We know that astronauts in the ISS feel "weightless" because they are in a perpetual free fall. The earth's gravity is the only force acting upon them, and it is accelerating them towards the earth's center at the same rate as the ISS.
However, now imagine a crew in a spaceship very far from any planet, star or massive object. So much so that there isn't any gravitational force acting upon them. The spaceship is also traveling at a constant velocity, so the crew has a zero acceleration.
In the two situations, the astronauts feel "weightless" and are "floating" in the same way. However the two situations are very much different in terms of forces and acceleration. How can we explain that?
Best Answer
The feeling of weight is just the feeling of "something" pushing on you. For example, stand in an elevator accelerating upwards, and you will feel heavier. Stand in an elevator accelerating downwards, and you will feel lighter.
In both scenarios you describe it is the case that there is nothing pushing on you to cause your acceleration. On the ISS you are at rest relative to the ISS, so nothing is pushing on you. If you were moving at a constant velocity with no forces acting on you, the same thing applies.
Note that this is related to the equivalence principle, which you might be interested in reading about.