Work, energy and power MCQs
MDCAT · Physics · 6 questions
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Revision summary
PMDC MDCAT syllabus: Work and energy
Work W = Fd cos θ: it is zero when the force is perpendicular to the motion (θ = 90°) and negative when it opposes it, as friction does. Energy is the capacity to do work, measured in joules like work. Kinetic energy KE = ½mv² grows with the square of speed, so doubling speed quadruples KE. Gravitational potential energy near Earth is PE = mgh. Energy is conserved: as a body falls, PE turns into KE, and mgh = ½mv² gives v = √(2gh) when air resistance is ignored. The work–energy theorem says the net work done on a body equals its change in kinetic energy. Power is the rate of doing work, P = W/t = Fv, in watts (1 W = 1 J/s); a horsepower is about 746 W. The electricity "unit" is the kilowatt-hour: 1 kWh = 3.6 × 10⁶ J. Watch the units in each option; many distractors differ only by a power of ten.
Questions with answers
Written by Parcha from the syllabus. Samples until a subject reviewer checks them; report anything wrong from the practice screen.
Q1 · Parcha original · Sample
The SI unit of power is the:
- A. Joule
- B. Newton
- C. Watt
- D. Pascal
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Answer: C. Watt
Power is the rate of doing work: 1 watt = 1 joule per second. The joule is the unit of work and energy, the newton of force, and the pascal of pressure.
Open this question on its own pageQ2 · Parcha original · Sample
When a force acts perpendicular to the displacement, the work done is:
- A. Maximum
- B. Negative
- C. Equal to the force
- D. Zero
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Answer: D. Zero
W = Fd cos θ. With θ = 90°, cos θ = 0, so no work is done. Example: the centripetal force on a body moving in a circle does no work.
Q3 · Parcha original · Sample
The kinetic energy of a 2 kg mass moving at 3 m/s is:
- A. 6 J
- B. 9 J
- C. 18 J
- D. 3 J
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Answer: B. 9 J
KE = ½mv² = ½ × 2 × 3² = 9 J.
Q4 · Parcha original · Sample
If the speed of a body is doubled, its kinetic energy becomes:
- A. Four times
- B. Twice
- C. Half
- D. Unchanged
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Answer: A. Four times
KE = ½mv² depends on v², so doubling v multiplies KE by 2² = 4.
Q5 · Parcha original · Sample
One kilowatt-hour (kWh) is equal to:
- A. 1000 J
- B. 3.6 × 10³ J
- C. 3.6 × 10⁶ J
- D. 6.0 × 10⁴ J
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Answer: C. 3.6 × 10⁶ J
1 kWh = 1000 W × 3600 s = 3.6 × 10⁶ J. It is the 'unit' on electricity bills.
Q6 · Parcha original · Sample
A 10 kg box is lifted 2 m vertically. Taking g = 10 m/s², the gain in potential energy is:
- A. 20 J
- B. 50 J
- C. 100 J
- D. 200 J
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Answer: D. 200 J
PE = mgh = 10 × 10 × 2 = 200 J.