Unit 4: Momentum and Unit 4: Momentum and EnergyEnergy

Chap. 7Chap. 7

MomentumMomentum

Which is harder to stop, a Which is harder to stop, a truck traveling at 55 mi/hr or a truck traveling at 55 mi/hr or a

small car traveling at 55 small car traveling at 55 mi/hr?mi/hr?

Why?Why?

Which is harder to stop, a Which is harder to stop, a slow moving large truck or a slow moving large truck or a very fast moving small car?very fast moving small car?

Why?Why?

What is What is MOMENTUMMOMENTUM??

A. A. MomentumMomentum: inertia in motion: inertia in motion

B. B. MomentumMomentum = mass x velocity (or = mass x velocity (or speed)speed)

p = m p = m .. v v

C. C. MomentumMomentum is greater if it has is greater if it has1. high speed1. high speed

2. large mass2. large mass

3. or both3. or both

What could you hypothesize if the mass What could you hypothesize if the mass of an object remained the same (same of an object remained the same (same object), but the momentum changed?object), but the momentum changed?

Acceleration occurs (velocity increases)Acceleration occurs (velocity increases)

What causes accelerations?What causes accelerations? ForceForce How do you get a large acceleration?How do you get a large acceleration? large forcelarge force

oror apply a smaller force for a longer period apply a smaller force for a longer period

of timeof time

ImpulseImpulse

ImpulseImpulse: force over a period of time: force over a period of time ImpulseImpulse = force x time = force x time

I = F I = F . . tt

The higher the impulse, the greater the The higher the impulse, the greater the change in momentumchange in momentum

ImpulseImpulse

The higher the impulse, the greater the The higher the impulse, the greater the change in momentum.change in momentum.

I = I = pp

F F .. t = t = (m (m .. v) v)

Would you rather hit a brick wall Would you rather hit a brick wall or a haystack?or a haystack?

WhyWhy It has to do with It has to do with impulseimpulse.. The impulse would be the same The impulse would be the same BUTBUT the the

forceforce and the and the timetime that make up the impulse that make up the impulse are notare not the same. the same.

You would be in contact with the haystack You would be in contact with the haystack longer than you would be in contact with the longer than you would be in contact with the brick wall.brick wall.

brick wall same impulsebrick wall same impulse haystackhaystack

t t .. FF = = II = = t t .. F F

Therefore, the Therefore, the force of the brickforce of the brick wall wall would be would be greatergreater than the than the force of the force of the haystackhaystack..

If you jump off a lab table, should you hit If you jump off a lab table, should you hit the ground with your legs straight or the ground with your legs straight or should you bend your legs?should you bend your legs?

Why?Why?

Which is better, to drop a ceramic dish on Which is better, to drop a ceramic dish on the sidewalk, or on thick carpet?the sidewalk, or on thick carpet?

Why?Why?

Which would be safer, a padded Which would be safer, a padded dashboard or a metal dashboard?dashboard or a metal dashboard?

Why?Why?

HOW IS BOUNCING HOW IS BOUNCING DIFFERENT?DIFFERENT?

Impulse is greater when something Impulse is greater when something bounces.bounces.

There are really 2 steps when bouncing:There are really 2 steps when bouncing: 1. impulse to bring momentum to a stop1. impulse to bring momentum to a stop 2. provide additional impulse to bring it back 2. provide additional impulse to bring it back

upup

HOW IS BOUNCING HOW IS BOUNCING DIFFERENT?DIFFERENT?

The impact force will still react the same.The impact force will still react the same. 1. If the impulse is over a long time, then the 1. If the impulse is over a long time, then the

impact force is small.impact force is small. Example - a circus netExample - a circus net

2. If the impulse is over a short time, then the 2. If the impulse is over a short time, then the impact force is large.impact force is large. Example - a body dumped from a 10 story windowExample - a body dumped from a 10 story window

What happens to momentum What happens to momentum during collisions?during collisions?

Conservation of MomentumConservation of Momentum Total momentum before equals total Total momentum before equals total

momentum after (when there are NO momentum after (when there are NO outsideoutside forces acting).forces acting).

In the absence of an external force, the In the absence of an external force, the momentum of a system remains unchanged.momentum of a system remains unchanged.

How do we know?How do we know?

Newton’s 3rd Law: for every action, there Newton’s 3rd Law: for every action, there is an equal and opposite reaction.is an equal and opposite reaction.

Types of CollisionsTypes of Collisions

ELASTICELASTIC 1. Objects hit but don’t stick together1. Objects hit but don’t stick together 2. Transfers momentum2. Transfers momentum

INELASTICINELASTIC 1. Objects hit and stick together1. Objects hit and stick together 2. Shares momentum2. Shares momentum