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Flashcards in KRS463E3CH16 Deck (61):

Newton's Laws of Motion

Law of inertia, acceleration, and reaction


Newton's First Law: Law of Inertia

“A body will maintain a state of rest or constant velocity unless acted on by an external force.”


Newton's 2nd Law: Law of Acceleration

“The acceleration of a body is proportional to the applied force and takes place in the direction in which the force is applied. It is inversely proportional to the mass of the body”


Force Formula

Force = Mass x Acceleration
Note: This applies to masses at rest and in motion.


Newton's 3rd Law: Law of reaction

“For every action there is an equal and opposite reaction”


Questions relating to Law #2:
Explain what happens when a small object traveling at high velocity collides with a disproportionally larger mass.

Only way to stop that person is to run faster because of force = mass x acceleration


Ground Reaction Force

This force is reactive force experienced when pushing against the ground during all forms of gait.


Ground Reaction Force is which of Newton’s laws is this a specific application of ?

Newton's 3rd Law: Law of Reaction


Applications of GRF

The result of taking longer strides when running.

High Jump: the object is to convert horizontal velocities to vertical velocities at the time of take off.
This is done by using GRF to change the direction of the component of force.


Mechanical Behavior of Bodies in Contact.

Frictional Forces
Know the difference between Static Friction (Fm) and Dynamic Friction (Fk)


Know the difference between Static Friction (Fm) and Dynamic Friction (Fk)

Static friction number is larger, once get something moving it’s smaller


Frictional Force =

lu x R


Where “lu” is the

coefficient of friction and is dependent on the nature of the surfaces in contact with each other.


“R” is the

perpendicular or “normal” reaction force.


coefficient of static friction is greater than

coefficient of kinetic friction


coefficient of static friction used to find

the minimum force necessary to start the sled moving.


coefficient of kinetic friction used to find

This is the minimum force necessary to keep the sled moving.


Pushing vs. pulling an object on the floor.
Resolving horizontal and vertical forces it is possible to determine which movement results in less friction.

Pulling is easier because pushing adds to the normal force (weight of table)


Linear Momentum

Defined as the amount/quantity of motion a moving body possesses.


Linear Momentum Formula

M = m x v (Mass x velocity)


Units of Momentum

Units of momentum – mass.velocity (ex. kg.m/sec


The Principle of “Conservation of Momentum”

Deals primarily with colliding bodies.
When colliding, the bodies can either be moving in opposite directions, or in the same direction.
The total “M” before the collision is equal to the “M” after the collision


Fomulas for conservation of momentum

For two bodies that collide and then separate.
m1.v1 + m2.v2 (Before Collision) = m1.v3 + m2.v4 (After Collision)


what variable for the conservation of momentum formula has changed and what has remained the same after the collision?

the velocity


Formulas for conservation of momentum: For two bodies that collide and coalese (stick together) after the collision.

m1.v1 + m2.v2 = (m1+ m2).v3



Defined as the product of a “force” and the “time” over which the force acts.
-also change in momentum


impulse formula

f x t


Theoretically, impulse can be increased by

increasing either force or time.


In reality, with many activities, such as jumping, applying the force over a prolonged time is



The same impulse can be obtained from:

A small force applied for a long time.
Or a large force applied for a brief time.



Defined as the “collision” between two bodies or objects.


in impact, The behavior of the two objects depends on:

The initial momentum.
Surfaces or materials that make up the objects.


To observe what happens after impact:

Objects are classified depending on the composition of the material of the body.
At one end of the continuum are Perfectly Elastic Bodies.
At the other end are Perfectly In-elastic (Plastic) Bodies.


For perfectly elastic bodies

The velocity of each object after collision is the same as before the collision.
This means that there was no distortion of either body at the time of impact.
No energy was lost through this distortion.


Think of an example of two “almost” perfectly elastic objects colliding with each other.
Hint: Perfectly in-elastic bodies do not separate after impact.

Steel bouncing on steel, least distortion the more it will bounce
-more distortion, will not bounce high


The coefficient of “restitution” (e)

Defined as the “index of the elasticity for colliding bodies”.
Can also be viewed as the capability of an object to rebound after being deformed at impact.
Assumes a number between zero and 1.


“e” can be measured in 2 ways:

Comparing the velocity of a dropped object the instant before it hits the surface, with the velocity of the object the instant it leaves the surface on the rebound.


e =

= Vs/ Va
Vs is the velocity of “separation”,
Va is the velocity of “approach”


Or e can be measured by:

Calculating the height the object rebounds to after being dropped from a known height.


e2 =

= h/ H
“h” is the height to which the object rebounds.
“H” is the height from which the object dropped.


Injury Prevention and Applied Forces

Injury-prevention in sports by the reduction of the “force of impact” will be examined at this time.


The areas discussed in this lecture are based on the following equations:

Force x distance
½ mv2


Force x distance
½ mv2

Question: What are these formulas?

Work = f x d
Kinetic energy = 1/2mv^2



Work has to be done on an

object for it to acquire kinetic energy.
Therefore, these principles apply to injuries resulting from “falls” and “collisions”.


Point # 1

The greater the mass of the object, the more severe the force of impact.


application of point #1

A heavier the person the harder the perceived force of impact, ie. when falling or colliding.


Point # 2

The greater the velocity during the fall, the greater the force of impact.
K.E.  v2


Point #2 important note

Important: Note that the change in kinetic energy is proportional not to the velocity but the “square” of the velocity!


Applications in Point # 2 Question: How does this point apply to contact sports?

Greater the distance, faster you go, more it will be felt


Point # 3

The greater the distance or time over which the body decelerates upon impact, the smaller the average force that will be felt.


Applications in Point # 3: Question: What is done to reduce the K.E. of a pole vaulter to reduce the force of impact at landing?
How about fielding a fast-ball?
How about running surface?

***how about running surface?
-easier on knees on a softer surface but need to work harder


Point # 4

The greater the area of impact, the less the force acting on any given point on the body.


Point #4 Applications

Applications: Falling in an “energy absorbing” manner.


How does point #4 Apply to Boxing?
Air bags in automobiles?
Martial arts?

Boxing? Bigger gloves less force of impact


how rolling friction differs from static and dynamic friction

coefficient of rolling friction is less than static and dynamic
- it is when one surface is rolling over another but not sliding across it


how anti-lock brakes work?

slowly brings the wheels to a stop by generating a precise amount of braking force to keep the tires just below the point of completely coming to a stop
-maintains static friction between tire and pavement to allow max breaking force while letting driver maintain control of vehicle


Which has greater traction on the road?
wide track tires vs. standard tires

wide-track tires


Friction and ballet dancing. Why rosin is used on dance floors?

Rosin: increases coefficient of static friction and decreases coefficient of dynamic friction, when standing you can grip the floor , once push off can easily move


rosin ? static friction and ? kinetic friction

increases static friction and decreases kinetic friction


coefficient of friction of turf compared to grass

less friction but may cause more injuries


Why driving with under-inflated tires uses more gas.
What type of friction is under consideration?

causes more friction and needs more gas to move the car
-rolling friction