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There are four fundamental forces, sometimes called fundamental interactions. The forces are called fundamental because there is no simpler way for physicists to understand what the forces do or how they do it (their action). They are called interactions because the action of one object on another is matched by a reaction from the other.

We feel the effects of gravity and electromagnetism all the time.

• The gravitational force is described by Einstein's general theory of relativity and is understood to be due to the curvature of spacetime by the mass of matter.
• The electromagnetic force is due to electric charge. Charge causes electric force and movement of charge causes magnetic force.

The strong and weak interactions are forces at the smallest distances and explain nuclear interactions.

• The strong force binds protons and neutrons together and also keeps the nuclei of atoms together.
• The weak force causes beta decay.