Astronauts orbiting the Earth experience sensations of weightlessness similar to that felt by anyone who has been temporarily suspended above their seat on any amusement park ride like the roller-coaster.
Before understanding weightlessness, you must be able to understand the difference between contact forces and non-contact forces. As an example, when you sit on a chair, you will experience two forces, that are the pull of gravity on your body downwards towards the Earth (non-contact force) and the force of the chair pushing you upwards also known as normal force (contact force). Bear in mind that contact forces can only result from the actual touching of the two interacting objects, which in this case is the chair and your rear end. The force of gravity (non-contact force), on the other hand, can never be felt. Therefore, without a contact force, there is no means of feeling the effect of non-contact force.
Weightlessness is simply a sensation experienced by an individual when there are no external objects touching one's body and exerting a push or pull on it. In other words, it occurs when all contact forces are removed like when you are momentarily (or perpetually) in a state of free fall where the only force acting on your body is the force of gravity (non-contact force).
Even when you are undergoing free fall while sitting on a chair, there will be no contact force exerted on you by the chair as it is falling at the same rate as your body. Similarly astronauts and objects in space experience weightlessness as they are constantly falling at the same rate and no contact forces are present.
Here is a video of a dog experiencing weightlessness momentarily as it travelled unbuckled in a plane. There is also a video of how water behaves under weightless conditions.
Click here to view flying dog video.
Click here to view suspended water video.
You may want to explore the internet further for information on what are the effects of weightlessness on human beings especially for astronauts who undergo long periods of weightlessness. You may also want to find out more about common misconceptions about weightlessness.
Adapted from http://www.physicsclassroom.com/Class/circles/U6L4d.html
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