Dear kushal,

You can''t think of any physics experiment that would demonstrate that gravity is related to rotational motion because it isn''t. If the Earth were not rotating relative to the stellar background, its gravitational field would be unchanged. Mind you, it IS true that the acceleration ``due to gravity'''' would be very slightly more than 9.8 m/s^2 anywhere but at the poles because there would no longer be any centrifugal force reducing the effect of gravity. But that isn''t what your students are saying anyway. (And we are ignoring any issues of general relativity here, as they are weensy corrections.)

You probably want a counter-argument to throw their way. Let''s see. You need first of all to convince them that the gravitational attraction between two bodies depends on the properties of both (e.g. the mass of both bodies appears in in Newton''s law of gravitation). Point out to them that the weight of any small body, which is the gravitational attraction between it and the Earth, varies with its size. Having two students step on the nearest scale will verify this. 

Now note that they have assumed that the gravitational attraction between a large body (the Earth) and a small body (a person or object on the Earth''s surface) depends on the spin of the LARGE body. (Because if the Earth were not spinning, there would be no such force, according to them.) 

And ask them why, in that case, the gravitational force between the small body and the large body does NOT depend on the spin of the SMALL body. (You can easily demonstrate that the time it takes a spinning object to fall is identical to the time it takes a nonspinning object to fall.)

You can also perform the thought experiment of considering two bodies near each other, one spinning planet, one nonspinning small object. Gradually you transfer mass from one to the other, keeping the angular momentum constant (which means the planet starts spinning faster and faster, incidentally, but perhaps irrelevantly). After a while the object will become as large as the planet was, and the planet as small as the object was. Is the gravitational attraction still due to the (very quickly) spinning ``micro''''planet left over? If so, then we should see that two spinning objects -- if necessary very quickly spinning objects -- should attract each other more strongly then they do when not spinning. And this is not, of course, observed. 

If their arguments are vaguely based on centrifugal forces, ask them why gravity does not vary near the poles, where you are going around the Sun once a year, but not being flung around in a circle every 24 hours.

I hazard that your students are suffering from a peculiarly American malady -- the tendency to confuse proximity to causation. The Earth does two interesting things as far as human beings are concerned: it spins, giving us day and night, and it exerts gravity, giving us up and down. (I suppose a case can be made for a third: it goes around the Sun tilted, which gives us summer and winter.) It is human, or perhaps more correctly American human nature, to assume that because the two phenomena occur on the same object one must cause the other. Gravity is the more mysterious fact, more in need of explanation, so it is assigned to be the effect, and spin assigned to be the cause.

The root cause is that American students, in particular, find it difficult to cope with the state of ``not knowing''''. They need to feel they know, even if they can''t possibly given the evidence available to them. They are happier feeling they know, and later finding themselves wrong, than in feeling ignorant, and later being enlightened. It''s an unfortunate tendency, which tends to hamper their scientific abilities, since a good scientist needs to cherish the feeling of ``not knowing'''' as long as possible. It is only in this mental silence, free of unjustifiable hypothesizing, that the truth, which always starts out as a tiny voice indeed, can be heard. 

Hence you might do them a favor if you explore with them first WHY they feel there is a connection between spin and gravity. If the answer is ``just because'''' or ``it seems sensible'''' or something equally data- and logic-free, then you can do them some good by discussing how very dangerous trusting that kind of unsupported evidence-free ``hunch'''' can be, not only scientifically, but in other areas. The tendency to judge too hastily and on too little actual data is a particularly unfortunate American vice. You can point out to them that the very best and most successful scientists and inventors are those who are the least inclined to think they know the answer before they are absolutely, positively, convinced-by-mountains- of-data sure they know. In science plausible hypotheses are a dime a dozen, but good, fact-supported theories are rare gems indeed. 

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