HERE'S HOW:

How would they start? We know that whatever the force is, it must balance the gravitational force of the Sun pulling on her. And that force we know a lot about. It depends on the mass of the Sun (which we know), the mass of the woman, the distance between them, and a well known number, called the gravitational constant, G. We must also be pretty sure that the gravitational force is the only one to consider or at least the only important one. [We might ask: Is electricity or magnetism important? We will assume not in this case.]

Now we pretend we can do what Sherlock Holmes does. That is, we make guesses based on some good experience and past experiments. We visit a biologist friend and ask, "What might the density of this green woman be?" The biologist (Ms. B.) would say, "She must be green for a reason. She's probably made out of plant life, so her density is about 0.9 that of water."

Now the physicist can use the density of the extraterrestrial woman to estimate her mass. How? We must know her size. We see that her brother is standing right on the Sun (and we know its size), so we can use that fact to estimate his size. Then we use that to compare to her size. [PAUSE: How can we do that, if we don't know if they are the same size? That's a little bit of a problem, but it's not a big problem, since we are only estimating. We say that men are usually bigger than women, even if not smarter. (Which one is likely to be smarter here? Keep in mind that he's standing on the Sun, and she apparently knows not to!)] We assume that she is about 75% of his size. So from the Sun's well-known size, we know his size by comparison, then we estimate her size from his (75%). And from her size and estimated density, we get her mass. All that just to get her mass, and it's only an ESTIMATE of her mass! Science is usually a lot of fun, but it often takes a lot of work. Anyway, we're almost there. [PAUSE: Think about why we didn't just estimate her size based on how big she appears to us. Remember we know how far the Sun is from us, but do we really know how far away she is from us?] Since we have a fairly good idea of her size, we make a direct comparison of the brother's and sister's sizes as they appear to us, to be sure they are at the same distance from us. If not, we adjust to what size she would appear to be if they were at the same distance, the Sun's distance. We use this and how far she is observed to be OFF the Sun's surface to estimate how far she really is from the Sun's center. Now we have all that we need to find the gravitational force on her, which points from her to the Sun's center. So the force which she must be using to keep from falling into the Sun, must be of that same magnitude but in the opposite direction. That's it!

Back to Activities page . Or use your "BACK" key to return to the question page.