Weightlessness 

A person in a descending lift or descending aeroplane experience a loss in weight at certain point or the other. The apparent weight is less than the weight of the person. At a point when the deceleration of the lift is equal to the acceleration due to gravity,  weightlessness is experienced.

When the lift is at rest or moving with uniform speed

Resultant force F=mg  (see fig 2 above)

When the lift moves upward, the resultant  force F that produces the acceleration is .

F-mg=ma , therefore F=ma+mg

When the lift moves downward the resultant force is

mg-F=ma, therefore  F = mg-ma , the apparent weight of the person is mg-ma =m(g-a)

weightlessness occur when g=a and the lift is descending.

At this point, resultant force =0 and the person appear to have no weight.

The apparent weight condition is also experienced in a descending aeroplane.

Example

A body of  mass 2kg is thrown vertically upward with a force of 100N . what is the acceleration of the body, assume air resistance is not neglected?  Take 

(see fig 1 above)

The resultant force is given by

Read more about PHYSICS for High Schools