Units (Energy)

3 marks · Normal

An electric motor lifts a 50 kg load a height of 5 m in 10 seconds. Gravitational field strength g = 10 N/kg.

(a)

Calculate the work done by the electric motor. Include the correct unit in your answer.

[2 marks]

(b)

Calculate the power output of the motor. State the correct unit.

[1 mark]

Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.

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(a) 2500 J

(b) 250 W

Also accept: 2500 J ; 250 W

Common mistakes: Forgetting to multiply by g; Using mass instead of weight for work

Show mark scheme and worked solution

Marking points

M1 (a) · 1 mark · Correctly calculates force as weight (50 kg × 10 N/kg) (follow-through)

M1 (a) · 1 mark · Correctly applies W = F × d to get work done (follow-through)

A1 (a) · 0 marks · Final answer: 2500 J (follow-through)

M1 (b) · 1 mark · Correctly divides work by time to find power (2500 J ÷ 10 s) (follow-through)

A1 (b) · 0 marks · Final answer: 250 W (follow-through)

Worked solution

a) Weight = mass × g = 50 × 10 = 500 N. Work done = force × distance = 500 N × 5 m = 2500 J.

b) Power = work done / time = 2500 J / 10 s = 250 W.