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I was explaining the air motion in the atmosphere, so I introduced Gravity, Pressure Gradient and Coriolis Forces. Then one of the Professors asked me this question, which I couldn't answer. He said that, as there are only four fundamental forces, each of the forces I mentioned must belong to which subset of these 4 fundamental forces?

So, in classical mechanics, which kind of macroscopic forces (pressure, contact/reaction forces, etc.) are gravitational? And what are the forces that can be experienced in the Earth gravity (because of the Earth gravity)?

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    $\begingroup$ What do you mean by "other forces"? Gravity is gravity, what kind of subdivision do you think we ought to make here? $\endgroup$ – ACuriousMind Jul 5 '15 at 14:09
  • $\begingroup$ Thanks for reply. I has been asked this question and I don't know the answer so only I asked it here. Sorry if the question don't seems relevant to you. $\endgroup$ – Gracy Jul 5 '15 at 14:13
  • $\begingroup$ For #1: there are no subdivisions. For #2: are you looking for Newtonian reaction forces? $\endgroup$ – Kyle Kanos Jul 5 '15 at 14:30
  • $\begingroup$ It's not that it's not relevant, it's that it's not clear. Like ACuriousMind said, gravity is gravity. $\endgroup$ – Javier Jul 5 '15 at 14:30
  • $\begingroup$ Actually I was explaining the air motion in the atmosphere. So I was introducing the Gravity, Pressure Gradient and Coriolis Force. Then one of the Professor has asked me this question, which I don't know so though of asking here. He also said that as there are only four fundamental forces so each of the force must belong to some subset of these 4 fundamental forces. $\endgroup$ – Gracy Jul 5 '15 at 14:34
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As said in the comments, there are only 4 known fundamental forces, gravity, strong and weak interaction, and electromagnetism. Since we are restricted to low energy classical mechanics, we can ignore two of them, and we are left only with gravity and electromagnetic forces (which includes, among others, magnetostatic and electrostatic forces). Moreover, in classical mechanics it makes sense to talk about inertial forces, which are apparent forces which appear in non-inertial frames. An example of inertial force is the Coriolis force. In general relativity these inertial forces are described as an effect of the space-time curvature.

In the end, in classical mechanics (non-relativistic, non-quantum), we are left on three kind of forces: gravity, electromagnetism, and inertial forces. All kind of forces in classical mechanics can be classified in these 3 categories, but in some cases this is not straightforward. For example: friction, tension, contact forces, capillary forces, pressure, and in general hydrostatic forces originate at a microscopic level from electrostatic forces. In some cases this classification is a little bit ambiguous. Atmospheric (and oceanic) pressure, for example, is proportional to and caused by the gravitational field of the Earth. However the pressure felt by an object in the atmosphere originates from the many microscopic collisions of atoms of the atmosphere on the object surface. These microscopic collisions are due to the electrostatic repulsion of the electrons in atoms. Another example is buoyancy, where the net force is a combination of gravity and pressure.

To make a long story short, gravity is gravity, that is, the only force in classical mechanics which originates solely from the Earth gravitational field is the weight of objects. Some hydrostatic forces (like atmospheric and oceanic pressure) are also gravitational in some sense, although one should have in mind that their microscopical description relies on the electrostatic repulsion between atoms. As a rule of thumb, any time a force in classical mechanics is not clearly gravitational or electromagnetic, there is a microscopic electrostatic origin.

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    $\begingroup$ To further nitpick, atmospheric (and oceanic water) pressure is the only form of hydrostatic pressure that is in any way related to gravity; for example, it is possible to have a pressurized system such as a balloon even ignoring gravity. $\endgroup$ – Asher Jul 5 '15 at 23:03
  • $\begingroup$ @sintetico thanks a lot for much clear detail answer. $\endgroup$ – Gracy Jul 6 '15 at 4:09
  • $\begingroup$ @Asher thank you for your comment and clarification. I think that all hydrostatic forces are a form of contact force. I update my answer following your suggestion. $\endgroup$ – sintetico Jul 6 '15 at 7:31
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    $\begingroup$ I would say that tidal forces also originate from gravity, namely its gradient. $\endgroup$ – fibonatic Jul 6 '15 at 13:04

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