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
View Answer

Answer: c
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.
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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

Answer: c
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
View Answer

Answer: d
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.
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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
View Answer

Answer: d
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
View Answer

Answer: b
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.
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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
View Answer

Answer: d
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
View Answer

Answer: c
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.
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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
View Answer

Answer: b
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
View Answer

Answer: a
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.
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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

Answer: b
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
View Answer

Answer: b
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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Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He is Linux Kernel Developer & SAN Architect and is passionate about competency developments in these areas. He lives in Bangalore and delivers focused training sessions to IT professionals in Linux Kernel, Linux Debugging, Linux Device Drivers, Linux Networking, Linux Storage, Advanced C Programming, SAN Storage Technologies, SCSI Internals & Storage Protocols such as iSCSI & Fiber Channel. Stay connected with him @ LinkedIn | Youtube | Instagram | Facebook | Twitter