chapter 6

A box is being pulled up a rough incline by a rope connected to a pulley. How many forces are doing work on the box?

3 N does no work

A truck, initially at rest, rolls down a frictionless hill and attains a speed of 20 m/s at the bottom. To achieve a speed of 40 m/s at the bottom, how many times higher must the hill be?

4x the height

f a car traveling 60 km/hr can brake to a stop within 20 m, what is its stopping distance if it is traveling 120 km/hr? Assume that the braking force is the same in both cases. a) 60 b) 30 c) 80 d) 100

80 m

A 35-N bucket of water is lifted vertically 3.0 m and then returned to its original position. How much work did gravity do on the bucket during this process? 4) A) 0 J B) 45 J C) 900 J D) 90 J E) 180 J

A

A force produces power P by doing work W in a time T. What power will be produced by a force that does six times as much work in half as much time? 40) A) 12P B) 6P C) 1/6P D) P E)1/12 P

A

Three cars (car F, car G, and car H) are moving with the same speed and slam on their brakes. The most massive car is car F, and the least massive is car H. If the tires of all three cars have identical coefficients of kinetic friction with the road surface, which car travels the longest distance to skid to a stop? 8) A) They all travel the same distance in stopping. B) Car F C) Car H D) Car G

A

Two cyclists who weigh the same and have identical bicycles ride up the same mountain, both starting at the same time. Joe rides straight up the mountain, and Bob rides up the longer road that has a lower grade. Joe gets to the top before Bob. Ignoring friction and wind resistance, which one of the following statements is true? 38) A) The amount of work done by Joe is equal to the amount of work done by Bob, but the average power exerted by Joe is greater than that of Bob. B) Bob and Joe exerted the same amount of work, and the average power of each cyclist was also the same. C) The amount of work done by Joe is greater than the amount of work done by Bob, and the average power exerted by Joe is greater than that of Bob. D) The average power exerted by Bob and Joe was the same, but Joe exerted more work in getting there.

A

Which one has larger kinetic energy: a 500-kg object moving at 40 m/s or a 1000-kg object moving at 20 m/s? 6) A) The 500-kg object B) The 1000-kg object C) Both have the same kinetic energy.

A

You throw a baseball straight up. Compare the sign of the work done by gravity while the ball goes up with the sign of the work done by gravity while it goes down. 5) A) The work is negative on the way up and positive on the way down. B) The work is positive on the way up and positive on the way down. C) The work is positive on the way up and negative on the way down. D) The work is negative on the way up and on the way down because gravity is always downward.

A

) A heavy dart and a light dart are launched horizontally on a frictionless table by identical ideal springs. Both springs were initially compressed by the same amount. Which of the following statements about these darts are correct? (There could be more than one correct choice.) 16) A) The darts both have the same kinetic energy just as they move free of the spring. B) The lighter dart leaves the spring moving faster than the heavy dart. C) Both darts had the same initial elastic potential energy. D) The heavy dart had more initial elastic potential energy th

A,B,C

Two identical grasshoppers jump into the air with the same initial speed and experience no air resistance. Grasshopper A goes straight up, but grasshopper B goes up at a 66° angle above the horizontal. Which of the following statements about these grasshoppers are correct? (There could be more than one correct choice.) 29) A) At their highest point, grasshopper A has more gravitational potential energy than grasshopper B. B) At their highest point, both of them have the same amount of gravitational potential energy. C) At their highest point, both of them have the same amount of kinetic energy. D) At their highest point, both of them have the same amount of mechanical energy. E) At their highest point, grasshopper B is moving faster than grasshopper A.

A,D,E

A stone is held at a height h above the ground. A second stone with four times the mass of the first one is held at the same height. The gravitational potential energy of the second stone compared to that of the first stone is 13) A) one-fourth as much. B) four times as much. C) one-half as much. D) twice as much. E) the same.

B

When you drop a pebble from height H, it reaches the ground with kinetic energy K if there is no air resistance. From what height should you drop it so it will reach the ground with twice as much kinetic energy? 19) A) 4H B) 2H C) 16H D) 2H E) 8H

B

When you drop a pebble from height H, it reaches the ground with speed V if there is no air resistance. From what height should you drop it so it will reach the ground with twice speed? 18) A) 8H B) 4H C) 16H D) 2H E) 2H

B

Which requires more work, increasing a car's speed from 0 mph to 30 mph or from 50 mph to 60 mph? 12) A) 0 mph to 30 mph B) 50 mph to 60 mph C) It is the same in both cases

B

A heavy frog and a light frog jump straight up into the air. They push off in such away that they both have the same kinetic energy just as they leave the ground. Air resistance is negligible. Which of the following statements about these frogs are correct? (There could be more than one correct choice.) 28) A) The heavier frog goes higher than the lighter frog. B) Both frogs reach the same maximum height. C) Just as they leave the ground, the heavier frog is moving faster than the lighter frog. D) Just as they leave the ground, the lighter frog is moving faster than the heavier frog. E) The lighter frog goes higher than the heavier frog. F) They both leave the ground with the same speed.

B,D

Two objects, one of mass m and the other of mass 2m, are dropped from the top of a building. If there is no air resistance, when they hit the ground 20) A) the heavier one will have four times the kinetic energy of the lighter one. B) the heavier one will have one-fourth the kinetic energy of the lighter one. C) the heavier one will have twice the kinetic energy of the lighter one. D) both will have the same kinetic energy. E) the heavier one will have half the kinetic energy of the lighter one

C

If the units of your answer are kg · m2/s3, which of the following types of quantities could your answer be? (There could be more than one correct choice.) 37) A) potential energy B) kinetic energy C) force D) power E) work

D

Joe and Bill throw identical balls vertically upward. Joe throws his ball with an initial speed twice as high as Bill. If there is no air resistance, the maximum height of Joe's ball will be 25) A) two times that of Bill's ball. B) roughly 1.4 times that of Bill's ball. C) equal to that of Bill's ball. D) four times that of Bill's ball. E) eight times that of Bill's ball.

D

A truck has four times the mass of a car and is moving with twice the speed of the car. If Kt and Kc refer to the kinetic energies of truck and car respectively, it is correct to say that 7) A) Kt = 4Kc . B) Kt = 1 2 Kc. C) Kt = Kc . D) Kt = 2Kc . E) Kt = 16Kc .

E

Jill does twice as much work as Jack does and in half the time. Jill's power output is 39) A) twice Jack's power output. B) one-fourth as much as Jack's power output. C) one-half as much as Jack's power output. D) the same as Jack's power output. E) four times Jack's power output.

E

Two men, Joel and Jerry, push against a car that has stalled, trying unsuccessfully to get it moving. Jerry stops after 10 min, while Joel is able to push for 5.0 min longer. Compare the work they do on the car. 2) A) Joel does 50% more work than Jerry. B) Joel does 75% more work than Jerry. C) Joel does 25% more work than Jerry. D) Jerry does 50% more work than Joel. E) Neither of them does any work.

E

When you throw a pebble straight up with initial speed V, it reaches a maximum height H with no air resistance. At what speed should you throw it up vertically so it will go twice as high? 17) A) 8V B) 2V C) 16V D) 4V E) sqrt 2V

E

wo blocks of mass m1 and m2 (m1 > m2) slide on a frictionless floor and have the same kinetic energy when they hit a long rough stretch (µ > 0), which slows them down to a stop. Which one goes farther?

M2

In a baseball game, the catcher stops a 90-mph pitch. What can you say about the work done by the catcher on the ball?

Negative work

mass attached to a vertical spring causes the spring to stretch and the mass to move downwards. What can you say about the spring' s potential energy (PEs) and the gravitational potential energy (PEg) of the mass?

PEs increases and PEg decreases

Two stones, one twice the mass of the other, are dropped from a cliff. Just before hitting the ground, what is the kinetic energy of the heavy stone compared to the light one?

Twice as much

You slam on the brakes of your car in a panic, and skid a certain distance on a straight level road. If you had been traveling twice as fast, what distance would the car have skidded, under the same conditions? 11) A) It would have skidded 4 times farther. B) It would have skidded one half as far. C) It would have skidded 1.4 times farther. D) It would have skidded twice as far. E) It is impossible to tell from the information given.

a

throw a ball straight up into the air. In addition to gravity, the ball feels a force due to air resistance. Compared to the time it takes the ball to go up, the time it takes to come back down is:

greater

Mike applied 10 N of force over 3 m in 10 seconds. Joe applied the same force over the same distance in 1 minute. Who did more work?

they do the same, it just takes joe longer