Weightlessness

Weightlessness is the experience (by people and objects) during freefall, of having no apparent weight. This condition is also known as microgravity (see below).

Overview

What humans experience as weight is not actually the force due to gravity (even though that is the technical definition of weight). What we feel as weight is actually the normal reaction force of the ground (or whatever surface we are in contact with) pushing upwards against us to counteract the force due to gravity, that is the apparent weight.

Related Topics:
Force - Reaction force - Apparent weight

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For example, a wood block in a container in free-fall experiences weightlessness. This is because there is no reaction to the wood block's weight from the container, as it is being pulled down with the same acceleration. The acceleration of the container equals the acceleration of the block, which equals the acceleration caused by gravity. When the container is at rest on the ground, however, the force on each piece of the block is not uniform. Because the block is not accelerating, there is also a force upward that arises because the block is a solid. Each horizontal cross section of the block experiences not only the force due to gravity on it, but also the weight of whatever portion of the block is above it. Part of feeling weight, then, is actually experiencing a pressure gradient within one's own body.

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There is another aspect of the feeling of weight that a pressure gradient does not account for, an example of which is the way that our arms are pulled downward with respect to our body. This effect comes from the fact that something hanging is not supported directly via a pressure from the ground. In fact the effect is almost the exact opposite of a pressure gradient, it is a tension gradient. It occurs because each cross section of a hanging object, a rope for instance, must support the weight of every piece below it.

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Hence, in short, weightlessness has nothing to do with whether we are under the influence of a gravitational force, but has to do with whether there are force gradients across our body. In free-fall, all parts of an object accelerate uniformly (assuming that there are no tidal forces, which is true to a very good approximation for human-scale objects in earth orbit)), and thus a human would experience no weight.

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