Power
5 marks · HardA passenger lift of mass 800 kg carries 5 people each of mass 60 kg to a height of 18 m in 22 s. Assume the upward force from the cables exactly balances the total weight as the lift moves at constant speed.
Calculate the total weight lifted by the cables.
Calculate the work done on the lift and passengers in raising them to the top floor.
Calculate the power developed by the lift motor.
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) 11000 N
(b) 198,000 J
(c) 9000 W
Common mistakes: Omitting the mass of the passengers; Using mass instead of weight in work done; Forgetting to divide by time for power
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Correct method for total weight: N
M2 (b) · 1 mark · Substitute correct values in work done formula (follow-through)
A1 (b) · 1 mark · Work done correctly calculated: 198,000 J (follow-through)
M3 (c) · 1 mark · Use correct power formula (follow-through)
A2 (c) · 1 mark · Power correctly calculated: 9,000 W (follow-through)
Worked solution
(a) Total mass = 800 kg + (5 × 60 kg) = 1100 kg
(b) Work done is force × distance:
(c) Power = work done / time: