Question

When an elevator starts to move down suddenly we experience 'weightlessness'. Explain.

Answer 1

When an elevator begins to move downwards in an accelerated mode, the forces acting on the body are the following:
a.Weight of the body acting downwards
b.Normal reaction of the floor acting upwards
c.The centrifugal force acting on the body, acting upwards.
Weight of the body is due to gravitational force on the body acting downwards.
Normal reaction is the force that is exerted by the elevator floor in response to the force with which the body presses itself against the floor.
The centrifugal force here  is fictious force that acts on the body in the direction opposite to the acceleration of the reference frame, here it is, the elevator floor. It is given by ma, where m is the mass of the body & a is the accelaration of the elevator floor. Centrifugal force is directed opposite to the acceleration of the elevator floor.
Weightlessness is the condition of the zero apparent weight.
When the acceleration of the elevator is such that the upward centrifugal force  Fc completely balances the downward weight Wt. of the body, the resultant normal reaction (N =Fc - Wt.) of the body is reduce dto zero. That's whene the body on the elevator floor will experience the state of weightlessness.