Question

A man of mass 70 kg stands on a weighing scale in a lift which is moving 

1. upwards with a uniform speed of 10 m s^-1 

2. downwards with a uniform acceleration of 5 m s^-2 

3. upwards with a uniform acceleration of 5 ms^-2. What would be the readings on the scale in each case? 

4. What would be the reading if the lift mechanism failed and it hurtled down freely under gravity?

Answer 1

The weighing machine measures the reaction R which is nothing but the apparent weight.

1. when the lift is moving upwards with uniform speed. 

R = mg = 70 × 9.8 = 686 N.

2. When lift moves downwards with an acceleration of 5m/s² 

R = m (g – a) = 70 (9.8 – 5) = 336 N.

3. When lift moves upwards with with an acceleration of 5m/s² 

R = m (g + a) = 70 (9.8 + 5) = 1036 N.

4. If the lift falls down freely under gravity 

R = m (g – g) = 0.