Have you ever gone bungee jumping or cliff diving? Roller coaster? Drop tower ride where you are just free-falling for a while? Doesn’t it seem as if you are weightless for a while? Your weight is defined as long as you have something supporting you to realize this weight. You feel the experience of weightlessness when there is no supporting force acting on your body.
Zero gravity is another term referring to the phenomenon of weightlessness. Bodies in free fall and those in orbit experience this. For example, people aboard the International Space Station (one of the man-made satellites in orbit around the earth) all experience zero gravity when they are on board. They are still under the earth’s sphere of influence and therefore gravity.
Zero gravity is not the absence of gravity as it is commonly perceived but just the experience of feeling weightless. So what is it called when there is no gravity then? Actually, there is no such thing as the absence of gravity, even in space.
For example, we know that the earth orbits the sun because of the gravitational pull of the sun, the moon is held in its orbit around the earth, because of the earth’s gravity. And yet when astronauts are in space, they have experienced weightlessness and go on spacewalks and such. NASA refers to this phenomenon as “microgravity”. There is gravity everywhere, but its influence is so small that it can be regarded as negligible.
Still don’t get the difference between zero gravity and microgravity?
Let me put it this way. There’s a weighing scale at the edge of a diving board near a swimming pool. You go and stand on it. Let’s say it reads 50 kgs. Your friend comes along behind you and shoves you along with the weighing scale into the pool. For those few seconds when you’re screaming and falling, although technically you are still on the scale, the scale reads zero! Since you are in free fall, you and the scale are both experiencing weightlessness!