A man stands in a lift going downwards with uniform velocity. He experiences a loss of weight at the start but not when lift is in uniform motion. Explain why ?

When the lift starts it accelerates downward and afterwards it moves with a constant velocity. Let the acceleration is a.
The the reaction force on the man by the floor of the lift is $=m(g-a)$
So the man feels weight loss. Once the lift achieves constant velocity the acceleration is zero hence he don't experience any weight loss.