Question

A man of mass M stands on a weighing machine in an elevator accelerating upwards with an acceleration a. Draw the free-body diagram of the man as observed by the observer A (stationary on the ground) and observer B (stationary on the elevator). Also, calculate the reading of the weighing machine.

Answer 1

Method 1 : Observation from observer A on ground
From FBD of man w.r.t. A, using Newton's second law,
$N-Mg=Ma\Rightarrow N=M(g+a)$
N is the reading of weighing machine.
Method 2 : Observer 'B' is moving in accelerated frame of reference. If he observes the man on weighing machine he will observe the man at rest w.r.t. elevator. If he is asked to draw a free-body diagram of M he will apply pseudo force on M apart from real forces acting on him.
From free-body diagram:
$N=Mg+ma\Rightarrow N=M(g+a)$
The value of N calculated from both the methods is same.