A boy of mass 40kg climbs up the stairs and reaches the roof at a height 8m in 5s. Calculate:
(i) The force of gravity acting on the boy,
(ii)The work done by him against gravity,
(iii)The power spent by the boy.
Take g = 10 ms-2
Given,
m = 40 kg
h = 8 m
t = 5 s
(i) Force of gravity acting on the boy
F=mgF=40×10=400NF = mg \\[0.5em] F = 40 \times 10 = 400N \\[0.5em]F=mgF=40×10=400N
(ii) Work done against gravity
W=F×SW=400×8=3200JW = F × S \\[0.5em] W = 400 \times 8 = 3200J \\[0.5em]W=F×SW=400×8=3200J
(iii)
Power spent=work donetime takenW=32005=640W\text{Power spent} = \dfrac{\text{work done}}{\text{time taken}} \\[0.5em] W = \dfrac{3200}{5} = 640 WPower spent=time takenwork doneW=53200=640W
