Review Questions: Momentum and Energy
Answers
1) Suppose you find yourself teetering on the edge of a cliff or the roof of a tall building with a heavy physics book in your hand. As you are about to fall, you suddenly remember how physics can save you! What law did you remember?
a) law of inertia
b) law of gravitation
c) law of conservation of momentum
d) law of conservation of energy
Reasoning: If you throw the books forward (!), then you will suffer a backward momentum because of the law of conservation of momentum.
2) A heavy truck has more momentum than a passenger car moving at the same speed because the truck
a) has greater mass
b) has greater speed
c) is not streamlined
d) has a large wheelbase
3) Momentum is conserved
a) in an elastic collision of two balls
b) in an inelastic collision of two balls
c) in the absence of an external force
d) in all of the preceding cases
4) A gun recoiling when it is fired is an example of
a) conservation of momentum
b) conservation of angular momentum
c) conservation of energy
d) none of the above
Reasoning: See page 93 of the text.
5) A heavy object and a light object have the same momentum. Which has the greater speed?
a) heavy object
b) light object
c) both have the same speed
d) the heavy object has a zero speed while the light object has a nonzero speed
Reasoning: Momentum = mass x velocity. Thus if two objects have the same momentum, the lighter object will have a larger velocity.
6) A 1500 kg truck traveling at 80 km/h collides with another car of mass 1000 kg traveling at 30 km/h in the same direction. The two cars stick together after the collision. Their speed immediately after the collision is
a) 40 km/h
b) 60 km/h
c) 110 km/h
d) 55 km/h
Reasoning: Use the conservation of momentum law.Study the examples on page 96-97 of the text.
7) A baseball player grabs a 200 gram baseball which is moving at speed of 30 m/s. The ball comes to rest in about 1/10 second. The force acting on the player’s hand is (Think of the relationship between change of the momentum and force),
a) 60 N
b) 200 grams
c) 30 m/s
d) 2000 N
Reasoning: See page 88-89 of the text. Using F = change in momentum/contact time, we get: F = (.2 kg x 30 m/s)/(.1 s) = 60 N.
8) A change in momentum may result from
a) an acceleration
b) a force
c) an impulse
d) all of the above
9) A boxer quickly learns to move his head backward when he sees he is going to receive a jab to the head. What does this head motion accomplish?
a) It increases the momentum and decreases the force
b) It increases the contact time which decreases the force
c) It decreases the contact time which decreases the force
d) It causes confusion on the opponent.
Reasoning: Using the relationship between force and momentum (discussed on question 7 and on page 89 of the text), as the time of contact increases the force will decrease.
10) Guardrails along roadsides collapse (bend and crumple) when a car runs into them. This is done
a) so they can replace them every year and provide jobs for unemployed workers
b) so the cars cannot rebound back to the middle of the road
c) to increase the contact time and decrease the crash force
d) to increase the momentum and decrease the crash force
11) A bazooka, or rocket launcher, is essentially a large tube, open at both ends, that a soldier holds on the shoulder when firing a heavy shell. There is a very little recoil on the person firing the shell. Why?
a) Momentum is not conserved in this case, and as a result the tube does not recoil.
b) Momentum is shared between the gas and the shell; the tube does not recoil.
c) Since the tube is held on the shoulder, its mass is high and it does not recoil.
d) There is a significant recoil; it is absorbed by the large mass of the tube.
12) In order to reduce the "sting" in catching a hard ball one usually,
a) increases momentum change
b) increases the contact force
c) increases the impulse
d) increase the contact time
13) For the same force, which one imparts a greater momentum to a cannonball?
a) a long cannon
b) a short cannon
c) either (a) or (b) depending on the air resistance
d) neither (a) nor (b). The momentum is independent of the length of the cannon
Reasoning: the force is constant. To increase the momentum of the outgoing shell, the time of contact must increase. Thus the long cannon produces a larger momentum.
14) Which of the following objects has the largest kinetic energy?
a. an 8-kg object moving at 2 m/s
b. a 4-kg object moving at 4 m/s
c. a 6-kg object moving at 2 m/s
d. a 6-kg object moving at 3 m/s
Reasoning: KE = 1/2 m v2 = (1/2)(4)(4)to the power 2 = 32 J. The others are all smaller than this number.
15) A 100-kg box is pushed with a force of 35 lb. If the box is displaced by 6 meters, the amount of work done on the box is
a. 3500 ft.lb
b. 600 kg.m
c. 210 Joules
d. 924 Joules
Reasoning: Work = force x distance. We need force in newtons. Since 1 lb = 4.4 newtons, thus 35 lb = 35 x 4.4 = 154 N. Now work = 154 x 6 = 924 J. See page 61).
16) If the box of the previous question is raised to a height of 1.5 meters, its potential energy is
a. 1470 Joules
b. 150 Joules
c. zero
d. 3500 Joules
Reasoning: PE = (m)(g)(h) = (100 kg)(9.8)(1.5) = 1470 J. (see page 107 of the text).
17) If the box of the previous question is dropped, its K.E. right before coming to rest is
a. 1470 Joules
b. 150 Joules
c. zero
d. 3500 Joules
Reasoning: This is the consequence of the law of conservation of energy.
18) A 200-kg rock is pulled out of a well of depth 20 meters at a constant speed in 4 seconds by a motor. The power used by the motor is
a. 4000 watts
b. 9800 watts
c. 1000 watts
d. 16000 watts
Reasoning: power = (work (or energy))/time. But work = force x distnace = (200 x 9.8)(20) = 39200 J. Thus power =( 39200 J)/(4 s) = 9800 watts.
19) A car traveling at a speed of 40 km/hr increases its speed to 80 km/hr. As a result its kinetic increases
a. 2 times
b. 4 times
c. 8 times
d. ....stays the same
Reasoning: Doubling the speed, increases the KE to 4 times.
20) A 3-kg bowling ball is sliding with a speed of 4 m/s. Its K.E. is
a. 48 Joules
b. 12 Joules
c. 24 Joules
d. 12 watts
Reasoning: Follow the reasoning presented for question 14.
21) A lawn mower engine is rated as 5 hp. This means that the engine
a. can perform 5 joules of work per second
b. can perform 3730 joules of work per second
c. can perform 550 joules of work per second
d. can perform 746 joules of work per second
Reasoning: 1 hp = 746 watts. Thus 5 hp = 5 x 746 = 3730 watts or 3730 joules/second.
22) The unit of energy in the metric system (SI) is
a. Newton
b. ft.lb
c. Joule
d. Watt
23) A 4-kg box is pushed against a flat surface with a force of 100 N for 5 meters. The work done on the box is
a. 500 Joules
b. 400 Joules
c. 392 Joules
d. 200 Joules
Reasoning: work = force x distnace = (100 N)(5 m) = 500 J
24) A garage door opener performs 1500 Joules of work to open a garage door in 2 seconds. The power used by the door opener is
a. 3000 ft.lb/sec.
b. 750 Joules
c. 1500 hp
d. 750 watts
25) An apple hanging from a limb has potential energy because of its height. If it falls, what becomes of this energy just before it hits the ground?
a. its total energy goes to zero
b. its P.E. is transformed into kinetic energy
c. its energy is transformed into the thermal energy of the apple and the ground
d. its P.E. increases and just before it hits the ground it is doubled