Review Questions - Work, Energy, and Power

SAT Physics Subject Test

Chapter 4 Work, Energy, and Power

Chapter 4 Review Questions

See Chapter 17 for solutions.

1. A force F of strength 20 N acts on an object of mass 3 kg as it moves a distance of 4 m. If F is perpendicular to the 4 m displacement, the work it does is equal to

(A) 0 J

(B) 60 J

(D) 600 J

(E) 2,400 J

2. Under the influence of a force, an object of mass 4 kg accelerates from 3 m/s to 6 m/s in 8 s. How much work was done on the object during this time?

(A) 27 J

(B) 54 J

(D) 96 J

(E) Cannot be determined from the information given

3. A box of mass m slides down a frictionless inclined plane of length L and vertical height h. What is the change in its gravitational potential energy?

(A) –mgL

(B) –mgh

(D) –mgh/L

(E) –mghL

4. An object of mass m is traveling at constant speed v in a circular path of radius r. How much work is done by the centripetal force during one half of a revolution?

(A) πmv²

(B) 2πmv²

(D) πmv²r

(E) 2πmv²r

5. While a person lifts a book of mass 2 kg from the floor to a tabletop, 1.5 m above the floor, how much work does the gravitational force do on the book?

(A) –30 J

(B) –15 J

(D) 15 J

(E) 30 J

6. A block of mass 3 kg slides down a frictionless inclined plane of length 6 m and height 4 m. If the block is released from rest at the top of the incline, what is its speed at the bottom?

(A) 5 m/s

(B) 6 m/s

(D) 9 m/s

(E) 10 m/s

7. A block of mass 3 kg slides down an inclined plane of length 6 m and height 4 m. If the force of friction on the block is a constant 16 N as it slides from rest at the top of the incline, what is its speed at the bottom?

(A) 2 m/s

(B) 3 m/s

(D) 5 m/s

(E) 6 m/s

8. As a rock of mass 4 kg drops from the edge of a 40-meter-high cliff, it experiences air resistance, whose average strength during the descent is 20 N. At what speed will the rock hit the ground?

(A) 8 m/s

(B) 10 m/s

(D) 16 m/s

(E) 20 m/s

9. An astronaut drops a rock from the top of a crater on the moon. When the rock is halfway down to the bottom of the crater, its speed is what fraction of its final impact speed?

(A)

(B) 1/4

(C)

(D) 1/2

(E)

10. A force of 200 N is required to keep an object sliding at a constant speed of 2 m/s across a rough floor. How much power is being expended to maintain this motion?

(A) 50 W

(B) 100 W

(D) 400 W

(E) Cannot be determined from the information given

11. The moon has mass M and radius R. A small object is dropped from a distance of 3R from the moon”s center. The object”s impact speed when it strikes the surface of the moon is equal to for k =

(A)

(B)

(C)

(D)

(E)

Keywords

force

energy

work

law of conservation of energy

first law of thermodymamics

conserved

total work

kinetic energy

work–energy theorem

potential energy

gravitational potential energy

conservative

mechanical energy

power

joule

watt

escape speed

Summary

· Work done by a constant force is the product of force and distance and the resulting change of energy W = Fd cos θ.

· Forward forces do positive work, backward forces do negative work, perpendicular forces do no work.

· Work done by a variable force is measured by graphing F versus the horizontal, and then finding the area bounded by the graph of F, the x-axis, and vertical lines indicating the beginning and end of the period of force.

· Kinetic energy refers to the energy an object possesses by virtue of its motion and equals mv².

· The work–energy theorem states that the total work done on an object is equal to the object”s change in kinetic energy.

· Potential energy is the energy an object has by virtue of its position. Work done on an object to put it in a given position is stored in the object that can be retrieved.

· Conservation of mechanical energy is the sum of an object”s kinetic and potential energies. Nonconservative forces, such as friction, are disregarded, so the initial mechanical energy is equal to the final mechanical energy.

· Power is the measure of work over time P = . It is the rate at which work is done.

· Gravitational potential energy comes into play when the height is large compared with the earth”s radius. In this case, gravitation is a variable force.