Laws of Motion
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Question : 13
Total: 40
A man of mass 70 kg stands on a weighing scale in a lift which is moving
(a) upwards with a uniform speed of 10 m s– 1
(b) downwards with a uniform acceleration of 5 m s– 2
(c) upwards with a uniform acceleration of 5 m s– 2
What would be the readings on the scale in each case?
(d) What would be the reading if the lift mechanism failed and it hurtled down freely under gravity?
(a) upwards with a uniform speed of 10 m s
(b) downwards with a uniform acceleration of 5 m s
(c) upwards with a uniform acceleration of 5 m s
What would be the readings on the scale in each case?
(d) 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 s– 2
The weighing machine in each case measures the reaction R i.e. the apparent weight.
(a) When the lift moves upwards with a uniform speed, its acceleration is zero.
∴ R = m g = 70 × 10 = 700 N
(b) When the lift moves downwards with a = 5 m s– 2
R = m ( g – a )
= 70 ( 10 – 5 ) = 350 N
(c) When the lift moves upwards with a = 5 m s– 2
R = m ( g + a )
= 70 ( 10 + 5 ) = 1050 N
(d) If the lift was to come down freely under gravity, downwardacceleration
a = g
∴ R = m ( g – a ) = m ( g – g ) = zero .
The weighing machine in each case measures the reaction R i.e. the apparent weight.
(a) When the lift moves upwards with a uniform speed, its acceleration is zero.
(b) When the lift moves downwards with a = 5 m s
(c) When the lift moves upwards with a = 5 m s
(d) If the lift was to come down freely under gravity, downwardacceleration
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