WORK AND ENERGY
MULTIPLE CHOICE
1. In which of the following sentences is work used in the everyday sense of the word?
a. Lifting a heavy bucket involves doing work on the bucket.
b. The force of friction usually does negative work.
c. Sam and Rachel worked hard pushing the car.
d. Work is a physical quantity.
2. Work is done when
a. the displacement is not zero.
b. the displacement is zero.
c. the force is zero.
d. the force and displacement are perpendicular.
3. If the sign of work is negative,
a. the displacement is perpendicular to the force.
b. the displacement is in the direction opposite the force.
c. the displacement is in the same direction as the force.
d. no work is done.
4. In which of the following scenarios is no net work done?
a. A car accelerates down a hill.
b. A car travels at constant speed on a flat road.
c. A car decelerates on a flat road.
d. A car decelerates as it travels up a hill.
5. A worker does 25 J of work lifting a bucket, then sets the bucket back down in the same place. What is the total net work done on the bucket?
a. -25 J c. 25 J
b. 0 J d. 50 J
6. A horizontal force of 200 N is applied to move a 55 kg television set across a 10 m level surface. What is the work done by the 200 N force on the television set?
a. 550 J c. 6000 J
b. 2000 J d. 11000 J
7. Which of the following energy forms is associated with an object in motion?
a. potential energy c. nonmechanical energy
b. elastic potential energy d. kinetic energy
8. Which of the following energy forms is not involved in hitting a tennis ball?
a. kinetic energy c. gravitational potential energy
b. chemical potential energy d. elastic potential energy
9. Ball A has triple the mass and speed of ball B. What is the ratio of the kinetic energy of ball A to ball B.
a. 3 c. 9
b. 6 d. 27
10. Which of the following equations expresses the work-kinetic energy theorem?
a.MEi = MEf c. ΔW = ΔKE
b.Wnet = ΔPE d. Wnet = ΔKE
11. The main difference between kinetic energy and potential energy is that
a. kinetic energy involves position, and potential energy involves motion.
b. kinetic energy involves motion, and potential energy involves position.
c. although both energies involve motion, only kinetic energy involves position.
d. although both energies involve position, only potential energy involves motion.
12. Gravitational potential energy is always measured in relation to
a. kinetic energy. c. total potential energy.
b. mechanical energy. d. a zero level.
13. Which of the following parameters does not depend on how resistant a spring is to being compressed or stretched?
a. compression distance c. spring constant
b. relaxed length d. stretching distance
14. If the displacement of a horizontal mass-spring system was doubled, the elastic potential energy in the system would change by a factor of
a. 1/4. c. 2.
b. 1/2. d. 4.
15. What is the potential energy of a 1.0 kg mass 1.0 m above the ground?
a. 1.0 J c. 10 J
b. 9.8 J d. 96 J
16. Which of the following is a true statement about the conservation of energy?
a. Potential energy is always conserved.
b. Kinetic energy is always conserved.
c. Mechanical energy is always conserved.
d. Total energy is always conserved.
17. Why doesn't the principle of mechanical energy conservation hold in situations when frictional forces are present?
a. Kinetic energy is not completely converted to a form of potential energy.
b. Potential energy is completely converted to a form of gravitational energy.
c. Chemical energy is not completely converted to electrical energy.
d. Kinetic energy is completely converted to a form of gravitational energy.
18. Which of the following refers to the sum of kinetic energy and all forms of potential energy?
a. total energy c. nonmechanical energy
b. Σ energy d. mechanical energy
19. Which of the following is a form of mechanical energy?
a. internal energy c. gravitational potential energy
b. chemical potential energy d. electrical energy
20. A 3.00 kg toy falls from a height of 1.00 m. What will the kinetic energy of the toy be just before the toy hits the ground? (Assume no air resistance and that g = 9.81 m/s2.)
a. 0.98 J c. 29.4 J
b. 9.8 J d. 294 J
21. Which of the following is the rate at which work is done?
a. potential energy c. mechanical energy
b. kinetic energy d. power
22. Which of the following are not units of power?
a. hp c. W
b. J d. J/s
23. What is the average power supplied by a 60.0 kg person running up a flight of stairs a vertical distance of 4.0 m in 4.2 s?
a. 57 W c. 560 W
b. 240 W d. 670 W
24. A more powerful motor can do
a. more work in a longer time interval.
b. the same work in a shorter time interval.
c. less work in a longer time interval.
d. the same work in a longer time interval.
SHORT ANSWER
1. In the following sentence, is the everyday meaning or the scientific meaning of work intended? A student works on a term paper.
2. In the following sentence, is the everyday meaning or the scientific meaning of work intended? A bulldozer does work lifting a load.
3. How is work related to force and displacement?
4. Name the two SI units for work.
5. A child pulls a toy across the floor. Is the work done on the toy positive, negative, or zero?
6. A car travels at a speed of 25 m/s on a flat stretch of road. The driver must maintain pressure on the accelerator to keep the car moving at this speed.
Are any forces doing work on the car? Explain your answer.
7. The car's engine is doing work on the car, yet the kinetic energy of the car is not changing. What is happening to the energy supplied by the engine?
8. What form of energy is stored in any stretched or compressed object?
9. State, in words, the work-kinetic energy theorem.
10. Describe the relationship between kinetic energy and gravitational potential energy during the free fall of a pencil from a desk.
11. An object is lowered into a deep hole in the ground. How does the potential energy of the object change?
12. A ski jumper has 1.2 x 104J of potential energy at the top of the ski jump. The friction on the jump slope is small, but not negligible. What can you conclude about the ski jumper's kinetic energy at the bottom of the jump? Explain your answer.
13. List three different forms of mechanical energy.
14. Write an equation that expresses the conservation of mechanical energy in a system where the only forms of mechanical energy are kinetic energy and gravitational potential energy.
15. Write an equation that expresses the conservation of mechanical energy in a system that involves kinetic energy, gravitational potential energy, and elastic potential energy.
16. A child does 5.0 J of work on a spring while loading a ball into a spring-loaded toy gun. If mechanical energy is conserved, what will be the kinetic energy of the ball when it leaves the gun?
17. How are work and power related?
18. How is a machine's power rating related to its rate of doing work on an object?
19. Which motor performs more work in the same amount of time-a 10 kW motor or a 20 kW motor? How much more work can it do?
PROBLEM
1. A worker pushes a box with a horizontal force of 40.0 N over a level distance of 4.0 m. If a frictional force of 27 N acts on the box in a direction opposite to that of the worker, what net work is done on the box?
2. A hill is 132 m long and makes an angle of 12.0o with the horizontal. As a 54 kg jogger runs up the hill, how much work does the jogger do against gravity?
3. A 31.0 kg crate, initially at rest, slides down a ramp 2.6 m long and inclined at an angle of 14.0o with the horizontal. Using the work-kinetic energy theorem and disregarding friction, find the velocity of the crate at the bottom of the ramp. (g = 9.81 m/s2)
4. A child riding a bicycle has a total mass of 49.0 kg. The child approaches the top of a hill that is 15.0 m high and 106.0 m long at 14.0 m/s. If the force of friction between the bicycle and the hill is 22.0 N, what is the child's velocity at the bottom of the hill? (Assume no air resistance and that g = 9.81 .)
5. An 97 kg climber climbs to the top of Mount Everest, which has a peak height of 8850 m above sea level. What is the climber's potential energy with respect to sea level? (Assume that g = 9.8 m/s2.)
6. A 2.74 g coin, which has zero potential energy at the surface, is dropped into a 12.2 m well. After the coin comes to a stop in the mud, what is its potential energy with respect to the surface?
7. A 53.0 N crate is pulled up a 5.6 m inclined plane at a constant velocity. If the plane is inclined at an angle of 34.0 to the horizontal and there is a constant force of friction of 15.0 N between the crate and the surface, what is the net gain in potential energy by the crate?
8. On a given occasion, Old Faithful geyser in Yellowstone National Park shoots water to a height of 47.1 m. With what velocity does the water leave the ground during this eruption? (Assume no air resistance and that g = 9.81 m/s2.)
9. A bobsled zips down an ice track, starting from rest at the top of a hill with a vertical height of 170 m. Disregarding friction, what is the velocity of the bobsled at the bottom of the hill? (g = 9.81 m/s2)
10. What is the average power output of a weightlifter who can lift 260 kg to a height of 1.9 m in 1.8 s?
11. A 1.71 kg sports car accelerates from rest to 25.8 m/s in 7.41 s. What is the average power output of the automobile engine?