Body weight, unlike weight, can will change as a result of acceleration. Little changes of weight can be felt, for example, at a start of motion or a stop of the elevator. The condition of total absence of weight is called zero gravity.
Zero gravity phenomenon
The physics gives definition to the weight as force with which any body affects a surface, a support or a suspension. There is a weight owing to a gravitational attraction of Earth. In number weight is equal to gravity, but the last is attached to the center of body weights, weight is attached to a support.
The zero gravity — zero weight, can arise if there is no gravity, that is the body is rather removed from massive objects which can attract it.
The International Space Station is at distance of 350 km from Earth. On such removal the acceleration of gravity (g) is 8.8 m / с2 that is only 10% less, than on the surface of the planet.
In practice you will seldom meet it — gravitational influence exists always. The astronauts who are at the ISS are still affected by Earth, however the zero gravity is present there.
Other case of zero gravity arises if gravity is compensated by other forces. For example, the ISS is subject to the gravity which is slightly reduced at the expense of distance, but also the station moves on a circular orbit with the first cosmic speed and centrifugal force compensates inclination.
Zero gravity on Earth
The phenomenon of zero gravity is possible also on Earth. As a result of acceleration the body weight can decrease, and even becomes negative. A classical example which is given by physicists — the falling elevator. If the elevator moves with acceleration down, then pressure upon an elevator floor, and, therefore, and weight, will be will decrease. And if acceleration is equal to acceleration of gravity, that is the elevator falls, the weight of bodies will become zero.
Negative weight is observed if acceleration of the movement of the elevator exceeds acceleration of gravity — the bodies which are inside "will stick" to a cabin ceiling.
This effect is widely applied to simulation of zero gravity when training astronauts. The plane equipped with camera for trainings rises by considerable height. Then dives down a ballistic trajectory, in fact, freely falls, at the Earth's surface the car is leveled. At dive from 11 thousand meters it is possible to receive 40 seconds of zero gravity which use for trainings. There is a delusion that similar planes carry out difficult figures, like "Nesterov's loop", for obtaining zero gravity. Actually for trainings modifed production passenger planes which are incapable of complex maneuvers are used.
Physical expression
The physical formula of weight (P) at the accelerated movement of a support, whether it be the falling bodice or the diving plane, has the following appearance: P=m(g-a) where m is body weight, g is acceleration of gravity, an is acceleration of a support. At equality of g and a, P=0, that is the zero gravity is reached.