Question

A man of 70kg stands on a weighing scale in a lift which is moving

a) upwards with uniform speed of 10m/s2.

b) downward with a uniform acceleration of 5m/s2.

c)upwards with a uniform acceleration of 5m/s2 . the reading of the scale in each case?

Answer 1

(a) Mass of the man, m = 70 kg

Acceleration, a = 0
Using Newton’s second law of motion, we can write the equation of motion as:
R – mg = ma
Where, ma is the net force acting on the man.
As the lift is moving at a uniform speed, acceleration a = 0
∴ R = mg
= 70 × 10 = 700 N
∴ Reading on the weighing scale = 700 / g = 700 / 10 = 70 kg

(b) Mass of the man, m = 70 kg
Acceleration, a = 5 m/s2 downward
Using Newton’s second law of motion, we can write the equation of motion as:
R + mg = ma
R = m(g – a)
= 70 (10 – 5) = 70 × 5
= 350 N
∴ Reading on the weighing scale = 350 g = 350 / 10 = 35 kg

(c) Mass of the man, m = 70 kg
Acceleration, a = 5 m/s2 upward
Using Newton’s second law of motion, we can write the equation of motion as:
R – mg = ma
R = m(g + a)
= 70 (10 + 5) = 70 × 15
= 1050 N
∴ Reading on the weighing scale = 1050 / g = 1050 / 10 = 105 kg

(d) When the lift moves freely under gravity, acceleration a = g
Using Newton’s second law of motion, we can write the equation of motion as:
R + mg = ma
R = m(g – a)
= m(g – g) = 0
∴ Reading on the weighing scale = 0 / g = 0 kg
The man will be in a state of weightlessness.

Answer 2

• R = 700 N = 70 KG
• R = 350 N = 35kg
• R = 1050 N = 105 kg
• R = 0