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[–] Tor1 0 points 2 points (+2|-0) ago 

You can experience a similar sensation of the orbiting ISS astronauts when you are temporarily suspended above your seat during a roller coaster ride.

The cause of the sensation of weightlessness in both cases are the same. You are in freefall and therefore not experiencing any contact force pushing or pulling at your body.

In each case, gravity is the only force acting upon the body. Being an action-at-a-distance force, it cannot be felt and therefore does not provide any sensation of weight.

In point of fact, the orbiting astronauts do weigh something; that is, there is a force of gravity acting upon their bodies. If it were not for the force of gravity, the astronauts would not be orbiting in circular motion. It is the force of gravity that distributes their weight and supplies the centripetal force required to produce the inward acceleration that is characteristic of their circular motion.

The force of gravity is the only force acting upon their body. The astronauts are in free-fall. Like the falling amusement park rider or a falling elevator rider, the astronauts and their surroundings are falling towards the Earth under the sole influence of gravity.

The astronauts and all their surroundings - the space station with its contents - are falling towards the Earth without colliding into it. Their tangential velocity allows them to remain in orbital motion while the force of gravity pulls them inward.

When you are accelerating upwards in an elevator, the contact force is greater than the usual amount. This gives you the sensation of weighing more than your usual amount. When accelerating downwards, the contact force is less than the normal amount. This gives you the sensation of weighing less than your usual amount. In all cases, your weight is not changing - you still weigh the same amount of Newtons you always have.