Question

A man weighs 70kg. He stands on a weighting machine in a lift, which is moving

(a) Upwards with a uniform speed of 10m/s.
(b) Downwards with a uniform acceleration of 5m/s2.
(c) Upwards with a uniform acceleration of 5m/s2. Take g = 9.8m/s2.

What would be the readings on the scales in each case what would be the reading if the lift mechanism failed and it came down freely under gravity?

CLASS - XI PHYSICS (Laws of Motion)

Answer 1

a) the forces acting on body = weight of person = 70*9.8 = 686 N
    weighing machine will show = 686/9.8 = 70 kg
b)force acting on body = weight of person + upward pseudo force
                                    = 70*9.8 - 70*5 = 336 N
  weighing machine will show = 336/9.8  = 34.28 kg
c)force acting on body = weight of person + downward pseudo force
                                    = 70*9.8 + 70*5 = 1036 N
  weighing machine will show = 1036/9.8  = 105.71 kg

Answer 2

Explanation:

a) Since the lift is moving with uniform velocity there will be no change in the reading of the weighing machine. As the net acceleration is 10 m/s^2 itself.

b) When the lift is moving downwards,

the net acceleration= g - a

= 5m/s^2

therefore net force = 70 X 5

= 350 N

therefore reading shown = 35 kg

C) When the lift is moving upwards

net acceleration= 15 m/s^2

therefore, net force = 70 X 15

= 1050 N.

hope you understand......