Problem 4 - A Complete Resource Book in Physics for JEE Main

Question

A girl of mass \(50 \mathrm{~kg}\) stands on a measuring scale in a lift. At an instant, it is detected that the reading reduces to \(40 \mathrm{~kg}\) for a while and then returns to original value. It can be said that (A) The lift was in constant motion upwards (B) The lift was in constant motion downwards (C) The lift was suddenly started in downward motion (D) The lift was suddenly started in upward motion

Step-by-Step Solution

Answer

The lift was suddenly started in downward motion (C)

Step 1: Identifying that the weight has changed

First, notice that the girl's weight has decreased according to the scale. This suggests that the force exerted by the girl on the scale (her weight) was less than usual. This means that she is either moving upwards or downwards.

Step 2: Understand the direction of motion

If a lift is moving downwards or just started moving downwards, people in the lift will feel lighter. This is because the force of gravity pulling them downwards is countered by the momentum of the lift. Hence, people are pulled away from the scale, causing a lower reading.

Step 3: Answer based on the observation

Since the reading on the scale went back to normal, it suggests that the downward motion was not constant but rather initiated at that particular point. Therefore, the lift was suddenly started in downward motion.