a man is moving from 1st floor to ground floor in a lift . what happens to his weight ? why ?
Answers
As per the question, a man is moving from first floor to ground floor in a lift.
The mass of a body is always constant [ for non relativistic motion]
Let the mass of the man is m .
The weight of a body is the normal reaction of the lift on the man.
Let the normal reaction is denoted as R.
Here, two cases arise.
CASE-1: Lift is moving down with acceleration a.
In this case the gravity is acting in vertical downward direction .
The gravity W = mg downward.
Here g is called acceleration due to gravity.
The normal reaction on the man = R . It acts in vertical upward direction.
Hence, the net force acting on the man is W-R.
From Newton's second law of motion, we know that net force is the product of mass with acceleration.Hence, the net force on man is -
W-R = ma
⇒R= W-ma
= mg-ma
= m( g-a )
Hence, from above, it is obvious that the apparent weight of the man is decreased.
CASE-2 Let the lift is under free fall i.e a = g.
The apparent weight of the man will be R = m( g-a )
= m( g-g )
= 0
Hence, the apparent weight will be zero under free fall of the lift.