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I know there is something wrong in the way I think about physics generally, but I don't know what it is.
Suppose there is a cube with a hook on one face and a rope attached to that hook (see diagram). Now let's say there is some force $F$ pulling the cube from the face opposite the hook & rope. The hook will force the rope to accelerate in the direction of $F$. The rope in reaction would exert the force $-F$ on the cube through the hook. Now there are two forces acting on the cube: $F$ and $-F$, which would cancel, and the cube should be at rest. But what happens really is that the block is accelerating in the direction of $F$.
Now, where am I going wrong in this problem? Or in thinking about forces, in general?