Forces occur due to interactions between two or more objects. Whenever one object exerts a force on another, the second object simultaneously exerts an equal and opposite force on the first. This is known as Newton's Third Law of Motion, which states that forces always come in pairs—an action and a reaction
. These interactions can be of two main types:
- Contact forces , which happen when objects physically touch each other, such as friction, tension, normal force, and applied forces
- Action-at-a-distance forces , which occur even without physical contact, such as gravitational, electric, and magnetic forces
At a fundamental level, all forces arise from four fundamental interactions in nature: gravitational, electromagnetic, strong nuclear, and weak nuclear forces. For example, friction is a manifestation of electromagnetic forces acting between atoms of surfaces in contact
. In summary, forces occur because objects interact with each other, either through direct contact or at a distance, and these interactions always produce paired forces equal in magnitude and opposite in direction.
