We have studied mainly contact forces, forces in which the two interacting objects are physically touching one another, up to this point. A book pushing down on a desk and the desk pushing back up are contact forces. Gravity and electric force can act over a distance. That means the interacting objects do not need to be physically touching one another for forces to be exerted. We know earth experiences a force from the sun and vice-versa even though they do not physically touch one another. This section will focus on non-contact or action-at a-distance forces.
Electrically charged particles, such as positively charged protons and negatively charged electrons, exert forces on one another. These forces are attractive, pulling the particles closer, for two oppositely charged particles. They are repulsive, pushing them apart, for two like charged particles. Just like the old saying goes, "opposites attract!"