**WORK, ENERGY & POWER**

QUESTIONS

**1.**A rod of mass m and length l is lying on a horizontal table. Work done in making it stand on one end will be:

a) mgl b)
mgl/2

c) mgl/4 d)
2mgl

**2.**An electric motor creates a tension of 4500 N in hoisting a cable and reels it at the rate of 2m/s. The power of the motor is:

a) 25 kW b) 9 kW

c) 225 W d) 900 hp

**3.**A 3 hp motor requires 2.4 kW to drive it. The efficiency of the motor is about:

a) 90% b) 75%

c) 660% d) 50%

**5.**An engine develops a power of 10 kW. How much time will it take to lift a mass of 200 kg to a height of 40 m? (g = 10 m/s

^{2})

a) 4 s b) 5 s

c) 8 s d) 10 s

**6.**When the KE of a body is increased by 300%, the momentum of the body is increased by:

a) 20% b) 50%

c) 125% d) 200%

**7.**If the linear momentum is increased by 50%, then KE will be increased by:

a) 50% b) 100%

c) 125% d) 25%

**8.**Two masses of 1 g and 4 g are moving with equal linear momenta. The ratio of their kinetic energies is:

a) 4:1 b)

c) 1:2 d) 1:16

**9.**Two masses of M and 4M are moving with equal kinetic energy. The ratio of their linear momenta is:

a) 1:8 b) 1:4

c) 1:2 d) 4:1

**10.**The KE of a body moving with a speed of 10 m/s is 30J. If its speed becomes 30 m/s, its KE will be:

a) 10 J b) 90 J

c) 180 J d) 270 J

**11.**A 4 kg of mass and a 1 kg of mass are moving with equal kinetic energies. The ratio of their momenta is:

a) 1:2 b) 1:1

c) 2:1 d) 4:1

**14.**A ball of mass 50 g is thrown upwards. It rises to a maximum height of 100 m. At what height its KE will be reduced to 70%?

a) 30 m b) 40 m

c) 60 m d) 70 m

**15.**It requires 20 turns of the stem of a watch to wind the main spring and this stores sufficient energy to keep the watch running for 30 h. If 10 turns are given to the stem, the watch will run for:

a) 20 h b) 15 h

c) 7.5 h d) 3.75 h

**17.**A body of mass 2 kg is projected vertically upwards with a speed of 3 m/s. The maximum gravitational potential energy of the body is:

a) 18 J b) 4.5 J

c) 9 J d) 2.25 J

**18.**The potential energy of a system increases if work done is:

a) on the system by a
non-conservative force

b) by the system
against a conservative force

c) by the system
against a non-conservative force

d) on the system by a
conservative force

**19.**A body projected vertically from the Earth reaches a height equal to Earth’s radius before returning to the Earth. The power exerted by the gravitational force is greatest:

a) at the highest
position of the body

b) at the instant just
before the body hits the Earth

c) it remains constant
all through

d) at the instant just
after the body is projected

**20.**A long spring when stretched by x cm, has a potential energy U. On increasing the stretching to nx cm, the potential energy stored in the spring will be:

a) U/n b) nU

c) n

^{2}U d) U/n^{2}**21.**An automobile moves along a straight horizontal road with a speed (i) v and (ii) 3v. Assuming identical conditions, the ratio of the shortest distances in which the automobiles can be stopped is:

a) 1:3 b) 1:9

c) 1:27 d) 1:81

**23.**A particle of mass M starting from rest undergoes uniform acceleration. If the speed acquired in time T is V, the power delivered to the particle is:

a) MV

^{2}/T b) MV^{2}/2T^{2}
c) MV

^{2}/T^{2}d) MV^{2}/2T**25.**A uniform chain of mass m and length l is lying on a table with (l/4) of its length hanging freely from the edge of the table. The amount of work done in dragging the chain on the table completely is:

a) mgl/4 b) mgl/8

c) mgl/32 d) mgl/16

**27.**A body of mass m is falling freely under gravity. If the velocity is v when it has lost potential energy equal to U, the mass m can be written as:

a) U

^{2}/v b) 2U^{2}/v^{2}
c)2U/v

^{2}d) v^{2}/U**28.**At sea level, a N

_{2}molecule in air has an average translational . Its mass is kg. If the molecule shoots up straight without any resistance, it will rise to a height of:

a) 135 km b)
13.5 km

c) 1.35 km d) 1350 km

**29.**Two bodies have their masses m

_{1}/m

_{2}= 3 and their kinetic energies E

_{1}/E

_{2}= 1/3. The ratio of their velocities is:

a) 1:1 b)
1:2

c) 1:3 d)
2:3

**30.**A tank of size 5 m*5 m*5 m is full of water and built on ground. The potential energy of water in the tank is:

a) 6250*10

^{3 }J b) 210*10^{3}J
c) 1250*10

^{3}J d) 3125*10^{3}J**32.**A particle moves X-axis from x = x

_{1}to x = x

_{2}under the influence of a force, F = 2x. The work done in the process is:

a) zero b)

c) 2x

_{2}(x_{2}-x_{1}) d) 2x_{1}(x_{1}-x_{2})**33.**Sand drops fall vertically at the rate of 3 kg/s on the conveyor belt moving horizontally with a speed of 0.25 m/s. The extra power required to keep it moving is:

a) (9/16)W b) (3/16)W

c) (5/16)w d)
(11/16)W

**34.**A block of mass 10 kg is moving in X-direction with a constant speed of 10 m/s. It is subjected to a constant retarding force, F = 0.1x J/m during its travel from x = 20 m to x = 30 m. Its final kinetic energy will be:

a) 475 J b) 450 J

c) 275 J d) 250 J

**35.**The power of a water pump is 2 kW. If g = 10 m/s

^{2}, the amount of water it can raise in 1 min to a height of 10 m, is:

a) 2000 litre b) 1000 litre

c) 100 litre d) 1200 litre

**36.**A bullet fired into a fixed target leson half of its velocity after penetrating 3 cm. How much further will it penetrate before coming to rest, assuming that it faces constant resistance to motion?

a) 1 cm b)
1.5 cm

c) 2 cm d) 3 cm

**40.**a body of mass 3 kg is under a constant force which causes a displacement s metre in it, given by the relation , where t is in seconds. Work done by the force in 2 s, is:

a) (19/5)J b) (5/19)J

c) (3/8)J d) (8/3)J

# ANSWERS

1. (b) 2.
(b) 3. (a) 4. (c) 5. (c) 6.
(c) 7. (c)

8.(a) 9.
(c) 10. (d) 11. (c) 12. (d) 13.
(b) 14. (a)

15. (c) 16.
(b) 17. (c) 18. (d) 19. (b) 20.
(c) 21. (b)

22. (c) 23.
(d) 24. (a) 25. (c) 26. (c) 27.(c) 28. (b)

29. (c) 30.
(d) 31. (c) 32. (b) 33. (b) 34.
(a) 35. (d)

36. (a) 37.
(a) 38. (c) 39. (d) 40. (d)

## No comments:

## Post a Comment