What is the definition of internal force?

By definition, internal force is whatever causes an object with mass to accelerate or move. This force is equal to the product of the mass of the object plus its rate of acceleration.

The first studies of force date back to the third century B.C. and the philosopher Archimedes. Some of his ideas are still found in modern mechanics and physics today. Years later, Aristotle's original findings about force were corrected by the discoveries of Isaac Newton. Isaac Newton's findings depict four fundamental kinds of force: strong, electromagnetic, weak and gravitational. His descriptions have remained largely unchallenged to this day.