Gravitational Forces
The gravitational force is the most effective over long distances.
The gravitational force between two objects increases as mass increases or distance decreases.
EX: As an astronaut travels far away from Earth, her weight decreases because gravity decreases.
The force that keeps an object moving in a circle is called centripetal force.
EX: The centripetal force acting on the moon continuously changes the direction of the moon’s motion.
EX: The centripetal force acting on a satellite in orbit; acts as an unbalanced force on the satellite. changes the direction of the satellite. is a center-directed force.
13.1 Fluid Pressure Pressure
Pressure is the result of force distributed over an area
In order to calculate pressure exerted on a surface, force is divided by the surface area. The formula, , is used to calculate Pressure
The SI unit of pressure is the pascal or Pa
A pascal is equal to 1 newton per meter squared
Q: A pressure of 10 N/m2 equals?
A: 10 Pa
Q: Express a pressure of 2500 N/m2 in kilopascals.
A: 2500 Pa = 2.5 kPa
Q: If the air inside a balloon exerts a force of 1 N on an area of 0.5 m2, what is the pressure inside the balloon?
A: 2 Pa
Q: The dimensions of a brick that weighs 21 N are 0.19 m 0.090 m 55 m.
What pressure does the brick exert on the ground if it is resting on its largest face? Show your work. A: 1200 Pa
Pressure in Fluids
A substance that flows and assumes the shape of its container is a fluid.
Gasses can be compressed but liquids cannot be compressed.
As a liquid is added to a beaker, the pressure exerted by the liquid on the bottom of the beaker increases.
The deeper under water you travel the greater the pressure.
The pressure exerted by a fluid at any given depth is exerted equally in all directions.
The pressure of a fluid at a specific depth depends only on the type of fluid.
Air Pressure and the Atmosphere
Atmospheric pressure is caused by the weight of the atmosphere above a particular location.
The pressure of air at sea level is approximately 101 kPa.
As your altitude increases, air pressure decreases.
13.2 Forces and Pressure in Fluids
Transmitting Pressure in a Fluid
Pascal’s principle states that a change in the pressure at any point in a fluid in a closed container is transmitted equally and unchanged in all directions throughout the fluid.
The operation of a hydraulic lift system is explained by Pascal’s principle.
If you squeeze the middle of an upright, closed soft-drink bottle the pressure will increase equally everywhere within the bottle.
Hydraulic Systems
A device that uses pressurized fluids acting on pistons of different sizes to change a force is called a Hydraulic System.
The hydraulic system of a dump truck is designed to multiply Force.
In a hydraulic lift system, the fluid pressure exerted throughout the system is constant. Q: A force of 1000 N is exerted on Piston 1 of the hydraulic lift shown. What force will be exerted on Piston 2?
A: 9000 N
Bernoulli’s Principle
Bernoulli’s principle states as the speed of a fluid increases, the pressure within the fluid decreases.
Due to the curvature of an airplane wing during flight air above the wing travels faster than air below the wing.
The upward force acting on the wing of an airplane in flight is called lift.
The downward force produced when air flows over the wing like spoiler on a race car is also an example of Bernoulli’s principle.
Q: To prevent a window from exploding outward during a strong windstorm, it is left slightly open. Explain why a slightly open window might not blow out.
A: A window may explode outward during a windstorm because the outside pressure is much less than the pressure inside the house. By opening the window, the difference in pressures is reduced.
13.3 Buoyancy
Buoyant Force
Buoyancy is the apparent loss of weight of an object in a fluid.
The upward force acting on an object submerged in a fluid is called buoyant force.
The direction of the buoyant force on an object placed in a fluid is always upward.
Even a rock at the bottom of a lake has a buoyant force acting upward on it.
Q: A brick weighs 21 N. Measured underwater, it weighs 12 N. What is the size of the buoyant force exerted by the water on the brick?
A: 9 N
The strength of the buoyant force acting on an object in a fluid depends on the object’s volume.
The buoyant force on an object in a fluid is equal to the weight of the fluid displaced by the object.
EX: A ball is floating partially submerged in a liquid. The buoyant force acting on the ball equals the weight of the ball.
Archimedes first stated the relationship between buoyant force and weight of a displaced fluid.
As you climb a high mountain, the buoyant force exerted on you by the atmosphere decreases.
Density and Buoyancy
If an object is less dense than the fluid it is in, it will float. If the object is more dense than the fluid it is in, it will sink.
Recall: D = m / V
The unit g/cm3 is often used to express density.
The density of water is 1.00 g/cm3.
A cork is floating in salty water. As more salt is added to the water to increase its density, the cork will float at a higher level in the water.
A submerged submarine alters its density to rise or fall in the water.
When the buoyant force is equal to the weight, and object floats or is suspended. When the buoyant force is less than the weight, the object sinks.
The weight of an object that sinks in a fluid is greater than the buoyant force acting on it.
Q: Why does a hot-air balloon float?
A: The weight of the balloon is less than the weight of the air displaced by the balloon.
Q: A cube of wood displaces half its volume when floating in water. When a 0.5-N washer is added to the cube, it floats just at the point where it is completely submerged in the water. What is the buoyant force acting on the cube when the washer is removed?
A: 0.5 N; because the 0.5-N washer and the cube floating on its own both displace the same volume, the 0.5-N force equals the buoyant force acting on the cube.
14.1 Work and Power
What is Work?
For work to be done on an object, the object has to move.
Any part of a force that does not act in the direction of an object’s motion does no work on an object.
EX: In which of the following is no work done?
a.
climbing stairs
c.
pushing a shopping cart
b.
lifting a book
d.
none of the above
*** A force acting on an object does no work if the force is not in the direction of the object’s motion.
Calculating Work
The SI unit of work is the Joule; a Joule is equal to a Newton times a meter.
You calculate work by multiplying the force acting in the direction of motion by the distance the object moves.
W = F d
EX: If you exert a force of 10.0 N to lift a box a distance of 0.75 m, how much work do you do?
EX: If you perform 30 joules of work lifting a 20-N box from the floor to a shelf, how high is the shelf?
EX: A worker uses a cart to move a load of bricks weighing 680 N a distance of 10 m across a parking lot. If he pushes the cart with a constant force of 220 N, what amount of work does he do? Show your work.
What is Power?
The rate at which work is done is called Power.
The SI unit of power is the Watt.
EX: The power of a machine measures its rate of doing work.
EX: To increase power, you can increase the amount of work you do in a given amount of time, or you can do a given amount of work in less time.
Calculating Power
P = W / t
EX: If you exert a force of 500 N to walk 4 m up a flight of stairs in 4 s, how much power do you use?
EX: A girl lifts a 100-N load a height of 2.0 m in a time of 0.5 s. What power does the girl produce? Show your work.
James Watt and Horsepower
The watt and the horsepower are both units of power.
About 746 watts equals one horsepower.
EX: A 750-W motor might also be rated as a 1-horsepower motor
14.2 Work and Machines
Machines Do Work.
A device that changes the size or direction of force used to do work is called a machine.
When a machine does work, it can;
Change the direction of a force.
Increase a force and change the distance a force moves.
Increase the distance a force moves and change the direction of a force.
Work Input and Work Output
A machine can make work easier for you by changing the direction of your force.
The force that is exerted on a machine is called the input force.
The force that the machine exerts on another object is the output force
Besides a reduction in friction, the only way to increase the amount of work output of a machine is to increase the work input.
*** The work output of a machine can never be greater than the work input.
14.3 Mechanical Advantage and Efficiency
Mechanical Advantage
The Mechanical Advantage of a machine is the number of times that the machine increases the input force.
The actual mechanical advantage of a machine is less than the ideal mechanical advantage of the machine because the ideal mechanical advantage does not take friction into consideration.
Actual Mechanical Advantage = Output force / Input force
If you know the input distance and output distance of a machine, you can calculate ideal mechanical advantage.
Ideal Mechanical Advantage = Input distance / Output distance
Calculating Mechanical Advantage
EX: If you have to apply 30 N of force on a crowbar to lift a rock that weighs 330 N, what is the actual mechanical advantage of the crowbar?
EX: A 16-N force applied to the handle of a door produces a 30-N output force. What is the AMA of the handle? Show your work.
EX: A 100-m long ski lift carries skiers from a station at the foot of a slope to a second station 40 m above. What is the IMA of the lift?
EX: The input force of a pulley system must move 6.0 m to lift a 3000-N engine a distance of 0.50 m. What is the IMA of the system? Show your work.
Efficiency
Efficiency = Work output / Work input x 100%
The mechanical efficiency of any machine is always less than 100 percent.
The efficiency of a machine is always less than 100 percent because some work input is lost to friction.
Reducing friction in a machine increases its efficiency.
EX: A mechanical device requires 400 J of work to do 340 J of work in lifting a crate. What is the efficiency of the device?
EX: A motor with an efficiency rating of 80 percent must supply 300 J of useful work. What amount of work must be supplied to the motor?
EX: A force of 12 N is applied to the handle of a screwdriver being used to pry off the lid of a paint can. As the force moves through a distance 0.3 m, the screwdriver does 3.2 J of work on the lid. What is the efficiency of the screwdriver? Show your work.
14.4 Simple Machines
Levers
A lever is a rigid bar that is free to move around a fixed point.
All levers have 3 main parts
The fulcrum is the fixed point that the bar rotates around.
The Input arm is the distance between the input force and the fulcrum.
The Output arm is the distance between the output force and the fulcrum.
First-Class Levers
The fulcrum is always between the effort force (input) and the resistance force (output) in a first-class lever.
Second-Class Levers
The bottle opener shown above is a second-class lever. The fulcrum is at one end with the output force in the middle.
Third-Class Levers
In a third-class lever the fulcrum is also at one end but with the input force in the middle.
Because of this the ideal mechanical advantage of a third-class lever is always less than 1.
Wheel and Axle
An automobile steering wheel is an example of a wheel and axle.
The ideal mechanical advantage of a wheel and axle is found by dividing the radius of the wheel by the radius of the axle.
Inclined Planes
An inclined plane reduces the effort force by increasing the distance through which the force is applied.
Wedges and Screws
An ax is an example of a wedge.
As the thickness of a wedge of given length increases, its IMA decreases.
A screw can be described as an inclined plane wrapped around a cylinder.
Pulleys
The ideal mechanical advantage of a pulley system is equal to the number of rope segments supporting the load.
Compound Machines
A machine is classified as a compound machine if it is made up of two or more simple machines that operate together.
A bicycle is an example of a compound machine.
A watch consists of a complex system of gears. Each gear acts as a continuous lever
15.1 Energy and Its Forms
Energy and Work
Work is a transfer of energy.
Energy of an object increases when work is done on the object.
Energy and work are measured in the SI unit called the Joule.
Energy is transferred by a force moving an object through a distance.
Kinetic Energy
The energy of motion is called Kinetic Energy. KE = ½ mv2
The kinetic energy of an object is proportional to the square of its speed.
If the mass of an object doubles, its kinetic energy doubles.
Q: A small 30-kilogram canoe is floating downriver at a speed of 2 m/s. What is the canoe’s kinetic energy?
A: 60 J
Q: A 12-kg sled is moving at a speed of 3.0 m/s. At which of the following speeds will the sled have twice as much kinetic energy? 1.5 m/s, 4.2 m/s, 6.0 m/s, 9.0 m/s
A: 4.2 m/s
Potential Energy
Energy that is stored due to position or shape is called potential energy.
PE = mgh
An object’s gravitational potential energy is directly related to;
Its mass.
The acceleration due to gravity.
Its height relative to a reference level.
Q: Why is the gravitational potential energy of an object 1 meter above the moon’s surface less than its potential energy 1 meter above Earth’s surface?
A: The moon’s acceleration due to gravity is less.
Q: A 4-kilogram cat is resting on top of a bookshelf that is 2 meters high. What is the cat’s gravitational potential energy relative to the floor if the acceleration due to gravity is 9.8 m/s2?
A: 78 J
Q: A 0.49-kg squirrel jumps from a tree branch that is 3.6 m high to the top of a bird feeder that is 1.5 m high. What is the change in gravitational potential energy of the squirrel? (The acceleration due to gravity is 9.8 m/s2.) Show your work.
A: 10J
When a pole-vaulter flexes the pole, the pole-vaulter increases the pole’s elastic potential energy.
Examples:
A wind up toy that has been wound up.
A compressed basketball.
A stretched rubber band.
Forms of Energy
All energy can be considered as kinetic energy, potential energy, or the energy in fields.
The sum of the kinetic energy and potential energy of an object is called its mechanical energy.
Mechanical energy does not include Thermal energy or Chemical energy.
The total potential and kinetic energy of all the microscopic particles in an object make up its Thermal energy.
Thermal energy increases when an object becomes warmer.
The energy stored in gasoline is Chemical energy.
15.2 Energy Conversion and Conservation
Energy Conversion
Energy can be converted from one form to another.
Walking converts chemical energy into mechanical energy.
Nuclear power plants are designed to convert nuclear energy into electrical energy.
Solar cells convert electromagnetic energy into electrical energy.
Wind turbines convert mechanical energy into electrical energy.
The process of changing energy from one form to another is called energy conversion.
Conservation of Energy
“Energy cannot be created or destroyed” is a statement of the law of conservation of energy.
Energy can be converted from one form to another.
Energy Conversions
The mechanical energy of an object equals its kinetic energy plus its potential energy.
(KE+ PE)beginning =
(KE + PE)end
When an apple falls from a tree to the ground, the apple’s beginning kinetic energy and ending gravitational potential energy are both equal to Zero.
The kinetic energy of the pendulum bob increases the most between locations B and D.
The kinetic energy of the pendulum bob decreases between locations B and E.
Energy And Mass
The equation E = mc2 relates energy and mass, c is the speed of light.
Einstein’s famous equation applies during nuclear fission and fusion reactions.
Consequences of the equation E = mc2:
Energy is released when matter is destroyed.
Mass and energy are equivalent.
The law of conservation of energy must be modified to state that mass and energy are conserved in any process.
Q: In a nuclear reaction, an amount of matter having a mass of 1.0 10–14 kg is converted into energy, which is released. How much energy is released? (The speed of light is 3.0 108 m/s.) Show your work.
A: 900J
15.3 Energy Resources
Nonrenewable Energy Resources
Energy resources that exist in limited amounts and, once used, cannot be replaced except over the course of millions of years are called nonrenewable energy resources.
Examples: oil, natural gas, coal, and uranium
Fossil fuels currently account for the majority of the world’s energy use because they are relatively inexpensive and readily available.
Renewable Energy Resources
Renewable energy resources include hydroelectric, solar, geothermal, wind, biomass, and possibly in the future, nuclear fusion.
Energy obtained from flowing water is known as hydroelectric energy.
A drawback of solar energy is that it depends on the climate.
Flat collector plates through which water flows are found in active solar energy systems.
Geothermal energy, in addition to being renewable, offers the benefit of being nonpolluting.
The chemical energy stored in living things is called biomass energy
A benefit of a hydrogen fuel cell is that its byproduct is water.
Conserving Energy Resources
Turning off unused lights or appliances is an example of energy conservation.
Manufacturers can increase a light bulb’s energy efficiency by using technology that increases the amount of electromagnetic energy the bulb converts from a given amount of electrical energy.
Example: fluorescent lighting.
Turning off lights can conserve energy when they are not in use.
Using mass transportation such as buses, streetcars, and trains to move more people at a time than a car would be able too can also conserve energy.
16.1 Thermal Energy and Matter
Work and Heat
From his observations of cannon drilling, Count Rumford concluded that heat could NOT be a form of matter.
Heat is the transfer of thermal energy from one object to another because of a difference in temperature.
Temperature
A measure of how hot or cold an object is compared to a reference point can be measured in units of Kelvin or degrees Celsius.
The property of an object that is related to the average kinetic energy of the particles in that object is called temperature
Thermal Energy
Thermal energy depends on an object’s; mass, phase (solid, liquid, or gas), and temperature.
As the temperature of an object rises, so does the thermal energy of the object.
EX: A hot dinner plate has greater thermal energy than a similar dinner plate at room temperature.
Thermal Contraction and Expansion
The decrease in volume of a material due to a temperature decrease is called thermal contraction.
The increase in volume of a material due to a temperature increase is called thermal expansion.
A thermometer is a device that is based on the property of thermal expansion.
Specific Heat
In the formula , c represents the specific heat, and is measured in units of J/gC.
Q: The specific heat of copper is 0.385 J/gC. What equation would you use to calculate correctly the amount of heat needed to raise the temperature of 0.75 g of copper from 10C to 25C?
A: Q = 0.75 g 0.385 J/g·C 15C
If the temperature change of an aluminum nail is negative, thermal energy is transferred from the nail to the surroundings.
Measuring Heat Changes
A calorimeter directly measures a change in temperature.
A sealed calorimeter is a closed system because thermal energy is not transferred to the environment.
In a calorimeter, the increase in the thermal energy of the water and the decrease in the thermal energy of the sample are equal.
16.2 Heat and Thermodynamics
Conduction
The transfer of thermal energy with no overall transfer of matter is called conduction.
A material that conducts thermal energy well is called a thermal conductor. Example: metal
A material that conducts thermal energy poorly is called a thermal insulator. Example: a wooden spoon
Convection
The type of thermal energy transfer that takes place in fluids is mostly Convection.
Radiation
The transfer of energy as waves moving through space is called radiation.
As an object’s temperature increases, the rate at which it radiates energy increases.
Energy from the sun reaches Earth mostly by radiation.
Radiation is the only form of energy transfer that does not require matter for the energy to travel through.
Thermodynamics
The study of the conversion between heat and other forms of energy is called thermodynamics.
According to the first law of thermodynamics, the amount of work done by a heat engine equals the amount of thermal energy added to the engine minus the waste heat.
This law applies to heating objects, transferring thermal energy, and doing work on a system.
The second law of thermodynamics states that thermal energy can flow from colder objects to hotter objects only if work is done on the system.
Thermal energy that is not converted into work by a heat engine is called waste heat.
Disorder in the universe increases because work produces waste heat, which leaves a system.
The third law of thermodynamics states that absolute zero cannot be reached.
One consequence of the third law of thermodynamics is that heat engines have efficiencies less than 100 percent.
16.3 Using Heat
Heat Engines
External Combustion Engines; Fuel is burned outside the engine, heat is converted into work, hot steam pushes a piston. AKA steam engine.
Internal Combustion Engines, example; In most automobile engines, the linear motion of the strokes is turned into rotational motion by the crankshaft.
In most four-stroke internal combustion engines, the piston moves downward during the power and intake strokes.
Heating Systems
In a hot-water heating system, room temperature is controlled by a device called a thermostat.
A steam central heating system is often used when heating many buildings from a central location.
A steam-heating system is most similar to a hot-water heating system.
Radiant heaters have the following advantages:
They are portable
They can easily be turned on or off
They direct warm air to where it is needed
Forced-air central heating systems involve a furnace and a blower.
In forced-air heating systems, warm-air vents are usually located near the floor. Having air cleaned as it passes through filters near the furnace is an advantage of a forced-air heating system.
Cooling Systems
A fluid that vaporizes and condenses inside the tubing of a heat pump is called the refrigerant.
In a heat pump the compressor increases the pressure and temperature of the refrigerant.
As the fluid in a heat pump evaporates, thermal energy is transferred from the surroundings to the fluid.
17.1 Mechanical Waves
What are Mechanical Waves?
A mechanical wave is a disturbance in matter that carries energy from one place to another.
The matter that a mechanical wave travels through is called a medium.
A mechanical wave generally does NOT move the medium from one place to another.
Every type of mechanical wave needs a source of energy to produce it.
EX: You can make a wave in a rope by adding energy at one end of the rope.
Types of Mechanical Waves
Transverse Waves are waves that cause the medium to vibrate at right angles to the direction in which the wave travels.
EX: Waves in a rope are transverse waves because the medium’s vibration is perpendicular to the direction in which the wave travels.
A Crest is the highest point of the wave above the rest position.
A Trough is the lowest point below the rest position.
A wave in a rope is a transverse wave, but a sound wave is a longitudinal wave.
A longitudinal wave causes the medium to vibrate only in a direction parallel to the wave’s motion.
Instead of crests and troughs, as in an ocean wave, a longitudinal wave has compressions and rarefactions.
EX: In an earthquake, a P wave is a longitudinal wave. It moves through soil and rock as a series of compressions and rarefactions.
Transverse and longitudinal waves both transfer energy through a medium.
The crest of a transverse wave is most similar to compression in a longitudinal wave.
A disturbance sends ripples across water in a tub. These ripples are an example of a surface wave.
A pebble drops straight down into a tub of water, setting off surface waves that travel between the water and air.
When a surfer rides an ocean wave (surface wave) on her surfboard, she is actually riding on a crest that is toppling over.
17.2 Properties of mechanical Waves
Frequency and period
Frequency is the number of complete cycles in a given time, usually measured in cycles per second.
A period is the length of time it takes for one complete cycle to pass a fixed point.
Wavelength
Wavelength is the measured length of one complete cycle.
In a transverse wave, wavelength is measured from crest to crest or from trough to trough.
Wave Speed
To determine the speed of a wave, you must know the wave’s wavelength and frequency.
To determine the speed of a wave, you would use the equation speed = wavelength ´ frequency
S = l x f
Q: If a wave has a wavelength of 2 m and a frequency of 3.0 hertz, what is the waves speed?
Hint hertz is equal to 1/s or one over a second.
A: 6 m/s
Amplitude
Amplitude measures the greatest displacement of a wave from the rest position.
To find amplitude, measure either from a trough to the rest position, or from a crest to the rest position.
Amplitude is related to the amount of energy carried by the wave and the maximum displacement from the rest position.
To compare the energy of different waves, measure the amplitude of the waves.
17.3 Behavior of Waves
Reflection
When a wave strikes a solid barrier, it behaves like a basketball hitting a backboard. This wave behavior is called reflection.
Reflection is the only property in which the wave does not continue moving forward.
Refraction
For refraction to occur in a wave, the wave must enter a new medium at an angle.
In refraction, when two parts of a wave travel through different mediums, the parts move at different speeds.
Ocean waves will not bend if they approach the shore head on.
Diffraction
Diffraction is the bending of a wave as it moves around an obstacle or passes through a narrow opening.
Wavelength is one property of a wave that determines how much it will diffract when it encounters an obstacle.
The wave with the longest wavelength will probably be diffracted the most.
Interference
Interference occurs when two or more waves overlap and combine together.
If two waves collide and form a temporary larger wave, the interference is constructive.
Destructive interference is when two waves collide and the temporary combined wave that results is smaller than the original waves.
Standing Waves
The formation of a standing wave requires interference between incoming and reflected waves.
At the node of a standing wave, there is no displacement from the rest position.
An antinode is a point where a crest or trough occurs midway between two nodes.
A standing wave forms only if half a wavelength or a multiple of half a wavelength fits exactly into the length of a vibrating cord.
Choosing a Frame of Reference
The motion of an object looks different to observers in different frames of reference.
EX: A passenger in the rear seat of a car moving at a steady speed is at rest relative to the front seat of the car. But is not at rest relative to: the side of the road, a pedestrian on the corner ahead, or even the rotating wheels of the car.
EX: Examine the figure. If you were standing under the tree, which object would appear to be moving?
Measuring Distance
The SI unit for measuring distance is the meter.
EX: One kilometer equals 1000 meters, because the prefix kilo- means 1000.
EX: Know what instruments & units you would use to measure different distances.
Q: Which distance can be most accurately measured with a ruler?
a. the length of a river
b. the width of a book
c. the distance between two cities
d. the size of an object under a microscope
Measuring Displacements
The direction and length of a straight line from the starting point to the ending point of an object’s motion is displacement.
Displacement and velocity are examples of vectors because they have both magnitude and direction.
EX: A person walks 1 mile every day for exercise, leaving her front porch at 9:00 A.M. and returning to her front porch at 9:25 A.M. What is the total displacement of her daily walk?
EX: A person drives north 3 blocks, then turns east and drives 3 blocks. The driver then turns south and drives 3 blocks. How could the driver have made the distance shorter while maintaining the same displacement?
Combining Displacements
The sum of two or more vectors is called the resultant vector.
EX: A ball is rolled uphill a distance of 3 meters before it slows, stops, and begins to roll back. The ball rolls downhill 6 meters before coming to rest against a tree. What is the magnitude of the ball’s displacement?
EX: Displacement vectors of 1 km south, 3 km north, 6 km south, and 2 km north combine to a total displacement of _____.
EX: A girl walks from her home to a friend’s home 3 blocks north. She then walks east 2 blocks to the post office, 1 block north to the library, and 1 block east to the park. From the park, she walks 2 blocks west to the movie theater. After the movie, she walks 4 blocks south to the pet store. What is the girl’s displacement from her starting point to the pet store? Where is the location of the pet store in relation to her home? Calculate the distance she walked in blocks.
11.2 Speed and Velocity
Speed
Speed is the ratio of the distance an object moves to the amount of time needed to travel the distance.
Speed is measured in base units of meters per second or m/s.
Speed can also be measured in other units of distance and time.
EX: What is the most appropriate SI unit to express the speed of a cyclist in the last leg of a 10-km race?
V=d/t is the equation that defines average speed.
EX: A car traveled 88 km in 1 hour, 90 km in the next 2 hours, and then 76 km in 1 hour before reaching its destination. What was the car’s average speed?
Instantaneous speed is measured at a particular instant.
EX: A car’s speedometer measures instantaneous speed.
Graphing Motion
The slope of a line on a distance-time graph is speed.
A constant slope on a distance-time graph indicates constant speed.
A horizontal line on a distance-time graph means the object is at rest.
EX: A distance-time graph indicates an object moves 20 km in 2 h. The average speed of the object is 10 km/h.
EX: What is the speed of a bobsled whose distance-time graph indicates that it traveled 100 m in 25 s?
EX: A distance-time graph indicates that an object moves 100 m in 4 s and then remains at rest for 1 s. What is the average speed of the object?
Velocity
Velocity is a description of both speed and direction of motion. Velocity is a vector because it has both magnitude and direction.
The difference between speed and velocity is that velocity indicates the direction of motion and speed does not.
Combining Velocities
Two or more velocities add by vector addition.
Vector addition is used when motion involves more than one speed in more than one direction, or more than one velocity.
EX: A river current has a velocity of 5 km/h relative to the shore, and a boat moves in the same direction as the current at 5 km/h relative to the river. How can the velocity of the boat relative to the shore be calculated?
Because its direction is always changing, an object moving in a circular path experiences a constant change in velocity.
11.3 Acceleration
What is Acceleration
The rate at which velocity changes is called acceleration.
A moving object does not accelerate if its velocity remains constant.
An object that is accelerating may be gaining speed, slowing down, or changing direction.
EX: a ball moving at a constant speed around a circular track is an example of acceleration because the direction of motion is changing.
Objects in free fall near the surface of the Earth experience constant acceleration.
Freely falling objects accelerate at 9.8 m/s2 because the force of gravity acts on them.
The velocity of an object moving in a straight line changes at a constant rate when the object is experiencing constant acceleration.
Calculating Acceleration
The acceleration of a moving object is calculated by dividing the change in velocity by the time over which the change occurs.
EX: A car that increases its speed from 20 km/h to 100 km/h undergoes positive acceleration.
EX: A train approaching a crossing changes speed from 25 m/s to 10 m/s in 240 s. How can the train’s acceleration be described?
EX: Suppose you increase your walking speed from 1 m/s to 3 m/s in a period of 2 s. What is your acceleration?
EX: An object moving at 30 m/s takes 5 s to come to a stop. What is the object’s acceleration?
Graphs of Accelerated Motion
A curved line on a distance-time graph represents accelerated motion.
The slope of a speed-time graph indicates acceleration.
EX: A speed-time graph shows that a car moves at 10 m/s for 10 s. The car’s speed then steadily decreases until it comes to a stop at 30 s. What would describe the slope of the speed-time graph from 10 s to 30 s?
Instantaneous Acceleration
Instantaneous Acceleration is how fast a velocity is changing at a specific instant.
An object at rest has an instantaneous acceleration of zero.
12.1 Forces
What is a Force?
A push or pull is an example of a force.
The SI unit of force is the newton.
1 N = 1 kgm/s2
The type of force measured by a grocery store spring scale is weight.
Combining Forces
The sum of all the forces acting on an object is called the net force.
If the forces acting on an object produce a net force of zero, the forces are called balanced forces.
EX: When a pair of balanced forces acts on an object, the net force that results is equal to zero.
When an unbalanced force acts on an object, the object accelerates.
Friction
The force that opposes the motion of objects that touch as they move past each other is called friction.
EX: As you push a cereal box across a tabletop, the sliding friction acting on the cereal box acts in the direction opposite of motion.
It usually takes more force to start an object sliding than it does to keep an object sliding because static friction is usually greater than sliding friction.
Fluid friction occurs as a fish swims through water.
Gravity
The two forces acting on a falling object are gravity and drag.
EX: The forces acting on a falling leaf are gravity and air resistance.
The drag force acting on a falling skydiver is also known as air resistance.
An open parachute increases air resistance of a falling skydiver by increasing surface area.
When a falling object reaches terminal velocity, the net force acting on it is zero.
Projectile Motion
The path of motion of a thrown javelin is an example of projectile motion.
The downward force of gravity and an initial forward velocity causes projectile motion.
The figure above shows the motion of three balls. The curved paths followed by balls B and C are examples of projectile motion.
12.2 Newton’s First and Second Laws of Motion.
Aristotle, Galileo, and Newton
Newton’s First law of Motion
According to Newton’s first law of motion, the state of motion of an object does not change as long as the net force acting on the object is zero.
The property of matter that resists changes in motion is called inertia.
EX: During a head-on auto collision, inertia causes a passenger in the front seat to continue moving forward.
EX: An orange might roll off your cafeteria tray when you stop suddenly because of the orange’s inertia.
Newton’s Second Law of Motion
According to Newton’s second law of motion, the acceleration of an object equals the net force acting on the object divided by the object’s mass.
a = F/m
EX: If a force of 10 N is applied to an object with a mass of 1 kg, the object will accelerate at ___.
EX: A crane exerts a net force of 900 N upward on a 750-kilogram car as the crane starts to lift the car from the deck of a cargo ship. What is the acceleration of the car during this time?
EX: A small engine causes a 0.20-kg model airplane to accelerate at a rate of 12 m/s2. What is the net force on the model airplane?
Weight and Mass
The force of gravity acting on an object is the object’s weight.
EX: Your weight equals your mass times the acceleration due to gravity.
The acceleration due to gravity at Earth’s surface is 9.8 m/s2
EX: The acceleration due to gravity on the surface of Mars is about one-third the acceleration due to gravity on Earth’s surface. The weight of a space probe on the surface of Mars is about one-third its weight on Earth’s surface.
EX: The mass of a newborn baby is 4.2 kilograms. What is the baby’s weight?
12.3 Newton’s Third law of motion and momentum.
Newton’s Third Law
Newton’s third law of motion describes action and reaction forces.
EX: When you push on a wall, the ____________________ pushes back on you.
EX: action reaction forces
When a tennis racket strikes a tennis ball
When stepping from a curb
When rowing a boat
Momentum
The product of an object’s mass and velocity is its momentum.
= mv
EX: If a golf ball and bowling ball are rolling at the same speed, the bowling ball has greater momentum.
EX: What is the momentum of a 50-kilogram ice skater gliding across the ice at a speed of 2 m/s?
EX: A small 32-kilogram canoe broke free of its dock and is now floating downriver at a speed of 2.5 m/s. What is the canoe’s momentum?
Conservation of Momentum
Momentum is conserved when two objects collide in a closed system.
In a closed system, the loss of momentum of one object equals the gain in momentum of another object.
12.4 Universal Forces
Electromagnetic Forces
The electric force is responsible for the repulsion between two positively charged particles.
The observation that a charged object can attract or repel other charged objects led scientists to conclude that there are two types of charges.
The magnetic force is the reason that when opposite poles of two magnets are brought together, the poles attract each other.
Nuclear Forces
The strong nuclear force acts only on the protons and neutrons in a nucleus.
The weak nuclear force is associated with nuclear decay.
Gravitational Forces
The gravitational force is the most effective over long distances.
The gravitational force between two objects increases as mass increases or distance decreases.
EX: As an astronaut travels far away from Earth, her weight decreases because gravity decreases.
The force that keeps an object moving in a circle is called centripetal force.
EX: The centripetal force acting on the moon continuously changes the direction of the moon’s motion.
EX: The centripetal force acting on a satellite in orbit; acts as an unbalanced force on the satellite. changes the direction of the satellite. is a center-directed force.
13.1 Fluid Pressure Pressure
Pressure is the result of force distributed over an area
In order to calculate pressure exerted on a surface, force is divided by the surface area. The formula, , is used to calculate Pressure
The SI unit of pressure is the pascal or Pa
A pascal is equal to 1 newton per meter squared
Q: A pressure of 10 N/m2 equals?
A: 10 Pa
Q: Express a pressure of 2500 N/m2 in kilopascals.
A: 2500 Pa = 2.5 kPa
Q: If the air inside a balloon exerts a force of 1 N on an area of 0.5 m2, what is the pressure inside the balloon?
A: 2 Pa
Q: The dimensions of a brick that weighs 21 N are 0.19 m 0.090 m 55 m.
What pressure does the brick exert on the ground if it is resting on its largest face? Show your work. A: 1200 Pa
Pressure in Fluids
A substance that flows and assumes the shape of its container is a fluid.
Gasses can be compressed but liquids cannot be compressed.
As a liquid is added to a beaker, the pressure exerted by the liquid on the bottom of the beaker increases.
The deeper under water you travel the greater the pressure.
The pressure exerted by a fluid at any given depth is exerted equally in all directions.
The pressure of a fluid at a specific depth depends only on the type of fluid.
Air Pressure and the Atmosphere
Atmospheric pressure is caused by the weight of the atmosphere above a particular location.
The pressure of air at sea level is approximately 101 kPa.
As your altitude increases, air pressure decreases.
13.2 Forces and Pressure in Fluids
Transmitting Pressure in a Fluid
Pascal’s principle states that a change in the pressure at any point in a fluid in a closed container is transmitted equally and unchanged in all directions throughout the fluid.
The operation of a hydraulic lift system is explained by Pascal’s principle.
If you squeeze the middle of an upright, closed soft-drink bottle the pressure will increase equally everywhere within the bottle.
Hydraulic Systems
A device that uses pressurized fluids acting on pistons of different sizes to change a force is called a Hydraulic System.
The hydraulic system of a dump truck is designed to multiply Force.
In a hydraulic lift system, the fluid pressure exerted throughout the system is constant. Q: A force of 1000 N is exerted on Piston 1 of the hydraulic lift shown. What force will be exerted on Piston 2?
A: 9000 N
Bernoulli’s Principle
Bernoulli’s principle states as the speed of a fluid increases, the pressure within the fluid decreases.
Due to the curvature of an airplane wing during flight air above the wing travels faster than air below the wing.
The upward force acting on the wing of an airplane in flight is called lift.
The downward force produced when air flows over the wing like spoiler on a race car is also an example of Bernoulli’s principle.
Q: To prevent a window from exploding outward during a strong windstorm, it is left slightly open. Explain why a slightly open window might not blow out.
A: A window may explode outward during a windstorm because the outside pressure is much less than the pressure inside the house. By opening the window, the difference in pressures is reduced.
13.3 Buoyancy
Buoyant Force
Buoyancy is the apparent loss of weight of an object in a fluid.
The upward force acting on an object submerged in a fluid is called buoyant force.
The direction of the buoyant force on an object placed in a fluid is always upward.
Even a rock at the bottom of a lake has a buoyant force acting upward on it.
Q: A brick weighs 21 N. Measured underwater, it weighs 12 N. What is the size of the buoyant force exerted by the water on the brick?
A: 9 N
The strength of the buoyant force acting on an object in a fluid depends on the object’s volume.
The buoyant force on an object in a fluid is equal to the weight of the fluid displaced by the object.
EX: A ball is floating partially submerged in a liquid. The buoyant force acting on the ball equals the weight of the ball.
Archimedes first stated the relationship between buoyant force and weight of a displaced fluid.
As you climb a high mountain, the buoyant force exerted on you by the atmosphere decreases.
Density and Buoyancy
If an object is less dense than the fluid it is in, it will float. If the object is more dense than the fluid it is in, it will sink.
Recall: D = m / V
The unit g/cm3 is often used to express density.
The density of water is 1.00 g/cm3.
A cork is floating in salty water. As more salt is added to the water to increase its density, the cork will float at a higher level in the water.
A submerged submarine alters its density to rise or fall in the water.
When the buoyant force is equal to the weight, and object floats or is suspended. When the buoyant force is less than the weight, the object sinks.
The weight of an object that sinks in a fluid is greater than the buoyant force acting on it.
Q: Why does a hot-air balloon float?
A: The weight of the balloon is less than the weight of the air displaced by the balloon.
Q: A cube of wood displaces half its volume when floating in water. When a 0.5-N washer is added to the cube, it floats just at the point where it is completely submerged in the water. What is the buoyant force acting on the cube when the washer is removed?
A: 0.5 N; because the 0.5-N washer and the cube floating on its own both displace the same volume, the 0.5-N force equals the buoyant force acting on the cube.
14.1 Work and Power
What is Work?
For work to be done on an object, the object has to move.
Any part of a force that does not act in the direction of an object’s motion does no work on an object.
EX: In which of the following is no work done?
a.
climbing stairs
c.
pushing a shopping cart
b.
lifting a book
d.
none of the above
*** A force acting on an object does no work if the force is not in the direction of the object’s motion.
Calculating Work
The SI unit of work is the Joule; a Joule is equal to a Newton times a meter.
You calculate work by multiplying the force acting in the direction of motion by the distance the object moves.
W = F d
EX: If you exert a force of 10.0 N to lift a box a distance of 0.75 m, how much work do you do?
EX: If you perform 30 joules of work lifting a 20-N box from the floor to a shelf, how high is the shelf?
EX: A worker uses a cart to move a load of bricks weighing 680 N a distance of 10 m across a parking lot. If he pushes the cart with a constant force of 220 N, what amount of work does he do? Show your work.
What is Power?
The rate at which work is done is called Power.
The SI unit of power is the Watt.
EX: The power of a machine measures its rate of doing work.
EX: To increase power, you can increase the amount of work you do in a given amount of time, or you can do a given amount of work in less time.
Calculating Power
P = W / t
EX: If you exert a force of 500 N to walk 4 m up a flight of stairs in 4 s, how much power do you use?
EX: A girl lifts a 100-N load a height of 2.0 m in a time of 0.5 s. What power does the girl produce? Show your work.
James Watt and Horsepower
The watt and the horsepower are both units of power.
About 746 watts equals one horsepower.
EX: A 750-W motor might also be rated as a 1-horsepower motor
14.2 Work and Machines
Machines Do Work.
A device that changes the size or direction of force used to do work is called a machine.
When a machine does work, it can;
Change the direction of a force.
Increase a force and change the distance a force moves.
Increase the distance a force moves and change the direction of a force.
Work Input and Work Output
A machine can make work easier for you by changing the direction of your force.
The force that is exerted on a machine is called the input force.
The force that the machine exerts on another object is the output force
Besides a reduction in friction, the only way to increase the amount of work output of a machine is to increase the work input.
*** The work output of a machine can never be greater than the work input.
14.3 Mechanical Advantage and Efficiency
Mechanical Advantage
The Mechanical Advantage of a machine is the number of times that the machine increases the input force.
The actual mechanical advantage of a machine is less than the ideal mechanical advantage of the machine because the ideal mechanical advantage does not take friction into consideration.
Actual Mechanical Advantage = Output force / Input force
If you know the input distance and output distance of a machine, you can calculate ideal mechanical advantage.
Ideal Mechanical Advantage = Input distance / Output distance
Calculating Mechanical Advantage
EX: If you have to apply 30 N of force on a crowbar to lift a rock that weighs 330 N, what is the actual mechanical advantage of the crowbar?
EX: A 16-N force applied to the handle of a door produces a 30-N output force. What is the AMA of the handle? Show your work.
EX: A 100-m long ski lift carries skiers from a station at the foot of a slope to a second station 40 m above. What is the IMA of the lift?
EX: The input force of a pulley system must move 6.0 m to lift a 3000-N engine a distance of 0.50 m. What is the IMA of the system? Show your work.
Efficiency
Efficiency = Work output / Work input x 100%
The mechanical efficiency of any machine is always less than 100 percent.
The efficiency of a machine is always less than 100 percent because some work input is lost to friction.
Reducing friction in a machine increases its efficiency.
EX: A mechanical device requires 400 J of work to do 340 J of work in lifting a crate. What is the efficiency of the device?
EX: A motor with an efficiency rating of 80 percent must supply 300 J of useful work. What amount of work must be supplied to the motor?
EX: A force of 12 N is applied to the handle of a screwdriver being used to pry off the lid of a paint can. As the force moves through a distance 0.3 m, the screwdriver does 3.2 J of work on the lid. What is the efficiency of the screwdriver? Show your work.
14.4 Simple Machines
Levers
A lever is a rigid bar that is free to move around a fixed point.
All levers have 3 main parts
The fulcrum is the fixed point that the bar rotates around.
The Input arm is the distance between the input force and the fulcrum.
The Output arm is the distance between the output force and the fulcrum.
First-Class Levers
The fulcrum is always between the effort force (input) and the resistance force (output) in a first-class lever.
Second-Class Levers
The bottle opener shown above is a second-class lever. The fulcrum is at one end with the output force in the middle.
Third-Class Levers
In a third-class lever the fulcrum is also at one end but with the input force in the middle.
Because of this the ideal mechanical advantage of a third-class lever is always less than 1.
Wheel and Axle
An automobile steering wheel is an example of a wheel and axle.
The ideal mechanical advantage of a wheel and axle is found by dividing the radius of the wheel by the radius of the axle.
Inclined Planes
An inclined plane reduces the effort force by increasing the distance through which the force is applied.
Wedges and Screws
An ax is an example of a wedge.
As the thickness of a wedge of given length increases, its IMA decreases.
A screw can be described as an inclined plane wrapped around a cylinder.
Pulleys
The ideal mechanical advantage of a pulley system is equal to the number of rope segments supporting the load.
Compound Machines
A machine is classified as a compound machine if it is made up of two or more simple machines that operate together.
A bicycle is an example of a compound machine.
A watch consists of a complex system of gears. Each gear acts as a continuous lever
15.1 Energy and Its Forms
Energy and Work
Work is a transfer of energy.
Energy of an object increases when work is done on the object.
Energy and work are measured in the SI unit called the Joule.
Energy is transferred by a force moving an object through a distance.
Kinetic Energy
The energy of motion is called Kinetic Energy. KE = ½ mv2
The kinetic energy of an object is proportional to the square of its speed.
If the mass of an object doubles, its kinetic energy doubles.
Q: A small 30-kilogram canoe is floating downriver at a speed of 2 m/s. What is the canoe’s kinetic energy?
A: 60 J
Q: A 12-kg sled is moving at a speed of 3.0 m/s. At which of the following speeds will the sled have twice as much kinetic energy? 1.5 m/s, 4.2 m/s, 6.0 m/s, 9.0 m/s
A: 4.2 m/s
Potential Energy
Energy that is stored due to position or shape is called potential energy.
PE = mgh
An object’s gravitational potential energy is directly related to;
Its mass.
The acceleration due to gravity.
Its height relative to a reference level.
Q: Why is the gravitational potential energy of an object 1 meter above the moon’s surface less than its potential energy 1 meter above Earth’s surface?
A: The moon’s acceleration due to gravity is less.
Q: A 4-kilogram cat is resting on top of a bookshelf that is 2 meters high. What is the cat’s gravitational potential energy relative to the floor if the acceleration due to gravity is 9.8 m/s2?
A: 78 J
Q: A 0.49-kg squirrel jumps from a tree branch that is 3.6 m high to the top of a bird feeder that is 1.5 m high. What is the change in gravitational potential energy of the squirrel? (The acceleration due to gravity is 9.8 m/s2.) Show your work.
A: 10J
When a pole-vaulter flexes the pole, the pole-vaulter increases the pole’s elastic potential energy.
Examples:
A wind up toy that has been wound up.
A compressed basketball.
A stretched rubber band.
Forms of Energy
All energy can be considered as kinetic energy, potential energy, or the energy in fields.
The sum of the kinetic energy and potential energy of an object is called its mechanical energy.
Mechanical energy does not include Thermal energy or Chemical energy.
The total potential and kinetic energy of all the microscopic particles in an object make up its Thermal energy.
Thermal energy increases when an object becomes warmer.
The energy stored in gasoline is Chemical energy.
15.2 Energy Conversion and Conservation
Energy Conversion
Energy can be converted from one form to another.
Walking converts chemical energy into mechanical energy.
Nuclear power plants are designed to convert nuclear energy into electrical energy.
Solar cells convert electromagnetic energy into electrical energy.
Wind turbines convert mechanical energy into electrical energy.
The process of changing energy from one form to another is called energy conversion.
Conservation of Energy
“Energy cannot be created or destroyed” is a statement of the law of conservation of energy.
Energy can be converted from one form to another.
Energy Conversions
The mechanical energy of an object equals its kinetic energy plus its potential energy.
(KE+ PE)beginning =
(KE + PE)end
When an apple falls from a tree to the ground, the apple’s beginning kinetic energy and ending gravitational potential energy are both equal to Zero.
The kinetic energy of the pendulum bob increases the most between locations B and D.
The kinetic energy of the pendulum bob decreases between locations B and E.
Energy And Mass
The equation E = mc2 relates energy and mass, c is the speed of light.
Einstein’s famous equation applies during nuclear fission and fusion reactions.
Consequences of the equation E = mc2:
Energy is released when matter is destroyed.
Mass and energy are equivalent.
The law of conservation of energy must be modified to state that mass and energy are conserved in any process.
Q: In a nuclear reaction, an amount of matter having a mass of 1.0 10–14 kg is converted into energy, which is released. How much energy is released? (The speed of light is 3.0 108 m/s.) Show your work.
A: 900J
15.3 Energy Resources
Nonrenewable Energy Resources
Energy resources that exist in limited amounts and, once used, cannot be replaced except over the course of millions of years are called nonrenewable energy resources.
Examples: oil, natural gas, coal, and uranium
Fossil fuels currently account for the majority of the world’s energy use because they are relatively inexpensive and readily available.
Renewable Energy Resources
Renewable energy resources include hydroelectric, solar, geothermal, wind, biomass, and possibly in the future, nuclear fusion.
Energy obtained from flowing water is known as hydroelectric energy.
A drawback of solar energy is that it depends on the climate.
Flat collector plates through which water flows are found in active solar energy systems.
Geothermal energy, in addition to being renewable, offers the benefit of being nonpolluting.
The chemical energy stored in living things is called biomass energy
A benefit of a hydrogen fuel cell is that its byproduct is water.
Conserving Energy Resources
Turning off unused lights or appliances is an example of energy conservation.
Manufacturers can increase a light bulb’s energy efficiency by using technology that increases the amount of electromagnetic energy the bulb converts from a given amount of electrical energy.
Example: fluorescent lighting.
Turning off lights can conserve energy when they are not in use.
Using mass transportation such as buses, streetcars, and trains to move more people at a time than a car would be able too can also conserve energy.
16.1 Thermal Energy and Matter
Work and Heat
From his observations of cannon drilling, Count Rumford concluded that heat could NOT be a form of matter.
Heat is the transfer of thermal energy from one object to another because of a difference in temperature.
Temperature
A measure of how hot or cold an object is compared to a reference point can be measured in units of Kelvin or degrees Celsius.
The property of an object that is related to the average kinetic energy of the particles in that object is called temperature
Thermal Energy
Thermal energy depends on an object’s; mass, phase (solid, liquid, or gas), and temperature.
As the temperature of an object rises, so does the thermal energy of the object.
EX: A hot dinner plate has greater thermal energy than a similar dinner plate at room temperature.
Thermal Contraction and Expansion
The decrease in volume of a material due to a temperature decrease is called thermal contraction.
The increase in volume of a material due to a temperature increase is called thermal expansion.
A thermometer is a device that is based on the property of thermal expansion.
Specific Heat
In the formula , c represents the specific heat, and is measured in units of J/gC.
Q: The specific heat of copper is 0.385 J/gC. What equation would you use to calculate correctly the amount of heat needed to raise the temperature of 0.75 g of copper from 10C to 25C?
A: Q = 0.75 g 0.385 J/g·C 15C
If the temperature change of an aluminum nail is negative, thermal energy is transferred from the nail to the surroundings.
Measuring Heat Changes
A calorimeter directly measures a change in temperature.
A sealed calorimeter is a closed system because thermal energy is not transferred to the environment.
In a calorimeter, the increase in the thermal energy of the water and the decrease in the thermal energy of the sample are equal.
16.2 Heat and Thermodynamics
Conduction
The transfer of thermal energy with no overall transfer of matter is called conduction.
A material that conducts thermal energy well is called a thermal conductor. Example: metal
A material that conducts thermal energy poorly is called a thermal insulator. Example: a wooden spoon
Convection
The type of thermal energy transfer that takes place in fluids is mostly Convection.
Radiation
The transfer of energy as waves moving through space is called radiation.
As an object’s temperature increases, the rate at which it radiates energy increases.
Energy from the sun reaches Earth mostly by radiation.
Radiation is the only form of energy transfer that does not require matter for the energy to travel through.
Thermodynamics
The study of the conversion between heat and other forms of energy is called thermodynamics.
According to the first law of thermodynamics, the amount of work done by a heat engine equals the amount of thermal energy added to the engine minus the waste heat.
This law applies to heating objects, transferring thermal energy, and doing work on a system.
The second law of thermodynamics states that thermal energy can flow from colder objects to hotter objects only if work is done on the system.
Thermal energy that is not converted into work by a heat engine is called waste heat.
Disorder in the universe increases because work produces waste heat, which leaves a system.
The third law of thermodynamics states that absolute zero cannot be reached.
One consequence of the third law of thermodynamics is that heat engines have efficiencies less than 100 percent.
16.3 Using Heat
Heat Engines
External Combustion Engines; Fuel is burned outside the engine, heat is converted into work, hot steam pushes a piston. AKA steam engine.
Internal Combustion Engines, example; In most automobile engines, the linear motion of the strokes is turned into rotational motion by the crankshaft.
In most four-stroke internal combustion engines, the piston moves downward during the power and intake strokes.
Heating Systems
In a hot-water heating system, room temperature is controlled by a device called a thermostat.
A steam central heating system is often used when heating many buildings from a central location.
A steam-heating system is most similar to a hot-water heating system.
Radiant heaters have the following advantages:
They are portable
They can easily be turned on or off
They direct warm air to where it is needed
Forced-air central heating systems involve a furnace and a blower.
In forced-air heating systems, warm-air vents are usually located near the floor. Having air cleaned as it passes through filters near the furnace is an advantage of a forced-air heating system.
Cooling Systems
A fluid that vaporizes and condenses inside the tubing of a heat pump is called the refrigerant.
In a heat pump the compressor increases the pressure and temperature of the refrigerant.
As the fluid in a heat pump evaporates, thermal energy is transferred from the surroundings to the fluid.
17.1 Mechanical Waves
What are Mechanical Waves?
A mechanical wave is a disturbance in matter that carries energy from one place to another.
The matter that a mechanical wave travels through is called a medium.
A mechanical wave generally does NOT move the medium from one place to another.
Every type of mechanical wave needs a source of energy to produce it.
EX: You can make a wave in a rope by adding energy at one end of the rope.
Types of Mechanical Waves
Transverse Waves are waves that cause the medium to vibrate at right angles to the direction in which the wave travels.
EX: Waves in a rope are transverse waves because the medium’s vibration is perpendicular to the direction in which the wave travels.
A Crest is the highest point of the wave above the rest position.
A Trough is the lowest point below the rest position.
A wave in a rope is a transverse wave, but a sound wave is a longitudinal wave.
A longitudinal wave causes the medium to vibrate only in a direction parallel to the wave’s motion.
Instead of crests and troughs, as in an ocean wave, a longitudinal wave has compressions and rarefactions.
EX: In an earthquake, a P wave is a longitudinal wave. It moves through soil and rock as a series of compressions and rarefactions.
Transverse and longitudinal waves both transfer energy through a medium.
The crest of a transverse wave is most similar to compression in a longitudinal wave.
A disturbance sends ripples across water in a tub. These ripples are an example of a surface wave.
A pebble drops straight down into a tub of water, setting off surface waves that travel between the water and air.
When a surfer rides an ocean wave (surface wave) on her surfboard, she is actually riding on a crest that is toppling over.
17.2 Properties of mechanical Waves
Frequency and period
Frequency is the number of complete cycles in a given time, usually measured in cycles per second.
A period is the length of time it takes for one complete cycle to pass a fixed point.
Wavelength
Wavelength is the measured length of one complete cycle.
In a transverse wave, wavelength is measured from crest to crest or from trough to trough.
Wave Speed
To determine the speed of a wave, you must know the wave’s wavelength and frequency.
To determine the speed of a wave, you would use the equation speed = wavelength ´ frequency
S = l x f
Q: If a wave has a wavelength of 2 m and a frequency of 3.0 hertz, what is the waves speed?
Hint hertz is equal to 1/s or one over a second.
A: 6 m/s
Amplitude
Amplitude measures the greatest displacement of a wave from the rest position.
To find amplitude, measure either from a trough to the rest position, or from a crest to the rest position.
Amplitude is related to the amount of energy carried by the wave and the maximum displacement from the rest position.
To compare the energy of different waves, measure the amplitude of the waves.
17.3 Behavior of Waves
Reflection
When a wave strikes a solid barrier, it behaves like a basketball hitting a backboard. This wave behavior is called reflection.
Reflection is the only property in which the wave does not continue moving forward.
Refraction
For refraction to occur in a wave, the wave must enter a new medium at an angle.
In refraction, when two parts of a wave travel through different mediums, the parts move at different speeds.
Ocean waves will not bend if they approach the shore head on.
Diffraction
Diffraction is the bending of a wave as it moves around an obstacle or passes through a narrow opening.
Wavelength is one property of a wave that determines how much it will diffract when it encounters an obstacle.
The wave with the longest wavelength will probably be diffracted the most.
Interference
Interference occurs when two or more waves overlap and combine together.
If two waves collide and form a temporary larger wave, the interference is constructive.
Destructive interference is when two waves collide and the temporary combined wave that results is smaller than the original waves.
Standing Waves
The formation of a standing wave requires interference between incoming and reflected waves.
At the node of a standing wave, there is no displacement from the rest position.
An antinode is a point where a crest or trough occurs midway between two nodes.
A standing wave forms only if half a wavelength or a multiple of half a wavelength fits exactly into the length of a vibrating cord.
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