Physics Chapter 7 Momentum. If a wagon were to crash into you, how much would it hurt?
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Transcript of Physics Chapter 7 Momentum. If a wagon were to crash into you, how much would it hurt?
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Physics
Chapter 7
Momentum
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Momentum
• If a wagon were to crash into you, how much would it hurt?
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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.
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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.
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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.”
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• 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.
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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!
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Example problem
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Impulse Changes Momentum
• Impulse is defined as a change in momentum.
• It can be calculated in one of two ways.
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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.
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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.
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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.
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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.
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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.
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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.
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ImpulseCase 2: Decreasing momentum
• A large force must be used if the stopping occurs over a short period of time.
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ImpulseCase 2: Decreasing momentum
• In order to use a smaller force (to prevent damage), the stopping must occur over a longer period of time.
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Bouncing
• When an object bounces, it experiences both a decrease in momentum, and then an increase in momentum in the opposite direction.
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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.
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Bouncing & The Pelton Wheel
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• 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!
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Law of Conservation of Momentum
• In the absence of an external force, the momentum of a system remains unchanged.
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Collisions
• The law of conservation of momentum is the most useful way to study collisions.
• There are 2 kinds of collisions:
elastic and inelastic.
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Elastic Collisions• When two objects collide with no
deformation, or heat, the collision is elastic.
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Inelastic Collisions
• Whenever two objects become tangled or coupled together, the collision is inelastic.
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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.
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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