A man of mass 70 kg stands on a weighing scale in a lift which is moving (a) upwards with a uniform speed of `10 m s^(-1)` , (b) downwards with a uniform acceleration of `5 m s^(-2)`, (c) upwards with a uniform acceleration of `5 m s^(-2)` . What would be the readings on the scale in each case? (d) What would be the reading if the lift mechanism failed and it hurtled 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 = 70 xx 10 = 700 N : ` Reading is 70 kg
(b) `70 xx 10 - N = 70 xx 5` : Reading is 35 kg
(c ) `N -70 xx 10 =70 xx 5` : Reading is 105 kg
(d) `70 xx 10 -N = 70 xx 10` : Reading would be zero, the scale would be read zero .