To learn the phenomenon of weightlessness, it is essential to know about the mass and the weight. The mass of a body is the amount of matter contained in it. In fact, it is the measure of inertia of the body. Weight of a body is the force with which a body is attracted towards the centre of the earth. Weightlessness occurs when there is no gravitational pull on a body. A man experiences weightlessness in a spacecraft. Weightlessness can also occur when a satellite orbits the earth. If the force of gravity is balanced by the centrifugal force, the man in the spacecraft comes in a state of weightlessness. A body falling freely under gravity also experiences weightlessness.

When spacecraft starts orbiting the earth, the force of gravity acting on it is balanced by centrifugal force and the main inside comes in a state of weightlessness.

Weightlessness causes problems in spacecraft’s. Anything that is not fixed or tied down just floats. Astronauts use special devices to enable themselves to eat and drink. The crew members have to learn to adjust the vigour of their actions to keep from crashing into the walls and equipment. Anything thrown from one person to other just floats with a little force. During the sleep also the astronauts feels instability.

Sometimes, weightlessness causes nausea and giddiness because of the working of the balancing organs in the inner ear gets upset. Particles floating in the aircraft do not settle down easily and may be harmful for health. To make up for the lack of gravity, regular exercise is essential in a spacecraft to keep the muscles in good condition. It has been observed that in the space astronauts tends to "grow’ taller. This is because the pads of cartilage between the bones of the spine are no longer under pressure from gravity, and they expand. This increase in height can be as much as 5 cm. However, the astronaut returns to his original heights when he returns to the earth’s atmosphere.

Under the conditions of weightlessness it is possible to conduct certain scientific experiments that are impossible on earth. Absolutely perfect crystals can be formed under such conditions. Alloys of very high homogeneity can be made under these conditions which are very difficult to make under the pull of gravity.

Scientists have been conducting experiments to find out whether a woman may become pregnant in weightlessness condition- and if so, what could be the effect of zero gravity on such children.