KIN 330 Exam 1 Study Guide

Chapter 1: Forces
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