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# What Is "Weightlessness"?

Media Type:
Video

Running Time: 1m 17s
Size: 1.8 MB

or

Source: ZOOM

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There's no need to don a space suit if you want to experience weightlessness. In this video segment adapted from ZOOM, two members of the cast drop a cup of water with holes in it to demonstrate how free fall can create a momentary condition of "weightlessness".

Background Essay

When we hear the word weightless, it's hard not to think of astronauts floating around in space. Surprisingly, however, an object does not need to be in outer space to experience the sensation of weightlessness. In fact, being in space, by itself, does not make a spacecraft or its occupants weightless.

The weight of an object is a function of its mass -- essentially how much stuff it's made of -- and how far it is from the center of a very large object, like the earth. All else being equal, the more mass an object has, the more it will weigh. Also, the closer an object is to the earth, the more it will weigh.

The space shuttle orbits at an altitude of about 320 kilometers (200 miles) above the earth's surface. At this distance, the spacecraft and the astronauts still weigh about 90 percent as much as they do on the ground. That is, the force of the earth's gravity acting on them is still 90 percent as strong as if they were on the earth's surface.

So, the astronauts experience weightlessness not because there is no gravity. Rather, weightlessness occurs because they and their spacecraft are free falling in gravity. Just as the ZOOM cast demonstrates in this activity, when two objects fall freely, one can float inside the other until they both reach the ground. The astronauts float inside the space shuttle just as the water floats and stays inside the perforated cup.

But what keeps the space shuttle from free falling to the earth like the cup of water? A spacecraft can maintain its free fall for a very long period of time by traveling fast enough -- about 7.5 kilometers (4.7 miles) per second -- horizontally, so that even though it is being pulled toward the center of the earth, its free-fall path is parallel to the earth's curvature. In other words, the spacecraft continually falls all the way around the earth.

Discussion Questions

• Explain how dropping a cup of water can demonstrate "weightlessness".
• In this activity, the water is not pushing against the open holes anymore. Do you think the water is pushing against the bottom of the cup? Suppose you placed a miniature scale in the cup. Would the water seem to have weight on that scale?

• Transcript

RACHEL: This phenom was sent in by David G. of Columbus, Ohio. Here's a cup with two holes in it. See how the water is pouring out of the holes?

KALEIGH: If we drop the cup what do you think will happen to the water coming out of the holes while the cup falls? Will the water come out of the holes faster? Will the water come out of the top of the cup? Or will the water stop pouring out of the cup altogether? Make a prediction. Okay, let's see if you were right.

RACHEL: Okay, here we go.

KALEIGH: Whoa!

RACHEL: Did you see how the water stopped pouring out of the holes? Was your prediction right? I'll explain. When I hold the cup the water pours out of the holes because of gravity. But when the water and the cup fall together, the water doesn't need to pour out of the holes to get down. It just falls with the cup. This is probably hard to see on TV, so try it at home.

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