⚡ Work and Energy – Class 9 Science MCQs

Section 3: Power, Commercial Unit of Energy & Conservation of Energy (Q51–Q75)

Q51. Power is defined as:

A. Rate of doing work
B. Work × time
C. Work/force
D. Force × displacement

✅ Correct Answer: A. Rate of doing work

Explanation:

• A. Correct, P = W/t.
• B. Wrong, should be W/t, not W × t.
• C. Not correct.
• D. Work = F × d, not power.

Q52. SI unit of power is:

A. Joule
B. Watt
C. Newton
D. Pascal

✅ Correct Answer: B. Watt

Explanation:

• 1 Watt = 1 Joule/second.

Q53. A man does 100 J work in 10 s. Power = ?

A. 1 W
B. 5 W
C. 10 W
D. 20 W

✅ Correct Answer: B. 10 W

Explanation:

• P = W/t = 100/10 = 10 W.

Q54. Which of these is larger unit of power?

A. Watt
B. Kilowatt
C. Megawatt
D. Both B & C

✅ Correct Answer: D. Both B & C

Q55. 1 horsepower = ?

A. 746 W
B. 1000 W
C. 746 J
D. 100 J

✅ Correct Answer: A. 746 W

Q56. If 200 J work is done in 4 s, power = ?

A. 25 W
B. 40 W
C. 50 W
D. 100 W

✅ Correct Answer: C. 50 W

Explanation:

• P = 200/4 = 50 W.

Q57. Work done = 500 J, power = 100 W. Time taken = ?

A. 2 s
B. 5 s
C. 10 s
D. 50 s

✅ Correct Answer: B. 5 s

Explanation:

• t = W/P = 500/100 = 5 s.

Q58. The commercial unit of energy is:

A. Joule
B. Watt
C. Kilowatt-hour
D. Newton

✅ Correct Answer: C. Kilowatt-hour

Q59. 1 kWh = ? Joules

A. 360 J
B. 3600 J
C. 36000 J
D. 3.6 × 10⁶ J

✅ Correct Answer: D. 3.6 × 10⁶ J

Explanation:

• 1 kWh = 1000 W × 3600 s = 3.6 × 10⁶ J.

Q60. A 100 W bulb glows for 1 hour. Energy consumed = ?

A. 0.1 kWh
B. 1 kWh
C. 10 kWh
D. 100 kWh

✅ Correct Answer: A. 0.1 kWh

Explanation:

• Energy = P × t = 100 W × 1 h = 0.1 kWh.

Q61. A 1 kW heater runs for 2 hours. Energy consumed = ?

A. 1 kWh
B. 2 kWh
C. 10 kWh
D. 20 kWh

✅ Correct Answer: B. 2 kWh

Q62. Which of the following is correct?

A. Energy = Power × Time
B. Power = Energy × Time
C. Energy = Power/Time
D. Work = Power × Velocity

✅ Correct Answer: A. Energy = Power × Time

Q63. An electric motor does 500 J work in 5 s. Its power = ?

A. 50 W
B. 100 W
C. 200 W
D. 250 W

✅ Correct Answer: B. 100 W

Explanation:

• P = 500/5 = 100 W.

Q64. Law of conservation of energy states that:

A. Energy can be created
B. Energy can be destroyed
C. Energy can neither be created nor destroyed
D. Energy is always lost

✅ Correct Answer: C. Energy can neither be created nor destroyed

Q65. In energy transformation, total energy:

A. Increases
B. Decreases
C. Remains constant
D. Becomes zero

✅ Correct Answer: C. Remains constant

Q66. Which is an example of transformation of KE to PE?

A. Falling stone
B. Moving car stops
C. Rising ball
D. Battery used

✅ Correct Answer: C. Rising ball

Explanation:

• KE of ball converts to PE at top.

Q67. Which is an example of PE to KE conversion?

A. Water falling from dam
B. Rising elevator
C. Stretched bow at rest
D. Sleeping person

✅ Correct Answer: A. Water falling from dam

Q68. In pendulum motion, energy oscillates between:

A. PE and KE
B. Heat and sound
C. Work and power
D. Electric and magnetic

✅ Correct Answer: A. PE and KE

Q69. Mechanical energy is conserved when:

A. Only conservative forces act
B. Friction acts
C. Air resistance acts
D. External force acts

✅ Correct Answer: A. Only conservative forces act

Q70. When a ball falls freely from height, at ground:

A. Only PE
B. Only KE
C. Both PE and KE
D. Neither

✅ Correct Answer: B. Only KE

Q71. At maximum height of projectile, energy is:

A. Only KE
B. Only PE
C. Both KE and PE
D. Zero

✅ Correct Answer: B. Only PE

Q72. Which of these is not an energy transformation?

A. Dynamo: mechanical → electrical
B. Solar panel: light → electrical
C. Battery: chemical → electrical
D. Car brakes: KE → PE

✅ Correct Answer: D. Car brakes: KE → PE

Explanation:

• Brakes convert KE → heat, not PE.

Q73. Which energy conversion occurs in microphone?

A. Sound → electrical
B. Electrical → sound
C. Mechanical → heat
D. Light → chemical

✅ Correct Answer: A. Sound → electrical

Q74. Which energy conversion occurs in electric bulb?

A. Heat → mechanical
B. Electrical → light + heat
C. Chemical → electrical
D. Sound → electrical

✅ Correct Answer: B. Electrical → light + heat

Q75. Which energy conversion occurs in hydroelectric power plant?

A. Heat → electrical
B. Chemical → electrical
C. Mechanical → electrical
D. Nuclear → electrical

✅ Correct Answer: C. Mechanical → electrical

Explanation:

• Flowing water KE/PE → turbine rotation (mechanical) → electrical.

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