We all might have experienced weightlessness without realizing it. Many amusement park rides especially the ones that operate vertically, roller coaster rides, jumping on a trampoline create brief experiences of weightlessness.
The term weightlessness is more relevant to the aeronautical industry. We generally think that there is no gravity in space. But it is not true. The earth’s gravitational pull is so strong that even at about 250 miles above the surface of earth, gravity is around 88% of that which exists at the surface. In fact, any orbiting spacecraft like space shuttle or space station is kept in the orbit around the earth by gravity.
Weightlessness is a condition in which an object appears to have no weight. It is more aptly termed 'microgravity'. The condition of microgravity occurs whenever an object is in a state of free fall. For example, one can experience microgravity inside a space shuttle. Whatever is not secured inside experiences microgravity; meaning everything seems to float. To move in any direction in microgravity, you must push against something so that it can push against you in the opposite direction. Furthermore, whatever you push on must be either anchored or more massive than you, such as the wall of the shuttle. For this reason, many restraints, hand-holds and foot-holds are put up throughout the cabin of the shuttle.
Microgravity causes various changes in the body, causing muscles and bones to weaken. People experiencing weightlessness have nausea, headaches, loss of appetite etc. Blood flow rushes towards the head as against gravity pulling blood to pool in the veins of legs on earth. To deal with fluid loss, weak muscles and bones, countermeasures are always undertaken inside the space shuttle to stay healthy especially on return to earth.