Forces are either contact or field forces. Contact forces act when objects are touching each other such as (a) the normal force that prevents two objects from occupying the same space, (b) the friction force that resists the sliding of objects on a surface, and (c) the tension exerted by a rope, wire, etc., on the objects attached to it. In contrast, field forces do not require physical contact. The four fundamental field forces are gravitational, electromagnetic, strong, and weak nuclear interactions. Gravitational force is the interaction between objects due to their masses. The electromagnetic force is the interaction between charged particles. The strong force acts between protons and neutrons of atoms and binds them together, while the weak force is responsible for the radioactive decay of atoms.