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Question
A boy of mass 40 kg climbs up the stairs and reaches the roof at a height 8 m in 5 s. Calculate:
- the force of gravity acting on the boy,
- the work done by him against the force of gravity,
- the power spent by boy. (Take g = 10 m s-2)
Solution
Mass of boy = 40 kg, Vertical height moved, h = 8 m, Time taken, t = 5s.
(i) Force of gravity on the boy F = mg
= 40 kg × 10 m/s-2
= 400 N
(ii) Work done against gravity, W = mgh
= 400 × 8
= 3200 J
(iii) Power = `"work"/"Time" = "mgh"/"t"`
`= (3200 " J")/(5 " s")`
= 640 W
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