A spring balance is attached to the ceiling of a lift. A man hangs his bag on the spring and the balance reads 49N, when the lift is stationary. If the lift moves downward with an acceleration of 5m/s2, the reading of the spring balance would be
A
24N
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B
74N
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C
15N
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D
49N
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Solution
The correct option is A24N When the lift is stationary, the spring force Fs in spring balance will be equal to the weight of bag.
∴Fs=W=49N
or, mg=49⇒m(9.8)=49
∴m=499.8=5kg
When lift is accelerating downwards, the acceleration of bag will be same as that of lift.
Applying Newton's 2nd law of motion,
mg−F′s=ma
49−F′s=5×5
∴F′s=49−25=24N
Thus, new reading of spring balance will be 24N.
Why this question ?Tip: Spring balance measures theweight of a body based on the springforce recorded by it.