Final answer:
In Coulomb's law, the role of mass from Newton's law of universal gravitation is played by electric charge. Unlike the gravitational force, which is always attractive, electrostatic forces can be both attractive and repulsive.
Step-by-step explanation:
In Newton's law of universal gravitation, the force between two masses is proportional to the product of their masses. This law posits that every mass attracts every other mass with a force that is inversely proportional to the square of the distance between their centers of mass and directly proportional to the product of the two masses.
In contrast, Coulomb's law, which deals with electrostatic forces, states that the force between two charged objects is directly proportional to the product of their electric charges and inversely proportional to the square of the distance between them. The equivalent of mass in Coulomb's law is electric charge.
This fundamental difference explains why gravitational forces are always attractive, while electrostatic forces can be either attractive or repulsive depending on the nature of the charges involved - like charges repel and opposite charges attract.