Question

A girl with a mass of 45 kg is standing on a scale in a lift. What is the weight of the girl when (i) the lift is accelerating upwards at 2.0 ms s −2 and (ii) the lift is moving down with a constant velocity. Calculate the inertial force in each case.

Answer 1

The weight of the girl in a lift

$$W=m(g \pm a)$$

(a)

Weight

$$W=45\:\rm{kg}\times(9.8\:\rm{m/s^2}+2.0\:\rm{m/s^2})=531\:\rm{N}$$

Inertial force

$$F_i=W-mg=531\:\rm{N}-441\:\rm{N}=90\:\rm{N}$$

(b)

Weight

$$W=45\:\rm{kg}(9.8\:\rm{m/s^2}+0\:\rm{m/s^2})=441\:\rm{N}$$

Inertial force

$$F_i=W-mg=441\:\rm{N}-441\:\rm{N}=0\:\rm{N}$$