Question

As i was standing on a weighing machine inside a lift it recorded 50 kg wt. Suddenly for few seconds it recorded 45 kg wt. What must have happened during that time? Explain with complete numerical analysis.

Answer 1

The weight recorded by weighing machine is always apparent weight and a measure of reaction force acting on the person. As the apparent weight (45 kg-wt) in this case is less than actual weight (50 kg-wt) the lift must be accelerated downwards during that time.

Numerical Analysis

1. Weight on the weighing machine inside the lift is recorded as 50 kg-wt

∴ mg = 50 kg-wt

2. This weight acts on the weighing machine which offers a reaction R given by the reading of the weighing machine

∴ R = 45kg-wt = 9/10 mg

3. The forces acting on person inside lift are as follows:

• Weight mg downward (exerted by the earth) 
• Normal reaction (R) upward (exerted by the floor)

 4. As, R < mg, the net force is in downward direction and given as,

5. Therefore, the elevator must be accelerated downwards with an acceleration of 1 m/s² at that time.

Answer 2

this is the answer 

by Aayush chauhan