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Question
How do you define force? What are the forces acting on a man standing in a lift? If the lift moves down with an acceleration of 2 ms−2, what is the net force on the man if his mass is 50 kg?
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Solution
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 = 390 N.
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