# Units 4 Review

Momentum

 Units 4 Review - Answers: [ #1 | #2 | #3 | #4 | #5 | #6 | #7 | #8 | #9 | #10 | #11 | #12 | #13]

1. Which of the following objects have momentum?

a. an electron orbitting the nucleus of an atom.

b. a UPS truck stopped in front of the school building.

c. a compact car moving with a constant speed.

d. a flea moving with constant speed.

e. Glenbrook South High School.

An object has momentum if it has its mass in motion. It matters not whether the object is of large mass or small mass, moving with constant speed or accelerating; if the object is MOVING, then it has momentum!

Momentum (7 seconds)

2. A freight train rolls along a track with considerable momentum. If it rolls at the same speed but has twice as much mass, its momentum is ____.
 a. zero b. quadrupled c. doubled d. unchanged

Momentum is directly related to the mass of the object. So for the same speed, a doubling of mass leads to a doubling of momentum.

Using the Momentum Equation as a Guide to Thinking (19 seconds)

3. Impulses are smaller when bounces take place.

 a. True b. False

Collisions involving rebounding are associated with a greater velocity change, a greater momentum change, a greater impulse and a greater force.

Real-World Applications (15 seconds)

4. A karate expert executes a swift blow and severs a cement block with her or his bare hand. The

1. impulse on both the block and the expert's hand have the same magnitude.
2. force on both the block and the expert's hand have the same magnitude.
3. time of impact on both the block and the expert's hand is the same.
4. all of the above.
5. none of the above.

In any collision, there are always four quantities which are the same for both objects involved in the collision. Each object experiences the same force (Newton's thrid law) for the same amount of time, leading to the same impulse, and subsequently the same momentum change. Only the acceleration and the velocity change can ever differ for the two objects. The lower mass object always receives the greater velocity change and acceleration.

Momentum Conservation Principle (16 seconds)

5. A rocket cannot accelerate in outer space because ...

1. there is no air in space
2. there is no friction in space
3. there is no gravity in outer space
4. both a and b
5. ... nonsense! Rockets do accelerate in outer space.

Rockets accelerate in outer space by means of Newton's thrid law of motion. It does not matter that there is no air outside of the rocket. Rockets produce their own gas by burning fuels. The combustion of rocket fuels produces gaseous products. The rocket's thrusters push these gases backwards (or rightwards, or leftwards, or ...) and the gases push the rocket forwards (or leftwards, or rightwards, or ...). Thus, rockets indeed can and do accelerate in outer space.

The Law of Action-Reaction (Revisited) (8 seconds)

6. In order to catch a ball, a baseball player moves his or her hand backward in the direction of the ball's motion. Doing this reduces the force of impact on the players hand principally because ...

1. the resultant velocity of impact is lessened.
2. the momentum change is reduced.
3. the time of impact is increased.
4. the time of impact is decreased.
5. none of these.

Increasing the time over which the ball's momentum is brought to 0 will decrease the force required to stop it. Suppose a ball is coming at you with 100-units of momentum. An impulse of 100-units would be required to stop the ball. Regardless of how the impulse is accomplished (big F, little t or little F, big t), there must be 100-units of it. Imparting such an impulse over a long time results in a small force.

Real-World Applications (15 seconds)

7. Paul D. Trigger fires a bullet. The speed of the bullet will be the same as the speed of the recoiling gun ______________.

1. because momentum is conserved
2. because velocity is conserved
3. because both velocity and momentum are conserved
4. only if the mass of the bullet equals the mass of the gun
5. none of these

Since both the bullet and the gun must encounter the same momentum change, the velocity change would only be the same if their mass was the same. Otherwise, the smaller-mass object receives a greater velocity change.

The Law of Action-Reaction (Revisited) (8 seconds)

8. You're driving down the highway and a bug spatters into your windshield. Which undergoes the greater change is momentum?
 a. the bug b. your car c. both the same

In any collision, there are always four quantities which are the same for both objects involved in the collision. Each object experiences the same force (Newton's thrid law) for the same amount of time, leading to the same impulse, and subsequently the same momentum change. Only the acceleration and the velocity change can ever differ for the two objects. The lower mass object always receives the greater velocity change and acceleration.

Momentum Conservation Principle (16 seconds)

9. You're driving down the highway and a bug spatters into your windshield. Which undergoes the greater force?
 a. the bug b. your car c. both the same

See explanation for question #8.

Momentum Conservation Principle (16 seconds)

10. You're driving down the highway and a bug spatters into your windshield. Which undergoes the greater impulse?
 a. the bug b. your car c. both the same

See explanation for question #8.

Momentum Conservation Principle (16 seconds)

11. You're driving down the highway and a bug spatters into your windshield. Which undergoes the greater acceleration?
 a. the bug b. your car c. both the same

See explanation for question #8.

Momentum Conservation Principle (16 seconds)

12. Three books, X, Y, and Z, rest on a table as shown in the diagram at the right. The weight of each book is indicated. The net or unbalanced force acting on book Y is

 a. 4 N down. b. 5 N down. c. 5 N up. d. 10 N up. e. zero.

If an object is at rest, then all the forces acting upon the object must be zero. The net force on any one of the books is 0 Newtons. Subsequently, in each case, the support force (which we have called the "normal force throughout this course) acting upwards on any of the books must be equal to the force of gravity on that book (i.e., the weight) plus the amount of load exerted from above (which would be equivalent to the weight of the other books located but the book). So for book Y, the support force acting upward would be equal to 9 N while the net force is still 0 Newtons. And for book Z, the support force is 19 N, sufficient to balance the 10-N gravitational force plus the 9-N of force resulting from the other two books bearing down on it.

Net Force (9 seconds)

13. Two identical freight cars roll without friction towards each other on a level track. One rolls at 2 m/s and the other rolls at 1 m/s. After the cars collide, they couple (attach together) and roll together with a speed of _____________.
 a. 0.5 m/s b. 0.33 m/s c. 0.67 m/s d. 1.0 m/s e. none of these

Use 5000 kg as the mass of the freight cars (or any number you wish) and then set the expression for initial total momentum equal to the expression for the final total momentum:

(5000 kg)*(2) + (5000 kg) *(-1) = (5000 kg) *v + (5000 kg) *v

Now solve for v using the proper algebraic steps.

(10000 kg•m/s) - (5000 kg•m/s) = (10000 kg) v

5000 kg•m/s = (10000 kg)v

(5000 kg•m/s) / (10000 kg) = v

0.5 m/s = v

Momentum Conservation Principle (16 seconds) | Using the Momentum Equation as a Guide to Thinking (19 seconds)