Question

A boy of mass 40 kg climbs up the stairs and reaches the roof at a height 8 m in 5 s. Calculate:

1. the force of gravity acting on the boy,
2. the work done by him against the force of gravity,
3. the power spent by boy. (Take g = 10 m s^-2)

Answer 1

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