Question

A man of mass 70 kg, stands on a weighing machine in a lift, which is moving
(a) upwards with a uniform speed of 10 ms
(b) downwards with a uniform acceleration of 5 ms
(c) upwards with a uniform acceleration of 5 ms”.​

Answer 1

(a). As the lift is moving at a uniform speed, acceleration a=0

R=mg

=70×10

⇒700N

Reading on the weighing scale

=700/g

=700/10=70kg

(b) Mass of the man, m=70kg

Acceleration, a=5m/s

2

downward

Using Newtons second law of motion, we can write the equation of motion as:

R+mg=ma

R=m(g−a)

=70(10−5)

=70×5

=350N

Reading on the weighing scale

⇒350g=350/10=35kg

(c) Mass of the man, m=70kg

Acceleration, a=5m/s

2

upward

Using Newtons second law of motion, we can write the equation of motion as:

R−mg=ma

R=m(g+a)

=70(10+5)=70×15

=1050N