The phenomenon of weightlessness
Physics defines weight as the force with which any body acts on the surface, prop or suspension. Occurs weight due to the gravitational attraction of the Earth. A numerical weight equal to the force of gravity, but the latter is applied to the center of mass of a body, the weight is applied to the support.
Zero gravity — zero weight, may occur if there is no force of gravity, that is, the body quite removed from the massive objects that can attract him.
The international Space Station is at a distance of 350 km from the Earth. At this 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 influence is always there. The astronauts are on the ISS, continued to operate in the Earth, but gravity is there.
Another case of weightlessness occurs when the force of gravity is offset by other forces. For example, the ISS is exposed to the force of gravity is slightly reduced by distance, but also the station is moving in a circular orbit with the first space velocity and the centrifugal force compensates gravity.
Weightlessness on Earth
The phenomenon of weightlessness possible in the Ground. Under acceleration the weight of the body can decrease and even becomes negative. A classic example, which lead to physics — falling Elevator.
If the Elevator moves down with acceleration, the pressure on the floor of the Elevator, and, consequently, the weight will decrease. Moreover, if an acceleration equal to the acceleration of free fall, the Elevator falls, the weight of the bodies are zero.
Negative weight can occur if the acceleration of the Elevator exceeds the acceleration of free fall of body inside "stick" to the ceiling of the cabin.
This effect is widely used to simulate weightlessness when training astronauts. The plane is equipped with a camera for training, rises to a considerable height. Then swoops down on a ballistic trajectory essentially in free fall, near the earth's surface the machine is aligned. When you dive with 11 thousand metres to 40 seconds of weightlessness that use them for training.
There is a misconception that such aircraft perform complex pieces like "the loop" for a zero gravity. In fact, for training uses modified production passenger aircraft, which are incapable of complex maneuvers.
Physical expression
Physical weight formula (P) in rapid movement of the support, whether it be the falling bodice or a diving plane, has the following form:
P=m(g-a),
where m is the mass of the body,
g – acceleration of free fall,
a – acceleration support.
In case of equal g and a, P=0, that is achieved by the weightlessness.