The phenomenon of weightlessness
Physics defines weight as a force with which any body acts on a surface, a support or a suspension. There is a weight due to gravitational attraction of the Earth. Numerically the weight is equal to the force of gravity, but the latter is applied to the center of mass of the body, the weight is attached to the support.

The International Space Station is located at a distance of 350 km from the Earth. At such a distance, the acceleration of gravity (g) is 8.8 m / s2, which is only 10% less than on the surface of the planet.

In practice, this is rarely seen - the gravitational effect exists always. The Earth is still operating on cosmonauts on the ISS, but there is zero-gravity there.

Another case of weightlessness occurs if gravity is compensated by other forces. For example, the ISS is subject to gravity, slightly reduced by distance, but also the station moves in a circular orbit with the first space velocity and centrifugal force compensates for gravity.

Weightlessness on Earth

The phenomenon of weightlessness is possible on the Earth. Under the influence of acceleration, the weight of the body can decrease, and even becomes negative. A classic example, which leads physicists - a falling elevator.

If the elevator moves down with acceleration, then the pressure on the floor of the elevator, and, consequently, the weight, will decrease. And if the acceleration is equal to the acceleration of free fall, that is, the elevator falls, the weight of the bodies becomes zero.

Negative weight is observed if the acceleration of the elevator movement exceeds the acceleration of free fall - the bodies inside are "stuck" to the ceiling of the cabin.

This effect is widely used to simulate weightlessness in the preparation of astronauts. The airplane equipped with a training chamber rises to a considerable height. After that, it dives down the ballistic trajectory, in fact, falls freely, at the surface of the earth the machine is leveled. When diving from 11 thousand meters, you can get 40 seconds of weightlessness, which they use for training.

There is a misconception that such aircraft perform complex shapes, like the "Nesterov loop," to obtain weightlessness. In fact, for training use the modified serial passenger aircraft, which are incapable of complex maneuvers.