# Newton's Third Law & Momentum Practice Questions

Directions
Choose the letter of the best answer for each of the questions. When you are finished, click on the button at the bottom of this page that says "Score the Quiz". Good Luck!

1. When you jump off a step, you usually bend your knees as you reach the ground. By doing this, the time of the impact is about 10 times longer than if you kept your knees locked. This also makes the average force on your body
a. 10 times smaller.
b. 10 times greater.
c. about the same.
d. none of the above.

2. A 0.1 kg snowball has a speed of 18 m/s. It strikes a 0.9 kg stationary skateboard and sticks to it. After the collision, how fast are the skateboard and snowball traveling?
a. 180 m/s.
b. 162 m/s.
c. 16.2 m/s.
d. 2.0 m/s.
e. 1.8 m/s.

3. Compared to falling on a tile floor, a glass may not break if it falls onto a carpeted floor because of the
a. lesser impulse in stopping.
b. longer time to stop.
c. both of these.
d. neither of these.

4. A little moth is hit by a garbage truck on the highway. The force of the impact is greater on the
a. moth.
b. garbage truck.
c. the same.

5. A rifle recoils from firing a bullet. The speed of the rifle's recoil is small because
a. the force against the rifle is relatively small.
b. the impulse on the rifle is less than the impulse on the bullet.
c. the rifle has a lot more mass than the bullet.
d. the momentum of the rifle is unchanged.
e. none of the above.

6. A ball is moving at 4 m/s and has a momentum of 48 kg m/s. What is the mass of the ball?
a. 4 kg.
b. 12 kg.
c. 48 kg.
d. 192 kg.
e. none of the above.

7. A 2 kg object is initially at rest when it is kicked. If the impulse of the kick on the object was 40 Ns, then what is the final momentum (in kg m/s) of the object?
a. 20.
b. 40.
c. 80.
d. can't tell - you need to know how long the kick lasted.

8. If you want to ship a fragile vase, you should wrap it in bubble-wrap and use packing peanuts in the box. All this wrapping material helps to protect the vase if it is dropped or kicked because of which of the following reasons?
a. While it can't really change the total impulse on the vase, it can increase the time the impulse is acting on the vase.
b. It makes the total impulse on the vase a lot less so that the force on the vase is less.
c. It lets the vase bounce instead of break.
d. It makes the package heavier, so that it resists sudden accelerations better.

9. What impulse would be needed to stop a 1500 kg car traveling at 20 m/s?
a. You need to know the force on the car to give an answer.
b. You need to know how long the force acted to give an answer.
c. 7500 Ns.
d. 15,000 Ns.
e. 30,000 Ns.

10. You are holding a heavy physics book in your hand. What is the reaction (Newton's 3rd Law) to the weight of the book?
a. The force of the book on the earth.
b. The force of your hand holding the book.
c. The force of the book on your hand.
d. The force of the earth on the book.

11. From Newton's Third Law, the "reaction" force does not cancel the "action" force because:
a. the action force is greater than the reaction force.
b. they are in the same direction.
c. the reaction force exists only after the action force is removed.
d. they act on different objects.
e. the reaction force is greater than the action force.

12. A box is on the floor, and you are pulling it to the right. What is the reaction force to this action?
a. gravity pulling the box down.
b. the normal force pushing up on the box.
c. the box pulling you to the left.
d. the force friction on the box, if there is any.
e. none of these are correct.

13. Which of the following are units of impulse?
a. Ns.
b. m/s.
c. kg m/s^2.
d. kg.
e. kg m.

14. Why is momentum important?
a. because it is something that is conserved.
b. because it sounds cool.
c. because it is mass times velocity.
d. because McClung said so.
e. I have no clue - mark me wrong and move on.

15. If you throw an egg at a concrete wall it will break. You can throw an identical egg at a sagging sheet with the same speed and it will not break. This is because
a. The sheet lessens the impulse acting on the egg.
b. The sheet has "give" and increases the time it takes to stop the egg.
c. The collision with the concrete wall is inelastic.
d. All of the above are valid reasons.

16. You are standing still when you decide to go for a walk, and start moving to the right. If momentum is conserved, from where did you get your momentum?
a. from your legs.
b. from the earth - you pushed it to the left.
c. conservation of momentum doesn't apply in this case.
d. you already had the momentum and were just saving it untill you wanted to use it.

17. The next five questions refer to the following:

A 4 kg cart is on a horizontal frictionless track and next to a 1 kg cart. There is an explosion between the two carts that causes them to go apart. 18. How much bigger is the change in momentum of the 4 kg cart compared to the 1 kg cart?
a. The change in momentum of the 4 kg cart is the same as the change in momentum of the 1 kg cart.
b. The change in momentum of the 4 kg cart is 4 times larger than the change in momentum of the 1 kg cart.
c. The change in momentum of the 4 kg cart is 4 times smaller than the change in momentum of the 1 kg cart.
d. It would actually depend on how long the explosion lasted, so you can't tell.

19. How much bigger is the change in velocity of the 4 kg cart compared to the 1 kg cart?
a. The change in velocity of the 4 kg cart is the same as the change in velocity of the 1 kg cart.
b. The change in velocity of the 4 kg cart is 4 times larger than the change in velocity of the 1 kg cart.
c. The change in velocity of the 4 kg cart is 4 times smaller than the change in velocity of the 1 kg cart.
d. The change in velocity of the 4 kg cart is 1/2 the change in velocity of the 1 kg cart.

20. How much bigger is the impulse on the 4 kg cart compared to the impulse on the 1 kg cart?
a. The impulse on the 4 kg cart is the same as the impulse on the 1 kg cart.
b. The impulse on the 4 kg cart is 4 times larger than the impulse on the 1 kg cart.
c. The impulse on the 4 kg cart is 4 times smaller than the impulse on the 1 kg cart.
d. The impulse on the 4 kg cart is 1/2 the impulse on the 1 kg cart.

21. How much bigger is the force of the explosion on the 4 kg cart compared to the force on the 1 kg cart?
a. The force is on the 4 kg cart is the same as the force on the 1 kg cart.
b. The force is on the 4 kg cart is 4 times larger than the force on the 1 kg cart.
c. The force is on the 4 kg cart is 4 times smaller than the force on the 1 kg cart.
d. The force is on the 4 kg cart is 1/2 the force on the 1 kg cart.

22. How much bigger is the acceleration of the 4 kg cart compared to the acceleration of the 1 kg cart?
a. The acceleration of the 4 kg cart is the same as the acceleration of the 1 kg cart.
b. The acceleration of the 4 kg cart is 4 times larger than the acceleration of the 1 kg cart.
c. The acceleration of the 4 kg cart is 4 times smaller than the acceleration of the 1 kg cart.
d. The acceleration of the 4 kg cart is 1/2 the acceleration of the 1 kg cart.