A boy having mass 40 kg is standing in a lift . . .

Question : A boy having mass 40 kg is standing in a lift. The force experienced by his feet will be maximum when lift is :

(a) stationary

(b) moving downward with velocity 4 m/s

(d) moving up with acceleration 4 m/s²

Doubt by Girisha

Solution :

Here
m=40 kg

We know, force experienced by his feet will be equal to the force of normal reaction (N or R) applied by the surface of the lift.

(a) When the lift is stationary then a=0
so
R=mg
R=40(9.8) = 392 N

(b) When the lift moves downward with velcity 4 m/s² then again a=0 so
R=mg
R=40(9.8) = 392 N

(d) When the lift moves upward with a=4 m/s² then

R=m(g+a)
R=40(9.8+4)
R=40×13.8
R=552 N

Cleary force experience by his feet will be maximum when the lift is moving up with an acceleration of 4 m/s².

So, (d) moving up with acceleration 4 m/s², would be the correct option.