When an elevator descends with a certain force F = ma

The image below shows the forces acting on a body in a descending elevator. The force at which the elevator descends will take a negative sign because it moves in a downward direction.

-Ma = N-Mg

N = Ma - Mg

Then N = M(a-g)

The expression shows that as the downward acceleration (deceleration) increases, the value of the normal force becomes reduced, which caused the body under the influence of the normal force to loss weight. When the value of the deceleration "a" becomes equal to the value of gravity "g", the value of normal force becomes zero and therefore, the body feels a complete loss of weight.