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 `ms^(2)` ,
(c) upwards with a uniform acceleration of 5 `ms^(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

Mass of person m= 70 kg
(a ) when it moves in upward directin with constant velocity
weight of person recorded in spring balance

W= mg
`=70 xx 10`
`=700 N`
Weighing scale will measure 70 kg weight
(b ) when lift is moving down with acceleration in upward direction resultant direction
resultant acceleration g= g- a

Weight recorded mg = mg - ma
`=70 (10-5)`
`=350N`
`: m = (mg )/( g ) = (350)/( 10)`
Weighing scale will measure 35 kg weight
(c ) When lift moves with upward acceleration will be in downward direction
Weight recorded in spring balance
`:. , mg = mg + ma`
`=70 (10 + 5)`
`70 xx 15`
1050 N
`:. = (mg)/( g) = (1050)/( 10) = 105 kg`
Weighing scale will measure 105 kg weight
(d ) when lift is falling freely then pseudo acceleration will be upward direction
`:.` Weight recorded
w= mg
`=m(0)`
`=0`
this is called weightlessness.