Recall that if the frictional effects of air are ignored, all bodies irrespective of their masses, fall with a constant acceleration known as acceleration due to gravity. Such bodies are called freely falling bodies and the net force acting on them is only due to their weight. They reach the ground at the same time. Hence, in the absence of contact forces, any freely falling body experiences weightlessness. For example, in a roller coaster ride, the rider is acted upon by the normal force and the gravitational force. When the car reaches the top of the loop, the rider is momentarily lifted off the seat, thus removing the normal force. The only force acting on the person at this position is his weight, which makes him feel "weightless". Weightlessness is merely a sensation as the weight of one's body is always acting when on Earth.