Define force

Force: a push or a pull

FORCE IS A VECTOR (HAS A SIZE & A DIRECTION)

Units: Newtons

1 N = the force required to accelerate a 1 kg mass 1 m/s2

1 N= 0.225 lbs

1 lb = 4.448 N

Classify forces

Internal Forces: forces that act within the object of system whose motion is being investigated

Pulling= tensile forces (putting the structure under tension)

Pushing= compressive forces (putting the structure under compression)

Internal forces hold things together when the structure is under tension or compression but sometimes the tensile of compressive forces acting on a structure are greater than the internal forces the structure can withstand leading to the structure to fail and break (muscle pulls, tendons rupture, ligaments tear, bones break)

Important for studying the nature and the causes of injuries

Muscles can only change our motion if they can apply force against some external object

Examples:

Running (pushing off the ground)

Kicking (applying force to the ball)

External Forces: forces that act on an object as a result of its interaction with the environment surrounding it

Either contact forces or noncontact forces

Touching vs. non-touching

Examples of contact forces:

Solids (ground) or fluid

Friction (horizontal)

Normal contact force (vertical)

Examples of noncontact forces:

Gravity

Magnetic forces

Electrical forces

Define friction force

Frictional force: the component of the contact force responsible for changes in the runner’s horizontal motion; primarily responsible for human locomotion

Dry Friction: acts between the non-lubricated surfaces of solid objects or rigid bodies in contact and acts parallel to the contact surfaces

Static friction: when dry friction acts between two surfaces that are not moving relative to each other

Dynamic friction (aka sliding friction or kinetic friction): when dry friction acts between two surfaces that are moving relative to each other

Dry friction is not affected by the size or the surface area in contact

static friction force = coefficient of static friction

= normal contact force

dynamic friction force = coefficient of dynamic friction

= normal contact force

Affected by weight and material type

Do we want to increase or decrease friction?

Skiing decrease friction

Putting chalk on hands for gymnastics increase friction

Athletic shoes have rubber soles increase friction

Define weight

Weight: the force of gravity acting on an object

Acceleration due to gravity is 9.81 m/s2

F= m*a

W= m*g

W= 9.81 N

Determine the resultant of two or more forces

Resultant force: vector addition of two or more forces

Net force: vector addition of all the external forces acting on an object

Can’t just add up all of the forces

Forces are vectors and need direction

Colinear Forces: forces that have the same line of action (work in the same direction)

Easiest to deal with

Can simply add or subtract depending if they are pushes or pulls

Concurrent Forces: forces that do not act along the same line but do act through the same point

Ways to calculate:

Use a free body diagram and a graphical representation of all forces

Use Pythagorean’s theorem:

Use trig (SOHCAHTOA)

To calculate angles if side lengths are known, use inverse function

Resolve a force into component forces acting at right angles to each other

Determine whether an object is in static equilibrium, if the forces acting on the object are known

If the object is at rest, the forces are in are in equilibrium, and the object is described as being in a static equilibrium

This means that the net forces are equal to zero

Determine an unknown force acting on an object, if all the other forces acting on the object are known and the object is in static equilibrium

Chapter 2: Linear Kinematics

Distinguish between linear, angular, and general motion

Linear: also referred to as translation; occurs when all points on a body or object move the same distance, in the same direction, and at the same time

Can happen in 2 ways:

Rectilinear translation: occurs when all points on a body or object move in a straight line so that the direction of motion does not change, the orientation of the object does not change, and all points on the object move the same distance

Curvilinear translation: occurs when all points on a body or object move so that the orientation of the object does not change and all points on the object move the same distance

Curved, so the direction of motion of the object is constantly changing even though the orientation of the object does not change

Examples:

Figure skater, gymnast on a trampoline, diver, ski jumper, skateboarder, in-line skater

Angular: also known as rotary motion or rotation; occurs when all points on a body or object move in circles about the same fixed central line or axis

Can occur about an axis within the body or outside of the body

Examples:

Child on swing (external to the body)

Ice skater in a spin (within the body)

General: combination of linear and angular motions; most common

Examples:

Angular motion at knee and hip can produce linear motion at the foot

Angular motion at the shoulder and elbow can produce linear motion at the hand

Running and walking; trunk moves linearly as a result of the angular motions of the legs and arms

Bicycling

Define distance traveled and displacement and distinguish between the two

Distance traveled: measure of the length of the path whose motion is being described from starting (initial) position to ending (final) position

DIRECTION OF TRAVEL IS NOT CONSIDERD

Displacement: the straight line distance in a specific direction from initial (starting) position to final (ending) position

Resultant displacement: the distance measured in straight line from the initial position to the final position

Displacement is a vector (associated with size and a direction); can be resolved into components

Formula: d= Δy; final position – initial position

Define average speed and average velocity and distinguish between the two

Average speed: distance traveled divided by the time it took to travel that distance

Units: meters per second

Average Velocity: displacement of an object divided by the time it took for that displacement

Vector (has a magnitude and a direction)

Units: meters per second

Define instantaneous speed and instantaneous velocity

Instantaneous speed: the speed of an object at a specific instant in time

Instantaneous velocity: the velocity of an object at an instant in time

Define average acceleration

Average acceleration: the change in velocity divided by the time it took for that velocity change to take place

Acceleration is a vector

Can be positive or negative

Units: m/s2

Define instantaneous acceleration

Instantaneous acceleration: the acceleration of an object at an instant in time (rate of change of velocity at that instant in time)

Use the equations of projectile motion to determine the vertical or horizontal position of a projectile given the initial velocities and time

Vertical position

Horizontal position

if initial position is zero:

Chapter 3: Linear Kinetics

Explain Newton’s three laws of motion

Newton’s 1st Law (law of inertia): if no net external force acts on an object, that object will not move if it wasn’t moving to begin with, or it will continue moving at constant speed in a straight line if it was already moving

Newton’s 1st Law applies:

If air resistance is negligible, the net horizontal force acting on a projectile is zero, so the horizontal velocity of the projectile constant and unchanging

External forces acting on an object only if the sum of those forces is zero (object is in a state of static equilibrium)

If an object is moving at constant velocity in a straight line, then the sum of all the external force acting on the object is zero

Reaction force: what is exerted against an object’s weight

Example: The force we exert on a 10 kg dumbbell to hold still

Summary of Newton’s 1st Law:

If an object is at rest and the net external force acting on it is zero, the object must remain at rest

If an object is in motion and the net external force acting on it is zero, the object must continue moving at constant velocity in a straight line

If an object is at rest, the net external force acting on it must be zero

If an object is in motion at constant velocity in a straight line, the net external force acting on it must be zero

Newton’s first law applies to the resultant motion of an object and to the components of this resultant motion

Conservation of Momentum

Linear momentum is the product of an object’s mass and its linear velocity

Momentum is a way of quantifying the motion and inertia of an object together in one measure

Applies to all 3 planes of movement

Total momentum of a system of objects is constant if the net external force acting on the system is zero

See this in collisions

Examples: ball with bat, ball with racket, ball with feet, person with person

Elastic Collisions: when momentum is transferred from one object to another

If objects are different sizes (quarter and a penny): smaller mass & larger mass quarter hits penny and penny accelerates more

Moving in opposite direction: equal and opposite

Same direction: Two objects moving in same direction at different velocities the faster object collides with the slower moving object if the two objects have the same mass, the momentum of the faster-moving object is completely transferred to the slower-moving object

Inelastic Collisions: momentum is still conserved and the objects in the collision stay together after the collision and move together with the same velocity

Example: NASCAR, football

Most collisions in the real world are not perfectly inelastic or perfectly elastic

Coefficient of Restitution: absolute value of the ratio of the velocity of separation to the velocity of approach

Velocity of separation is the difference between the velocities of the two colliding objects just after the collision (how fast they’re moving from each other)

Velocity of approach is the difference between the velocities of the two colliding objects just before the collision (how fast they’re moving towards each other)

Helps determine how elastic a collision is

No units

For perfectly elastic collisions, the coefficient of restitution is 1

For perfectly inelastic collisions, the coefficient of restitution is 0

Newton’s 2nd Law (law of acceleration): any time an object starts, stops, speeds up, slows down, or changes direction, it is accelerating and a net external force is acting to cause this acceleration

OR

If a net external force is exerted on an object, the object will accelerate in the direction of the net external force, and its acceleration will be directly proportional to the net external force and inversely proportional to its mass

Applies to all 3 planes of movement

Ex. Riding an elevator and how it effects how your weight feels

Finding acceleration with known vertical forces:

When lifting an object vertically, more force must be applied larger than the weight of the ball

Newton’s 3rd Law (law of action-reaction): if an object exerts a force on another object, the other object exerts the same force on the first object but in the opposite direction

Example: pushing on a wall or standing on the floor

The force you exerted against the wall does not act on you, so it can’t counteract the effect of the force the wall exerts on you

Apply Newton’s second law of motion to determine the acceleration of an object if the forces acting on the object are known

Apply Newton’s second law of motion to determine the net force acting on an object if the acceleration of the object is known

Define impulse

Impulse: the product of force and the time during which the force acts

Define momentum

Momentum: the product of an object’s mass and its linear velocity

Explain the relationship between impulse and momentum

The average net force acting over some interval of time will cause a change in momentum of an object

Using impulse to increase momentum:

Examples in sport where an object starts at zero of low velocity and we apply force to get it to a high velocity

Throwing a tennis ball in the air and hit with the racket

Golf ball on tee and the gets hit by golf club

Following through in sports

Techniques in sports such as throwing or jumping are largely based on increasing the time of force application to obtain a large impulse

Accelerate something for the longest period of time with the greatest force possible OR apply the greatest force possible for the longest amount of time possible

Using impulse to decrease momentum:

Examples in sports:

Flexing arms or legs help with decreasing momentum and prevent injuries by increasing

Increasing impact time () decreases the average impact force ()

This plays a role in equipment design; example: shoulder pads

Describe the relationship between mass and weight

Newton’s Law of Universal Gravitation: states that all objects attract each other with a gravitational force that is inversely proportional to the square of the distance between the objects

Also states that this force of gravity is proportional to the mass of each of the two bodies being attracted to each other

becomes

Chapter 4: Work, Power, and Energy

Define mechanical work

Work: product of force and the amount of displacement in the direction of the force; it is the means by which energy is transferred from one object or system to another

Units: Nm or J (joules)

work done on an object = average force exerted on an object = displacement of an object along the line of action of the average force

To determine work, must know:

Average force exerted on the object

Direction of the force

Displacement of an object along the line of action of the force during the time the force acts on the object

Distinguish the differences between positive and negative work

Positive work: when the object is displaced in the same direction of the force

Negative work: when the object is displaced in the opposite direction of the force

Catching

Lowering a weight

Landing

Skiing down hill

Define energy

Energy: capacity to do work

2 forms of energy: kinetic and potential

Define kinetic energy

Kinetic energy: a moving object has the capacity to do work due to its motion

Units: kg(m2/s2) = Nm = Joule

Define gravitational potential energy

Potential energy: the energy that an object has due to its position

2 types of potential energy: gravitational potential energy and strain energy

Gravitational potential energy: potential energy due to an object’s position relative to the earth

OR

Define strain energy

Strain energy: energy due to the deformation of an object

Examples: fiberglass vaulting pole bends, archer draws his bow, a diver deflects a diving board

= stiffness or spring constant of material

= change in length of deformation of the object from its undeformed position

Explain the relationship between mechanical work and energy

Work is the means by which energy is transferred from one object or system to another

Doing work to increase energy:

Impulse

A large change in KE requires that a large force be applied over a long distance

Ex. Shot put

Define power

Power: rate of doing work or how much work is done in a specific amount of time

Units: Watts

= Power

= work done

= time taken to do the work

Chapter 5: Torques and Moments of Force

Define torque (moment of force)

Torque: a turning effect produced by a force; angular or rotary force; also called a moment of force

Torque is a vector

Positive= counter clockwise

Negative= clockwise

Units: Nm

moment arm or perpendicular distance

Examples:

Torque wrench

Rowing sports (rowing, canoeing, and kayaking)

Striking sports (golf, baseball, tennis)

Any sport where we turn, spin or swing something

Wrestling

Muscular torque: most muscular contractions produce torque about our different joints

Size of movement arm changes as we move, this is partially why our muscles are stronger in some joint positions than others

Centric force: an external force directed through the center of gravity of an object

Effect of a centric force is linear movement

Eccentric force: an external force not directed through the center of gravity of an object

Effect of an eccentric force is to change both the linear and angular motions of an object

Force couple: a pair of forces which are equal in size both opposite in direction and non-colinear

Effect of a force couple is to cause a change in only the angular motion of an object

No linear change based on Newton’s 1st Law

Define static equilibrium

The external forces must sum to zero and the external torques must sum to zero as well

List the equations of static equilibrium

Determine the resultant of two or more torques

Determine if an object is in static equilibrium, when the forces and torques acting on the object are known

Linear: for an object to be at static equilibrium the external forces acting on it must sum to zero

Angular: for an object to be at static equilibrium the external torques acting on it must sum to zero

For an object to be at equilibrium the external forces must sum to zero and the external torques (about any axis) must sum to zero

Determine an unknown force (or torque) acting on an object, if all the other forces and torques acting on the object are known and the object is in static equilibrium

Define center of gravity

Center of gravity: the point at which the entire mass or weight of the body may be considered to be concentrated

the movement arm of the entire weight of the object

Estimate the location of the center of gravity of an object or body

COG of the Human Body

55% of height in females & 57% for height in adult males

Males more than females because shoulders are wider

Child’s COG would be higher because their head us bigger than their body

In high jumper, COG below the bar

In pole vaulter, below chest area and in the front

COG and Performance

Think about volleyball and basketball players

Think about VJ test

How do some athletes appear to hang in the air?

Gymnast leaping

COG and Stability

Stability is the capacity of an object to return to equilibrium or to its original position after it has been displaced

When do we want to be stable?

Football linemen

Basketball post players

Wrestling

When do we want to be unstable? (able to move quickly)

Receiving a serve

Sprinter

Goalie

Skiing

Swimmer

Factors affecting stability

The stability of an object is affected by the height of the COG, the size of the base of support, and the weight of the object

Base of support is the area within the liens connecting the outer perimeter of each of the points of support

toppling force = moment arm of toppling force

= weight of object = moment arm of the book

Stability and Potential Energy

The most stable stance or position that an object or person can be in is the one that minimizes potential energy

Lower h

COG, Stability, and Human Movement

Stability is maintained only as long as the line of gravity falls within the base of support

Walking is a series of falls and catches

Sometimes you want to maximize stability in a certain direction

Examples:

Defensive lineman

Catching a ball

Posting up in basketball

What do you want to minimize stability (increase mobility)?

Swimming

Sprinting