Astronauts --- Weightlessness --- Encountering Microgravity

Weightlessness is more correctly termed microgravity. Astronauts are not actually weightless, because the Earth's gravity is holding astronauts and everything in the shuttle in orbit. They are actually in a state of free-fall, much like jumping from an airplane except that people are moving so fast horizontally (8 kilometers per second) that, as someone fall, he/she never touch the ground because the Earth curves away form him/her. It's like this: When a man stands on a bathroom scale, as the scale is resting on the ground, it pushes up on him with a force equal to the man's weight. So the weight is measured (see Newton's Third Law). However, if he jump off a cliff while standing on a bathroom scale, both he and the scale will be pulled down equally by gravity (and fall with the same velocity). The scale and he will not push against each other. Therefore, his weight will read zero.

Because the shuttle and all of the objects in it are falling around the world at the same rate, everything in the shuttle that is not secured floats. So if astronauts pour a glass of water out, it assumes a large spherical drop that they can break up into separate smaller drops; If the astronauts are not secured to something, they float! Therefore, NASA has placed many restraints, hand-holds and foot-holds throughout the cabin of the shuttle.

Encountering Microgravity How Astronauts Feel in Mircogravity Countermeasures to deal with Microgravity