The power required to lift a mass of 8.0 kg up a vertical distance of 4 m in 2 s is (taking acceleration due to gravity as 10 m/s²) :
- A.80 W
- B.160 W
- C.320 W
- D.640 W
Correct answer
Explanation
The correct answer is B, 160 W. Power is the work done divided by the time taken, and here the work is the potential energy gained by the mass.
Work done in lifting = mgh = 8.0 × 10 × 4 = 320 joules. This work is done in 2 seconds, so power = work ÷ time = 320 ÷ 2 = 160 watts. One watt is one joule of work per second, and the unit is named after James Watt, whose steam engine improvements made power a practical measure. The same idea is used when comparing motors and pumps: lifting the same load faster needs more power, even though the work done stays the same.
A is wrong because 80 W would mean the lift took 4 seconds, or the height was halved. C is wrong because 320 is the work in joules, not the power. D is wrong because 640 W doubles the work instead of dividing it by the time.
Exam tip: power P = W/t = mgh/t; 1 watt = 1 joule per second, and 1 horsepower ≈ 746 W.
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