Final answer:
The repulsive force between two magnets with their south poles facing each other would go from 18 N to 2 N if the distance between them is tripled, based on the inverse square law.
Step-by-step explanation:
When two magnets with the same poles facing each other, such as south to south or north to north, they exert a repulsive force on each other. In this case, two bar magnets with their south poles facing each other exerted an initial force of 18 N. According to the inverse square law, which applies to both gravitational and electrostatic forces, when the distance between the poles is tripled, the force exerted between them is reduced by a factor of 32, or 9. Therefore, the new force when the distance is tripled would be 18 N divided by 9, which equals 2 N.