Warp wrote:
>   When I was in high-school I had a friend who didn't quite understand
> correctly the concept of forces, in particular the attracting force of
> gravity. (Simpler times were those, in high-school, where the world was
> purely Newtonian, but that's not important here...)

I miss those times too.  I still haven't managed to find an 
interpretation of QM that I'm philosophically satisfied with, and it 
makes it tricky to think about other philosophical problems when I'm 
still not solid on something that fundamental.

>   No he was not a stupid person. In fact he was quite clever. It just
> seemed that he didn't quite fully grasp the concept of forces and what
> it actually means. (His reasoning would have been correct if the mountains
> were in vacuum, but he seemed to seriously think that since mountains
> attract each other because of gravity, they also move towards each other,
> even though they are sitting firmly on the surface of the Earth.)
> 
>   It's difficult to explain. It made much sense in the situation.

Your friend's mistake actually makes sense to me from how you explain 
it.  I suppose he knew that F=m*a, and thus any force implies some 
acceleration no matter what the mass, so his reasoning was actually 
largely correct.  Really, it seems that his main misconception was not 
directly about what forces do, but more about failing to account for all 
the places they can come form (for instance, the resistive force due to 
the cohesion of the rocks), and that it's only the net force that matters.

Still, I can see how an intelligent person could make that mistake, 
since it's rather surprisingly close to correct.  That said, without 
hearing the reasoning I would have been absolutely baffled by the error.

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