Physics

Chapter 7

Momentum

Momentum

• If a wagon were to crash into you, how much would it hurt?

Momentum

• If a wagon were to crash into you, how much would it hurt?

• It would depend on

1) how massive the wagon is.

2) how fast the wagon is.

Momentum

• The momentum of an object helps us to consider both the speed at which an object is moving, and how massive it is, at the same time.

Momentum

• If either the mass or the speed is large, the object can have a large momentum.

• As the textbook notes, “a weighty ship moving at a small speed and a lightweight bullet moving at a high speed can both have the same large momentum.”

• Think back to Newton’s Third Law - action and reaction• Why does a rifle recoil when fired?• If the forces are equal and opposite, why

does the rifle experience less acceleration than the bullet?

• We can also consider this question in terms of momentum.

Action and Reaction

• If the rifle and bullet have a combined momentum of zero before the shot… their combined momentums after the shot must add up to zero.

• They must be equal and opposite!

Example problem

Impulse Changes Momentum

• Impulse is defined as a change in momentum.

• It can be calculated in one of two ways.

Impulse Changes Momentum

• Impulse is defined as a change in momentum.

• It can be calculated in one of two ways.

- final momentum minus initial momentum.

Impulse Changes Momentum

• Impulse is defined as a change in momentum.

• It can be calculated in one of two ways.

- final momentum minus initial momentum.

- force times time.

ImpulseCase 1: Increasing momentum

• To increase the momentum of an object, a force must be applied to it.

• The more the force, the greater the impulse.• The longer the time that the force is applied, the

greater the impulse.

ImpulseCase 1: Increasing momentum

• To increase the momentum of an object, a force must be applied to it.

• The more the force, the greater the impulse.• The longer the time that the force is applied, the

greater the impulse.

Impulse

• This is why you “follow through” in sports – volleyball, baseball, tennis, hockey, boxing, etc. It increases the amount of time that the force is applied.

ImpulseCase 2: Decreasing momentum

• To decrease the momentum of an object, a force must be applied to it.

• In order to stop a moving object quickly, a large force is applied to it for a short time.

• In order to use a smaller force (to prevent damage), the stopping must occur over a longer period of time.

ImpulseCase 2: Decreasing momentum

• A large force must be used if the stopping occurs over a short period of time.

ImpulseCase 2: Decreasing momentum

• In order to use a smaller force (to prevent damage), the stopping must occur over a longer period of time.

Bouncing

• When an object bounces, it experiences both a decrease in momentum, and then an increase in momentum in the opposite direction.

Bouncing• An impulse is

required to stop a falling object…

• another impulse is required to throw that object back into the air!

• Therefore, it takes more impulse to bounce than just to stop.

Bouncing & The Pelton Wheel

• Think back to Newton’s Second Law

F=ma; if you wish to change the velocity of an object, you must apply a force.

• This can also be stated; if you wish to change the momentum of an object, you must apply an impulse.

• If no outside force is present, the total momentum cannot change!

Law of Conservation of Momentum

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

Collisions

• The law of conservation of momentum is the most useful way to study collisions.

• There are 2 kinds of collisions:

elastic and inelastic.

Elastic Collisions• When two objects collide with no

deformation, or heat, the collision is elastic.

Inelastic Collisions

• Whenever two objects become tangled or coupled together, the collision is inelastic.

Homework

• Chapter 7 Review Questions, page 99 #1-22 do in class Tuesday, 11/15

• If you aren’t here on Tuesday, it is assigned homework, due when you come back.

Credits• Many images taken from Hewitt textbook• Sports images taken from ESPN• Tigger from drtiggerishere.com, image probably owned by Disney• Trampoline image from http://powder.physics.sunysb.edu/Peter/Peter.htm• Einstein photo public domainl