Question

A girl of mass 40 kg takes a staircase of 15 steps,

each of height 20 cm. If she utilizes a power of

80 W to climb the staircase, the time taken by her

is [Take g = 10 m/s2]

Answer 1

Given data:
Mass = 40 kg
Power = 80 W
Height = 20 cm = 20 / 100 = 0.2 m
No. of steps = 15
So, total height will be 0.2 x 15 = 3 m
We have to calculate the time taken by her to climb the staircase.
i-e, Time = ?
Solution:
We know that,
Power = Work done / Time  .......... (1)
First we calculate the work done.
Work done = F . d
Work done = ma . h 
Replacing acceleration by gravity:
Work done = m.g.h
Work done = (40) . (10) . (3)    (given that g = 10 m/sec²)
Work done = 1200 N
Put this value in equation (1), we get:
Power = Work done / Time
80 = 1200 / Time
Time = 1200 / 80
Time = 15 sec
which shows that the girl needs 15 seconds to climb the staircase of 15 steps