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Simply because the density of the air decreases with altitude, and so there are fewer molecules, and therefore fewer collisions. You seem to be talking about the molecular collision theory here, which has nothing to do with the "air has weight" dilemma I have to explain how the molecules up in the air can "communicate" their weights to the Earth's surface. I think I've already covered this in my other responses.

I have tried to show that this "air has weight" theory for atmospheric pressure is wrong and should be deleted from wherever it appears.

Sorry, but what is this non-existent theory, and please could you refer me to the standard physics explanation? I agree that atmospheric pressure is caused by molecular collisions with the Earth's surface. But I find it difficult to understand how the weight of molecules throughout the height of the air column can affect the surface. What is the mechanism? If an air molecule "sits" on the surface, OK its weight is loaded on the surface and so may contribute to the pressure, but most of the other air molecules are still up there in the air, not contacting the surface. Their weights do not add up together in the air! How can they? I cannot see the mechanism. Further, if you refer back to my earlier post dealing with a simple thought experiment, I think you might then be convinced that the only viable explanation for atmospheric pressure is the molecular collision one.

Here you are involving the "molecular collision" ideas and not the simple "air has weight" theory, which is what the thought experiment is about.

According to the "air has weight" theory, the weight of molecules below the inclined plane do not act upon the plane, and so they are not relevant to this thought experiment.

Many thanks. That is in line with my thinking

There seems to be a fairly general acceptance of the idea that atmospheric pressure is caused by the weight of the molecules in the atmosphere. This idea is supported even by Wikipedia. I am not convinced that this is true. I think that atmospheric pressure is caused by "air" molecules colliding with the Earth's surface. This is not a new idea, of course, but it may not be widely held. If true, however, surely the "air has weight" explanation should be put to rest, as in Wiki and other websites. If I am wrong in believing the "molecular collisions" explanation, I should welcome comments that would put me right. However, in order to support my rejection of the "air has weight" theory, please consider the following thought experiment. According to this idea, all the air molecules are subjected to the force of gravity, and so the weight forces of the molecules act vertically downwards towards the centre of the Earth. So far, so good. Now, suppose a smooth, plane surface is placed on the Earth's surface, but at an angle to the surface. This plane will intercept the downward weight forces of the molecules, and so will cause a sideways force as well a a reduced vertically downwards force, by a simple resolution of forces. In the absence of friction, the inclined plane will then move sideways, and, hey, we've got a way of getting energy from the atmosphere!! I wish. This also begs another question about the "air has weight theory, but maybe better to leave this for another time.