Case (1): When the lift is accelerated upward:
Suppose, ‘R’ be the upward thrust of the floor on the man (normal reaction) and ‘mg’ is the weight of the man acting downwards.
Hence, Unbalanced force = mass × acceleration
Or, R = mg + ma
Or, R = m (g + a)
Thus, if the man is standing on weighing machine, it will show a larger weight than ‘mg’.
Case (2): When the lift is accelerate downwards:
R+F = mg
Or, R = mg – ma = m (g – a)
Case (3): When the lift moves with uniform velocity (or is at rest):
In this case, a = 0 So, R = mg.
In the case of free fall of the lift, a = g,
Then, r = m (g – a), i.e. the man will feel weightlessness.