Pk4: CH3 Linear Kinetics

Kinetics

branch dynamics concerned with the forces that cause linear motion

Newtons 1st Law: Law of Inertia

everybody stays in a state of rest, unless compelled to changed by forced impressed upon it (how hard it is to move and stop)

Does this apply on earth?

Yes the world is always in s=constant motion with gravity, and air resistance

Properties of Horizontal Motion

• gravity affects it

• horizontal velocity is constant and motion is a straight line *wont slow down or speed up ie: any object in flight

• if Air resistance is significant it CANNOT be considered a projectile

Zero Net Force

• if NO external forced act, the net external force is ZERO

• Objects at rest or moving in a straight line at. constant velocity

sumF=R+(-W)=0

ex: lifting a 10lbs fumbler at a constant velocity

Newtons 2nd Law: Law of Acceleration

if a net external forced act is exerted on an object, the object will accelerate in the direction of the external force and its accerlation will be proportional to the net external force

sumF=(m)(a)

Vertical Motion of a Porjection

• sumF(y)=(m)a(y)/ W=(m)a(y)

Throw a ball straight up in the air

• neg. acceleration

• it slows down in the upward direction and speeds up in the downward direction

• acceleration does not change due to no change in mass

true or False: vertical acceleration always constant and downward

true for all projectiles regardless of mass

True or False: Weight affects. vertical acceleration

False, weight does not affect it. It will always be -9.81m/s*2 in the downward direction

Combined Hotizontal and Vertical Motion

projectiles vertical velocity is constantly decreased by 9.81 m/s for each second of its flight upward

and constantly increased by 9.81 m/s for each second of its flight downward.

Newtons 3rd Law: Law of Motion (action-reaction)

for every action there is an equal opposite reaction

• forced are equal but opposite, not the effects of the forces

• the large the mass the greater the effect

Impulse

product of force multiplied by the time a force acts

Momentum

quantity of motion, measured as the product of a body’s mass and its velocity [M = mv]

Formula fo impulse and momentum

sumFxDt=m(V(f)-V(i))

do static objects have momentum?

No velocity it ZERO

Linear Momenum

product of an objects mass and its linear velocity

ex: the faster an object move the more momentum it has

to change momentum it requires a change in velocity

How can you change you velocity ?

1. impose a larger net force

2. increase the time during which net forces act

How do you use impulse to increase momentum?

a large change in velocity is made by a large average net force acting over a long-time interval

ex: the number one thing they tell you in softball is to follow through

How can you use impulse to decrease momentum?

object has a fast intial velocity and you want to descrease it to increase the time comment to decrease the average impact force

ex: padding in football or landing mats in gymnastics

If delta T is short= sumF will be longer

if delta T is long = sumF will be smaller because it is dispersed longer

Impact (collisions)

Collision of two bodies characterized by the exchange of a large force during a small time interval

Perfectly Elastic Collision

ex: a super bouncy ball in contact with a hard surface

Perfectly Inelastic (Plastic) Collision

• at least one bodie deforms and does not regain original shape and do not separate

ex: dropping clay on a flat surface

True or False: Most impacts are either perfectly elastic or plastic

FALSE, most impacts are NOT perfectly elastic or plastic

perfectly elastic collision formula

m1u1+m2u2+m1v1+m2v2

plastic collision

m1v1+m2v2= (m1+m2)(v)

Coefficient of Restitution

describe the relative elasticity of an impact

• the closer to 1, the more elastic the impact, and the closer to 0, the more inelastic/plastic is the impact

Newtons Law of Impact

e= relative velocity after impact (vf) / relative velocity before impact (vi)

Impact of a moving object and a stationary one

e= square root of H(bounce)/H(dropped)

Does E describe the interaction between two bodies or describe ant single object?

it describes the interaction between two bodies