Question

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

• A. 18 kgf
• B. 37.5 kgf
• C. 50 kgf
• D. 67.5 kgf

Answer 1

Answer: (C)

50 kgf

Answer 2

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 $N^{'}-mg=m \frac{g}{4}$ $N^{'}=5 \frac{mg}{4}=\frac{5}{4}\,\left(40\,kg\right)g$ $\Rightarrow\quad N^{'}=50 \,kgf.$