Physics Test Notes on Forces

Physics Notes

* If the vector of all sums is precisely zero the object could be still moving * IF an elevator fell , the passengers would be trapped to the ceiling of the elevator seeing as there is no air resistance , false * Constant speed = no acceleration = sum of all net forces is zero * An object in orbit around the earth is not truly weightless * Sitting half as far away from your class mate the gravitation force is four times as strong. (re2) * Newton’s First Law- inertia an object that is not moving will remain at rest (ex: and apple hanging from a tree) * The reason you head feels like it jerks back ward is when pulling away from a stop sign is due to Newtons first law * If two forces that are identical in strength but exactly opposite in direction act on the same object that object will not necessarily be motionless * When drawing a FBD, the net force must not be included * 2 people pushing an object across a frictionless surface will result in twice the acceleration than if only one was pushing. Assume the object slides in both cases and both people push with same force (true) *

Types of forces: * Dynamics: the study of motion * No movement forces are balanced * Applied Force: a force that results when one object makes contact with another (pushes or pulls it) * Tension: a pulling force, with a rope/ string * Normal force: only when in contact with something ie. The ground. Points away from surface * Friction: opposes the sliding of two surfaces cross one another. Act opposite to the motion/ attempt at motion (electrostatic forces) * Force diagram-show all forces acting on object

Newtons first Law: * Inertia: proportional to mass of the object, every object wants to maintain their state of motion, resists change in motion (stay in constant motion) * Law: an object will remain at rest or continue to move at a constant velocity when the net force on the object is 0. * 1st law means that there are no external forces (no friction), seat belt, the table cloth example. Ice skater, between skates and ice is minimal therefore moves a farther distance. * Non zero, net force will change velocity, change in direction or both * Net force is not required to maintain velocity * External forces are needed to change the motion of an object . ( internal no effect on motion) * Velocity constant = net force zero * Seat Belt: no seat belt you continue moving until you hit a external force ex: windshield. Sudden decrease the pendulum attached to a gear, locks due to inertia allowing the teeth of the gear to lock * Headrest: no head rest head will snap back due to the fact that when in a collision, your body goes forward, and seat exerts a force onto torso, making your head appear to snap back causing injury. Head rest brings head forward with rest of body.

Newton’s Second Law * Law: if there is a net force (external force) an object will accelerate in the direction of the net force; the magnitude of the acceleration is proportional to the net force and inversely proportional to the mass ( f=ma) * Light car with a powerful engine, that can exert a large force will experience large accelerations. * Boxes on sled; acceleration decreases , mass increases * Acceleration increases as net force increases

Newton’s Third Law: * Law: for every action force there is a simultaneous reaction force that is equal in magnitude, but opposite direction * Each action force has a reaction force (equal in opposite direction * Push on wall with 100 N, wall pushes back with 100N * Person rests on skateboard, takes steps forward, the skateboard and the person accelerate at different directions * Space Shuttle: gases at the bottom : action force, hot gas up , reaction force. Action force caused by the expanding gases is equal to the magnitude of the reaction force of the expanding hot gases on the rocket ship (reaction force = greater than gravity rocket goes up)

Using Newton’s Law * How to measure tension exactly, use a sensor or a spring scale * Tension internal force: 2 objects, one string, only drag one forward the other will move. Therefore the 2 objects will have the same acceleration when in a straight line * When asking for the tension on rope a use the weight of rope b

Gravitational Force near the Earth * Mass doesn’t change as it is the matter you are made up of * Weight is the measure of force on an object * “Weightless”= no force * Un even spacing when falling is constant acceleration * Even = constant velocity (net force) another force called friction is acting on it balances air and gravity * Free Fall: the motion of a falling object where the only force acting on it is gravity * Air resistance depends on, cross sectional area, speed, air resistance acts opposite to direction if no wind * Terminal Speed: the maximum constant speed of falling object * Force Field: a region of space surrounding an object that can exert a force on other objects those are placed within that region and are able to interact with that force. * Gravitational Field Strength: the force per unit mass acting on an object when placed in a gravitational field. ( No matter how big or small the object will always attract to the Earth, when it hits the force field) * Magnitude decreases as it moves away from the earth * Mass : quantity of matter in an object , to change add or remove matter, does not change due to location * Weight: Force of gravity on an object, depends on the location and the magnitude of the Earth’s gravitational force, is a vector * Gravity keeps astronauts in motion

Friction: * Static Friction: a force of friction that prevents the sliding of 2 surfaces relative to one another * Kinetic Friction: the force exerted on a moving object by a surface opposite to the direction of motion of the object * Friction: resists motion and acts in a direction opposite to the direction of motion * Starting: amount of force that must overcome to start an object moving * If kinetic friction and applied force are the same then there is a constant velocity * Three types of kinetic friction : sliding (sleds), rolling (bikes), and fluid (boats) * Coefficient of friction: the ratio of the force of friction to the normal force * Friction depends on : surface, mass and materiel * Coefficient of friction are determined experimentally and depend on materiel * An elevator moves vertically ward, with a constant speed. The vector sum of all the forces acting on the elevator is zero ( true ) * Newton’s first law : kicking feet against solid to remove * Feeling as being rocked from side to side * An apple from hanging from a tree * Head jerks from pulling away at a green light * Elevator: only tension force and gravity * Two bows fired, first arrow exerts twice, the average force as the bow that fires arrow B. ( arrow A has twice the acceleration of arrow B) * Four Fundamental forces: Nuclear Weak and Nuclear Strong, gravity, magnetic * Weakest is gravity * Strongest is Nuclear strong * Constant speed = no acceleration = force net to equal zero * Fnet is responsible for an objects acceleration * Gravitational law: force that the earth exerts on a group of astronauts gets weaker as the rocket rises * At all times the astronauts exert as much force on the earth as the earth on them ( third law ) * The rockets accelerates upward and the engines combined thrust overcomes gravity (second law) * Astronauts feel as though they’re being pushed back into their seats during launch ( first law) *