Q: A spring balance is attached to the ceiling of a lift. A man hangs his bag on the spring and the spring reads 49 N, when the lift is stationary. If the lift moves downward with an acceleration of 5 m/s2 , the reading of the spring balance will be
(a) 49 N
(b) 24 N
(c) 74 N
(d) 15 N
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Ans: (b)
Sol: $ m = \frac{49}{9.8} = 5 kg $
mg – R = m a
R = m g – ma
R = 49 -5 × 5 = 24 N