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

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

A boy stands on a weighing machine inside a lift.

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Concepts Used:

Laws of Motion

The laws of motion, which are the keystone of classical mechanics, are three statements that defined the relationships between the forces acting on a body and its motion. They were first disclosed by English physicist and mathematician Isaac Newton.

Newton’s First Law of Motion

Newton’s 1^st law states that a body at rest or uniform motion will continue to be at rest or uniform motion until and unless a net external force acts on it.

Newton’s Second Law of Motion

Newton's 2^nd law of motion deals with the relation between force and acceleration. According to the second law of motion, the acceleration of an object as built by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.

Newton’s Third Law of Motion

Newton's 3^rd law of motion states when a body applies a force on another body that there is an equal and opposite reaction for every action.

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

The Correct Option is C

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