Page 99 - Schaum's Outline of Theory and Problems of Applied Physics

84 ENERGY [CHAP. 7

7.12. The KE of a 900-kg car whose velocity is 60 km/h is
(a) 12.8 kJ (c) 1.23 MJ
(b) 125 kJ (d) 1.62 MJ

7.13. The velocity of a 5000-kg truck whose KE is 360 kJ is
(a) 12 km/h (c) 43 km/h
(b) 31 km/h (d) 144 km/h
7.14. The KE of a 2400-lb car whose velocity is 40 mi/h is
3
4
(a) 1.9 × 10 ft·lb (c) 7.0 × 10 ft·lb
5
4
(b) 6.0 × 10 ft·lb (d) 1.3 × 10 ft·lb
7.15. A 50-kg mass has a PE of 4.9 kJ relative to the ground. The height of the mass above the ground is
(a) 10 m (c) 960 m
(b) 98 m (d) 245 km

7.16. A 16-slug mass is raised by 10 ft. The PE of the mass increases by
(a) 5 ft·lb (c) 160 ft·lb
(b) 20 ft·lb (d) 5120 ft·lb
7.17. A girl on a swing varies in height from 60 cm above the ground to 180 cm. The girl’s greatest velocity

(a) is 1.5 m/s (c) is24m/s
(b) is 4.8 m/s (d) depends on her mass

7.18. The 2.50-kg head of an ax exerts a force of 80 kN as it penetrates 18 mm into the trunk of a tree. The velocity of the
ax head when it strikes the tree is
(a) 1.2 m/s (c) 34 m/s
(b) 3.4 m/s (d) 107 m/s

7.19. The mass equivalent of 6.0 MJ is
(a) 6.7 × 10 −11 kg (c) 6.7 × 10 −3 kg
(b) 5.4 × 10 −9 kg (d) 5.0 × 10 −2 kg

Supplementary Problems

7.1. Under what circumstances (if any) is no work done on a moving object even though a net force acts on it?

7.2. A golf ball and a Ping-Pong ball are dropped in a vacuum chamber. When they have fallen halfway to the bottom,
how do their velocities compare? Their kinetic energies? Their potential energies?

20
9
7.3. The earth exerts a gravitational force of 2 × 10 N on the moon, and the moon travels 2.4 × 10 m each time it orbits
the earth. How much work does the earth do on the moon in each orbit?
7.4. (a) How much work must be done to raise a 1100-kg car 2 m above the ground? (b) What is the car’s potential energy
afterward?
7.5. A 20-lb object is raised to a height of 40 ft above the ground. (a) How much work was done? (b) What is the potential
energy of the object? (c) If the object is dropped, what will its kinetic energy be just before it strikes the ground?