Question

A boy weighing 45 kg stands on spring scale in a lift that is going down with a constant speed of 3 m/s. If the lift is brought to halt by a constant deceleration, in a distance of 10 m, the scale reading during this period is

• A. 47 kg
• B. 55 kg
• C. 44 kg
• D. 50 kg

Answer 1

The correct option is A 47 kg

Acceleration of the lift can be found using
$v^2-u^2=2as$
$v=0$, $u=3m/s$, $s=10m$
Thus, $a=-0.45m/s^2$

The normal force by the scale on the boy will be given by
$F=m(g+a)$

$mg=45$

$m=\frac{45}{g}$

$F=\frac{45}{g}(g+a)$

$g=10m/s^2$ and $a=-0.45m/s^2$

Thus, $F=47.025$
Hence, scale reading is $47kg$.