The astronauts are weightless while in orbit around the earth. But, intrestingly, this is precisely because there IS gravity. When an object is in orbit it is actually falling. But as with any thrown object as it falls vertically, it also travels horizontally. When you throw a rock, for example, it travels horizontally because you threw it and it travels vertically because gravity is acting upon it.
What would happen if you could throw a rock so hard horizontally that as it fell vertically the curve of its arc exactly matched the curve of the earth? If that happened the object would never hit the ground would it? That's orbit!
So, even though the astronauts are weightless, there is gravity. Still, there is a problem.
They are 'floating' around the spaceship because, well, they are not actually floating. They are falling! They are falling just like your rock. But, not only are they falling, their spaceship is also falling. Since everything is falling together it looks like they are floating.
Imagine yourself in a falling elevator. If the elevator were falling with you in it and you were all falling at exactly the same rate it would look like you were floating in the elevator. Have you ever been on the ride at Six Flags where you sit in the chair and you are dropped. People on this ride often place a penny in front of there face just as they are released to fall. Since the penny and the rider fall at the same rate, it looks like the penny is floating!
So, the problem is, if everything is falling together, how can food 'fall' through the digestive system?
The simple answer is that it dosen't! It dosen't in space and it dosen't on earth either. The food in your mouth is pushed to the stomach by muscles in the lining of the tube between mouth and your stomach. Then, in your stomach the digested food is pushed again into your intestines. While in the intestines it is pushed all the way to the, well, to the end.
As a simple experiment, try drinking water while standing on your head. You will be amazed that you can indeed swallow the water. You could even eat a full meal on your head and you would have no problem getting your food where it belongs. You might want to get on your feet, though, before it all gets to. . . . the end!
Answered by: Tom Young, B.A., Science Teacher, Whitehouse High School, Texas