Final answer:
Astronauts train in water tanks to simulate reduced weight due to buoyancy, which lowers apparent weight but not mass. They also train in aircraft that create short-term weightlessness through free-fall. The 'weightlessness' experienced is due to continuous free-fall in orbit, providing a sensation of microgravity.
Step-by-step explanation:
Astronauts sometimes train in water tanks because the buoyancy of water exerts a force that opposes the pull of Earth's gravity. In the tank, the apparent weight of the astronauts and their equipment decreases, but the mss which affects how hard it is to move them, stays the same.
To simulate the apparent weightlessness of space orbit, astronauts are trained in the hold of a cargo aircraft that is accelerating downward at g. They will appear to be weightless, as measured by standing on a bathroom scale, in this accelerated frame of reference because both they and the scale are in free-fall experiencing the same acceleration due to gravity, thus no force registers on the scale. There is indeed a difference between their apparent weightlessness in orbit and in the aircraft: in orbit, the condition is constant whereas in the aircraft it is for short durations during parabolic flight maneuvers.
On the International Space Station, there is plenty of gravity; the astronauts and objects onboard are in a kind of free-fall, too, and feel nearly weightless. This feeling is due to being in a constant state of free-fall while orbiting Earth, giving the experience of microgravity, not the absence of gravity.