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

Options

A. upwards with a uniform speed of `ms^(-1)`
B. downwards with a uniform acceleration of `5ms^(-2)`
C. upwards with a uniform acceleration of `ms^(-2)`. What would be the readings on the scale in each case?
D. What would be the reading if the lift mechanism falled and it hurted down freely under gravity?

Explanation

The reading on the scale is a measure of the force on the floor by the man. By the Third Law, this is equal and opposite to the normal force N on the man by the floor.
(a) `N=70xx10=700N`, Reading is 70kg
(b) `70xx10-N=70xx5`, Reading is 35kg
(c) `N-70xx10=70xx5`, Reading is 105kg
(d) `70xx10-N=70xx10`, Reading would be zero, the scale would read zero.