### Multiple Choice Questions on POWER (Physics)

54. Power is defined as

(a) Time taken per joule                                                            (b) work done or energy converted per meter

(c) Work done or energy converted per unit time        (d) distance covered per joule

55.   Power is a measure of the

(a) Rate of change of momentum                                  (b) force which produces motion

(c) Change of energy                                                     (d) rate of change of energy

56. We can say that one watt is defined as

(a) Rate of work done                                      (b) rate of time of one second per joule

(c) Rate of distance of one meter per joule         (d) rate of work done per meter

57. In symbols, one watt is equal to

(a) 1 J s-1                      (b) 1 s J-1                      (c) 1 m J-1                     (d) 1 J m-1

Explanation: As power = work done/ time taken

Therefore, watt = J/s = J s-1

58. An electric iron is rated at 1000 W. Amount of heat generated in 15 minutes is equal to

(a) 66.67 J                    (b) 15 KJ                      (c) 900 KJ                     (d) None of above

Solution: Power = (Energy transfer)/time

Given that Power = 1000 (this means 1000 J of energy is transferred in one second)

If time = 15 minutes = 15 * 60 = 900 seconds, then,

Energy transferred = 900 * 1000 J = 900 KJ

59. If a bulb uses energy of 100 J and remains on for 25 s, power consumed by bulb will be

(a) 12 W                       (b) 4 W                                    (c) 220 W                     (d) 5 W

Power = Rate of work done = Energy transferred in one second = 100 / 25 = 4 W

60. A filament bulb of 100 W consumes energy of 1000 J, time taken by bulb is

(a) 900 s                       (b) 1100 s                     (c) 10 s                         (d) 100000 s

61. A boy walks up staircase of 20 m with a force of 600 N; time taken by boy is 8 minutes. Power consumed by boy is

(a) 25 W                       (b) 16 W                       (c) 220 W                     (d) 110 W

Do yourself.

Hint: Convert 8 minutes to seconds.

62. A machine is able to lift a mass of 200 kg vertically up to height of 30 m above ground in 50 seconds. Take g = 10 ms-2. Power of machine is

(a) 0.12 kW                  (b) 1.2 kW                   (c) 6.0 kW                    (d) 300 kW

Hint: The body is raised vertically up; work is done against the gravitational force= mgh. Therefore, work done is 200 * 10 * 30. Divide by time and convert watts to kilowatts.

63.   3730 watts = ______hp.

(a) 1.5                          (b) 2.2                          (c) 3.746                                   (d) 5

Hint: How many watts is one horse power?

64.   1.5 kW = ______ watts

(a) 1500                                    (b) 150                         (c) 15000                      (d) 15

Hint: How many watts is one kilowatt?

65.   Name the physical quantity which is equal to the product of force and velocity.

(a) Work                                   (b) energy                     (c) power                     (d) acceleration

Solution

66. Which of the following is not a unit of power?

(a) Joule-second                       (b) watt                                    (c) N m/s                      (d) horsepower

Explanation

Horse power is the British engineering system unit of power.

67. Force exerted by bat on ball if it strikes a ball of mass 0.16 kg initially hits bat with speed of 25 m s-1 with time impact of 0.003 s is

(a) 145 N                      (b) 1333.33 N               (c) 1456.7 N                 (d) 6543 N

Solution: F = p/∆t = (0.16 * 25) / 0.003 = 4/0.003 = 1333.33 N

68. Momentum of electron having mass 9.1 × 10-31 kg and velocity 2.1 × 107 is

(a) 1.91 * 10-23                       (b) 2.34 * 10-23              (c) 3.11 * 10-19              (d) 7.88 * 10-34

Solution           m = 9.1 * 10-31 kg         v = 2.0 * 107 m/s           p = mv

p = 9.1 * 2.1 * 10-31 * 107 = 19.11 * 10-23+7 = 19.1 * 10-24 = 1.91 * 10-23 kg m/s