Question

a man has power of 90W and mass 60kg runs up a staircase in 40sec. if each step of staircase is 20cm high, calculate the no of steps.

Answer 1

so obviously, displacement of the man is equal to the height of the no.stairs he runs up.
let no.stairs be y
hence,
h=20y cm
h=0.2y m

W=mgh
W= 60 × 10 × 0.2 y
W= 120y

P= W/t
P= 120y/40
90= 3y
y= 30

hence number of stairs is 30

Answer 2

Power = Energy / Time

m = 60 kg
g = 10 m/s
h = ?
Energy = P.E = mgh
= 60 × 10 ×h
= 600 h

Power = 90W

time = 40 sec

90 W = 600 h ÷ 40 sec
600 h = 40 ×90
h = 3600 ÷ 600

h = 6 metres = 600 cm


Height of 1 step = 20 cm

Height of steps the man ran = 600 cm

No : of steps = 600 ÷ 20
= 30 steps