We often speak about astronauts being weightless and it
is important to understand exactly what we mean by this.
Since weight is a force,
weightlessness implies the absence of this force. But this will only be true for a point out in
space infinitely distant from any other body. The condition could also apply at a point where
two or more gravitational fields cancel each other out. We usually describe a feeling
of weightlessness when there is no reaction on a body from the floor.
When we
jump of a stool onto the ground we feel weightless for the fraction of a second that
we are falling - the same is true for a free fall parachutist but for much longer. In orbit
astronauts feel weightless because both they and the spacecraft are falling round the earth
and have the same centripetal acceleration.
Astronauts train in aircraft that are
diving along a parabolic path (See Figure 1). The plane and its occupants are effectively in
free fall and so there is no reaction between the astronaut and the floor of the plane. They
feel weightless.
