Net force acting on a body is equal to mass into acceleration or we may define force as a push or pull which changes or tends to change the state of rest or of uniform motion in a straight line of the body on which it acts.
The forces acting on a man in a lift are
(i) weight of the man acting vertically downwards,
(ii) the reaction of the floor acting vertically upwards.
In a lift accelerating downwards the net force on the man is
F = m(g – a) = 50(9.8 – 2)
= 50 × 7.8 = 390N.
