weight of the body is zero in free fall why?

The actual weight of a body is W = mg.When this force acts on the weighing machine ,the machine offers a  reaction force called the normal force (Due to Newton's third Law).This reaction is measured in the weighing machine which is called the apparent weight. If we are falling freely ,eg. a freely falling lift, the body is not able to exert a  force on the weighing machine because the machine itself would be falling with the same acceleration 'g' .Hence, the normal reaction is zero and you feel weightless. It is to remember that the actual weight  does not go to Zero rather the weight you feel viz. Apparent Weight goes to Zero