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  4. A boy stands on a weighing machine inside a lift. When the lift is going down with acceleration g/4, the machine shows a reading 30 kgf. When the lift goes upwards with acceleration g/4, the reading would be

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Q. A boy stands on a weighing machine inside a lift. When the lift is going down with acceleration $g/4$, the machine shows a reading $30 \,kgf$. When the lift goes upwards with acceleration $g/4$, the reading would be

AIIMSAIIMS 2018Laws of Motion

Solution:

While going down, machine reading is 30 kgf.
$\Rightarrow \quad mg-N=m\left(\frac{g}{4}\right);$
$mg-30g=\frac{mg}{4}\,;3 \frac{mg}{4}=30g$
$m=\frac{30\times4}{3}=40\,kg$
While going up, the reaction would be
$N^{'}-mg=m \frac{g}{4}$
$N^{'}=5 \frac{mg}{4}=\frac{5}{4}\,\left(40\,kg\right)g$
$\Rightarrow \quad N^{'}=50 \,kgf.$

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