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In the real world, action-reaction pair always make an angle of exactly at 180ᵒ. But what would happen if instead it was something like, say 179.9ᵒ. What would be the weird effects?

I started off by thinking about a block lying on a surface. There is a force mg acting on it in downward direction. So the block exerts a normal force on the surface in downward direction. But now the reaction normal force by the surface on the block would make an angle with vertical.

The thing is that the forces on the block are not balanced (there is still a net force). Its pointing somewhere downwards. Now exactly what will happen? Horizontal force make it move sideways is okay but vertical forces not being balanced will result in what?

Free Body Diagram of the situation

Also as the block start sliding, the friction also do weird things. And things happening between the individual atoms of the block are just on a whole different level.

Can anyone give me a big picture (and some details if possible) of what that world behave like. This thought is not letting me sleep.

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The lack of symmetry if the reaction was not in the opposite direction to the action suggests this could not be a physical law (how would you choose the plane in which the action and reaction lie ?).

However, if it were a physical law then internal forces within objects would not cancel out - we can only ignore internal forces because they form action/reaction pairs which have no net effect on the object as a whole. So all objects would move about randomly as a result of un-matched internal forces between their constituent atoms or molecules.

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