# Physics Questions and Answers – Power

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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

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

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 v2) x 6] / 1
m = 5 x 10-3 kg
v = 600 m/s
P = (1/2 x 5 x 10-3 x 6002 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

Explanation: Power = Force x Velocity
= (Mass/Time) x Velocity2
= Rate of flow x Velocity2
Rate of flow = Area x Velocity
Doubling the rate of flow will double the velocity.
P = Rate of flow x Velocity2
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/s2)
a) 1000/3 W
b) 3000/3 W
c) 5000/3 W
d) 10000/3 W

Explanation: Density of water = 1000 kg/m3
Mass of water in 10m3 = 1000 x 10
= 10,000 kg
Volume of water filled in 1s = 10/(5×60)
= 1/30 m3
Mass filled in 1s = 1/30 x 1000
= 100/3 kg
Power = 100/3 x 10 x 5; [g = 10m/s2]
= 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/s2)
a) 1 second
b) 2 seconds
c) 3 seconds
d) 4 seconds

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/s2)
a) 220 kW
b) 240 kW
c) 420 kW
d) 440 kW

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/s2)
a) 2
b) 4
c) 6
d) 8

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 v2) x v
= 10,000/2 x v3
v3 = 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/s2)
a) 10 kW
b) 20 kW
c) 30 kW
d) 40 kW

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/s2)
a) 25 kW
b) 50 kW
c) 100 kW
d)125 kW

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

Explanation: v = u + at
30 = a x 6
a = 5 m/s2
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

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 82 and an integer such that;
1000/102 = P/82
P = 640 W.

Sanfoundry Global Education & Learning Series – Physics – Class 11.

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