Earth and Space


Physics Narrative for 11-14 Supporting Physics Teaching

Feeling weightless

An interesting challenge for teaching in this area is prompted by those pictures of astronauts in spacecraft, orbiting the Earth, who float around inside the craft and appear to be weightless.

A common term for this experience is that the astronauts are in a zero-g environment. Yet this cannot be so as the strength of the gravitational field at that height of orbit is only about 12 % less than that at the Earth's surface! So why do the astronauts appear to be weightless?

This is because there is something we all experience which is the sensation of weight. This is not the same as the force of gravity. The sensation of weight is the feeling we get from the Earth pushing up on us stopping us from falling. It is the feeling that we take for granted as the floor pushes up on our feet.

If you removed the floor instantly, you would not feel this sensation, and instead, would feel weightless. Such an effect happens if you are unfortunate enough to be in a lift and the cable breaks or if you sky-dive out of an aeroplane.

In a lift

If the lift cable breaks, both you and the lift will drop towards the ground at the same rate and you will feel no push up from the floor, so you will experience weightlessness.

In the case of the orbiting spacecraft, both the astronaut and the spacecraft are accelerating towards the Earth at the same rate. The floor is falling away from the astronauts as they fall towards the Earth, so they feel weightless. See episode 02 to find out why orbiting involves falling.

This is exactly like being in the falling lift and there is no sensation of weight. So a push on the floor of the spacecraft sends you moving up towards the ceiling just as it would do in the lift. This state is often called microgravity or weightlessness as it appears as if there is no gravity.

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