Question

A man weighs 70kg. He stands on a weighting machine in a lift, which is moving

(a) Upwards with a uniform speed of 10m/s.  

(b) Downwards with a uniform acceleration of 5m/s².

(c) Upwards with a uniform acceleration of 5m/s². Take g = 9.8m/s².

What would be the readings on the scales in each case what would be the reading if the lift mechanism failed and it came down freely under gravity?  

Answer 1

Here, m = 70kg, g = 9.8m/s² 

(a) When the lift moves upwards with a uniform speed, its acceleration is zero.

R = mg = 70 x 9.8 = 686 N

(b) When the lift moves downwards with

a = 5m/s² 

R = m (g – a) = 70(9.8 + 5) = 336N 

 (c) a = 5m/s² 

R = m (g + a) = 70 (9.8 + 5) = 1036N 

If the lift falls freely under gravity, a = g 

R = m (g – a) = m (g – g) = 0