Question

A man has power of 90W and mass 60 kg runs up a staircase in 40 seconds . If each step is 20m high , calculate the number of steps

Answer 1

Given power= 90 W
Mass = 60 kg
Time Taken = 40 Seconds 
Each step height = 20 cm

Let x  = number of stairs in staircase
g = acceleration due to gravity = 10 m/s square (approx)
power  = work done/ time
work done  = p x t  = 90 x 40 = 3600 J 
work done = f x s = mg  x s = 60 x 10 x (X x 20)
3600 = 600 x 0.2 x x
x = 3600 / (600 x 0.2) = 20

so x = 20

number of stairs are 20.