Question

A person of mass 50 kg stands on a weighing scale on a lift . If the lift is ascending upwards with a uniform acceleration of 9 m s^-2, what would be the reading of the weighing scale ? ( take g= 10 m s^-2)
A. 50 kg
B. 60 kg
C. 95 kg
D. 100kg

Answer 1

Hint: Use Newton’s second law to solve this problem. Normal reaction and the gravitational force are the two forces acting on the person on the lift.

Complete step by step solution:
Mass of the person = m = 50 kg
Acceleration of the lift = a = 9 m s^-2
Acceleration due to gravity = 10 m s^-2
When the person steps on the weighing scale, the surface of the weighing scale exerts normal reaction on the person. The magnitude of the normal reaction is measured with the help of calibrated reading on the weighing scale. So, we are supposed to find out the normal reaction of the person.
Applying Newton’s second law on the person,
F_net = ma

Gravitational force, mg , is acting along the downward direction on the person . Normal reaction, N, is acting along the upward direction on the person. Let us take upward direction as positive and downward direction as negative.
F_net = N - mg = ma
(50 × 10) N = (50 × 9) N
N - 500 N = 450 N
N = 500 N + 450 N = 950 N
If the normal reaction is 950 N , then the reading shown in the weighing scale is also equal to 950 N . When the same reading is shown in terms of kg , then it would be equal to 950/g = 950/10 =95 kg .

Hence, the correct choice for the given question is option C .

Note: If the lift is moving downwards with a uniform acceleration, then the reading on the weighing scale will be less than the actual weight of the person.