This set of Physics Multiple Choice Questions & Answers (MCQs) focuses on “Power”.

1. What are the units of power?

a) Newton

b) Joule

c) Watt

d) No units

View Answer

Explanation: The SI unit of power is “Watt”. It is named after the famous inventor “James Watt” who is widely remembered for his improvements on the steam engine.

1 Watt = 1 Joule per second.

2. A machine gun fires 360 bullets per minute. Each bullet has a mass of 5 grams and travels at 600 m/s. What is the power of the gun? (Assume no loss of energy and 100% power transmission)

a) 300 W

b) 600 W

c) 900 W

d) 1800 W

View Answer

Explanation: No. of bullets fired per second = 360/60 = 6

Power of the gun is completely transmitted to bullets.

Power (P) = Power of bullets in 1 second

= [(1/2 x m x v

^{2}) x 6] / 1

m = 5 x 10

^{-3}kg

v = 600 m/s

P = (1/2 x 5 x 10

^{-3}x 600

^{2}x 6) / 1

= 900 W.

3. A motor is used to deliver water through a pipe. Let the motor have a power P initially. To double the rate of flow of water through the pipe, the power is increased to P’. What is the value of P’/P?

a) 2

b) 4

c) 6

d) 8

View Answer

Explanation: Power = Force x Velocity

= (Mass/Time) x Velocity

^{2}

= Rate of flow x Velocity

^{2}

Rate of flow = Area x Velocity

Doubling the rate of flow will double the velocity.

P = Rate of flow x Velocity

^{2}

P’ = (2 x Rate of flow) x (2 x Velocity)

^{2}

= 8P

P/P’ = 8.

4. A motor is used to pump water from a depth of 5 m to fill a volume of 10 cubic meters in 5 minutes. If 50% of the power is wasted, what is the power of the motor? (Assume g = 10m/s^{2})

a) 1000/3 W

b) 3000/3 W

c) 5000/3 W

d) 10000/3 W

View Answer

Explanation: Density of water = 1000 kg/m

^{3}

Mass of water in 10m

^{3}= 1000 x 10

= 10,000 kg

Volume of water filled in 1s = 10/(5×60)

= 1/30 m

^{3}

Mass filled in 1s = 1/30 x 1000

= 100/3 kg

Power = 100/3 x 10 x 5; [g = 10m/s

^{2}]

= 5000/3 W

Since 50% of power is wasted, power of motor = 5000/3 x 2

= 10000/3 W.

5. A machine has a power of 20kW. How long will it take for it to lift a body of mass 10kg from the ground to a height of 100m? (Assume g = 10m/s^{2})

a) 1 second

b) 2 seconds

c) 3 seconds

d) 4 seconds

View Answer

Explanation: Power (P) = Energy (E) x Time (t)

20,000 = (m x g x h) x t

= (10 x 10 x 100) x t

t = 2 seconds.

6. A 2-ton vehicle travels at 20m/s on a road where the frictional force is 10% of the weight of the vehicle. What is the power required? (Assume g = 10m/s^{2})

a) 220 kW

b) 240 kW

c) 420 kW

d) 440 kW

View Answer

Explanation:Weight of car = m x g

= 2,000 x 10

= 20,000 N

Frictional force = (10/100) x (20,000)

= 2,000 N

Total force (F) = 20,000 + 2,000

= 22,000 N

Velocity (v) = 20 m/s

P = F x v

= 22,000 x 20

= 4,40,000 W

= 440 kW.

7. 1000 kW of power is supplied to a motor. 90% of this is transmitted to a machine operating at 80% efficiency. The machine lifts an object of 10-tons with a velocity _____ m/s. (Assume g = 10m/s^{2})

a) 2

b) 4

c) 6

d) 8

View Answer

Explanation: Power transmitted to machine = (90/100) x (1000) kW

= 900 kW

Power used to lift object = (80/100) x 900 KW

= 720 kW

7,20,000 = F x v

= (1/2 x m x v

^{2}) x v

= 10,000/2 x v

^{3}

v

^{3}= 216

v = 6 m/s.

8. A 200 kg wagon climbs up a hill of slope 30-degrees in 1 minute at a speed of 10 m/s. How much power is delivered by the engine? (Assume g = 10m/s^{2})

a) 10 kW

b) 20 kW

c) 30 kW

d) 40 kW

View Answer

Explanation: Weight of car = m x g

= 2,00 x 10

= 2,000 N

Total force (F) = 2,000

Velocity (v) = 10 m/s

P = F x v

= 2000 x 10

= 20,000 W

= 20 kW.

9. A 200 kg wagon climbs up a hill of slope 30-degrees in 1 minute at a speed of 36 km/h. How much power is delivered by the engine if the frictional force is 25% of the weight of the car? (Assume g = 10m/s^{2})

a) 25 kW

b) 50 kW

c) 100 kW

d)125 kW

View Answer

Explanation: Weight of car = m x g

= 200 x 10

= 2,000 N

Frictional force = (25/100) x (2,000)

= 500 N

Total force (F) = 2000 + 500

= 2,500 N

Velocity (v) = 36 km/h

= 36 x (1000/3600)

= 10 m/s

P = F x v

= 2500 x 10

= 25 kW.

10. A body of mass 3 kg starts from rest with a uniform acceleration of unknown magnitude. If the body has a velocity of 30 m/s in 6 seconds, what is the power consumed in 3 seconds?

a) 125 W

b) 225 W

c) 325 W

d) 425 W

View Answer

Explanation: v = u + at

30 = a x 6

a = 5 m/s

^{2}

Force (F) = m x a

= 3 x 5

= 15 N

v’ = u + at’

v’ = 5 x 3

= 15 m/s

Power consumed in 3 seconds = F x v’

= 15 X 15

= 225 W.

11. The force required to tow a vehicle at constant velocity is directly proportional to the magnitude of velocity raised to the first power. It requires 1000 W to tow with a velocity of 10 m/s. How much power is required to tow at a velocity of 8 m/s?

a) 320 W

b) 640 W

c) 160 W

d) 80 W

View Answer

Explanation: Since force is directly proportional to velocity, the power required will be directly proportional to the square of the velocity.

Power = Force x Velocity

Hence, power is a multiple of 8

^{2}and an integer such that;

1000/10

^{2}= P/8

^{2}

P = 640 W.

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