A man of mass 70 kg stands on a weighning scale in a lift which is moving (a) upwards with a uniform speed of `10 ms^(-1)`
(b) downwards with a uniform acceleration of ` 5 ms^(-2)`
(c ) upwards with a uniform acceleration of ` 5 ms^(-2)`
(d) What would be the reading if the lift mechanism failed and it hurtled down freely under gravity ?

Explanation

Here, m = 70 kg , ` g = 9.8 m//s^(2)`
The weighing machine in each case measures the reaction R i.e . The apparent weight
(a) When lift moves upwards with a uniform speed , its accelerations is zero .
`R = mg = 70 xx 9.8 = 686 N `
(b) When the lift moves dowwards with `a = 5 m//s^(2)`
`R = m(g-a) = 70 (9.8 - 5) = 336 N `
(c) When the lift moves upwards with `a = 5 m//s^(2)`
`R = m(g +a) = 70 ( 9.8 +5) = 1036 N `
If the lift were to down freely under gravity , downward acc. a= g
` :. R = m(g-a) = m(g-g) = zero `