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Question
A man of mass 70 kg stands on a weighing scale in a lift which is moving
- upwards with a uniform speed of 10 m s-1
- downwards with a uniform acceleration of 5 m s–2
- upwards with a uniform acceleration of 5 m s–2. What would be the readings on the scale in each case?
- What would be the reading if the lift mechanism failed and it hurtled down freely under gravity?
Solution
Here, m = 70 kg, g = 10 m/s2
In each instance, the weighing machine measures the reaction R, which represents the apparent weight.
(a) When the lift moves upwards with a uniform speed, acceleration a = 0.
R = mg = 70 x 10 = 700 N
(b) When the lift moves downward with a = 5 ms-2
R = m (g - a) = 70 (10 - 5) = 350 N
(c) When the lift moves upwards with a = 5 ms-2
R = m (g + a) = 70 (10 + 5) = 1050 N
(d) If the lift were to come down freely under gravity, downward acceleration. a = g
∴ R = m (g - a) = m (g - g) = zero.
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