__ __

These questions are given to you to help you focus on the important information from chapter 7. When you can answer all of these questions without reference to your notes or the book you are ready for the test. If you can answer only half of these questions in this way do not expect to pass the test! In addition to being able to answer these questions you also need to be able to solve the PAC7 19-22 on your own without reference to your notes or your previously done work.

7.1: *Momentum*

- What does Mr. Hewitt refer to as ‘inertia in motion’?

- Write the equation used to calculate momentum. Remember what the abbreviation for momentum is.

- Describe how it is possible for an object with a small mass to have a very large momentum.

- How much momentum does an object at rest have? Use the momentum equation to prove your answer.

7.2: *Impulse Changes Momentum*

- If the momentum of an object changes what two quantities could be changing?

- If you change your momentum which of the two quantities from #5 is changing?

- So, if
velocity changes what is required? (Remember Newton’s 2
^{nd}law!)

- If you apply a force to make an object accelerate, does the amount of time the force is applied matter?

- What is ‘impulse’?

- What happens to the change in momentum if the impulse is large; if the impulse is small?

- Write the equation that shows the relationship between impulse and change in momentum.

- Take another look at the equation from #11: Show that the SI units are the same for both sides of the equation. (I.e. show that Ns = kgm/s)

- OK, when you change momentum you are really changing velocity. So, take another look at the equation from #11: using this equation explain how a small force can cause a huge change in velocity.

- Why do coaches tell their athletes to “follow through”?

- Now, let’s look at the equation from #11 from the other side: If the change in velocity is set and cannot be changed, how can you reduce the force needed to change the velocity?
- Using the equation from #11 explain why you bend your knees when you jump off a table?

- Using the equation from #11 explain why a class dish will break when it hits concrete but will not break when it lands on a carpet?

- A boxer’s face is going to stop a punch, so the change in momentum is set in stone. Which is it better for the boxer to do: (a) move his face into the punch, or (b) move his face away from the punch? Using the Impulse – change in momentum equation explain how you know.

- Speaking of violent contact sports, why is the floor of a wrestling ring made of padded material?

- It hurts a lot to fall onto a concrete floor; it hurts less to fall on a wooden floor. Using the equation from #11 explain why.

7.3: *Bouncing*

- Which does it take longer to do: (a) catch something, or (b) catch something and then throw it back?

- Consider you answer to #21: In which case is more impulse required; is which case is the change in momentum greater?

- What does the Pelton Wheel do that flat water wheels can not do?

- What advantage does the Pelton Wheel have over flat water wheels?

7.4: *Conservation of Momentum*

- Is it possible to change the momentum of your car while sitting in your car?

- Is it possible to change the momentum of something from inside that something?

- So, to change momentum, from where must the force come?

- What is the momentum of a cannonball while it is sitting inside the barrel of the cannon?

- What is the momentum of the cannon while it is sitting on the field of battle?

- If the momentum of the cannon-cannonball system is zero before the cannon is fired what must the momentum of the cannon-cannonball system be after the cannon is fired?

- So, what is the net change in momentum of the cannon-cannonball system before the cannon is fired and after the cannon is fired?

- Consider the cannon-cannonball system: Was any external force involved before, during or after the cannon was fired?
- Now
you can finish this statement:
*If no net force or net impulse acts on a system, _____*.

- What is the only way to change the momentum of a system?

- State the Law of conservation of momentum.

7.5: *Collisions*

- The two cars crash into each other the momentum before the collision and the momentum after the collision must be the same; but only under what condition?

- Define what an elastic collision is.

- When a collision results in the objects that collided become distorted or if heat is generated, what kind of collision occurred?

- In our experience perfectly elastic collisions do not happen, why not?

- Mr. Hewitt claims that perfectly elastic collisions do actually occur! Describe a perfectly elastic collision that is truly perfectly elastic.

7.6: *Momentum Vectors*

- Is momentum a scalar or a vector?

- Consider the momentum of a firecracker before it explodes and after it explodes. In one case is the momentum greater than in the other case? Explain how you know.