g A model for how repulsive forces between two particles can be mediated by a third particle would be

Two people in two canoes, throwing a ball back and forth.

Explanation:

The complete question is shown in the image below.

For the two particle to be said to exert a repulsive force on one another, they must be moving away from themselves. In the case of the two people in two canoes, the throwing of the ball from one person to the other will exert a backward recoil on the boat of one away from the other (due to conservation of momentum), further increasing their distance apart. This distance will keep increasing if they continue to throw the ball back and forth between themselves. But nonetheless, the two persons (considered to be the particles in this case) still have a mediating particle (the ball) between them, which is the mode of contact and force transmission between them.