Under what condition do the forces of attraction between particles become noticeable?

The force of attraction between particles, and indeed between any bodies, directly depends on the distance between them. The greater the distance between them, the less the force, and at large distances there is none at all. As for the particles, at distances much smaller than the size of the particles, this force is perceptible.
Answer: at distances much less than the size of the particles themselves.

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