# Newton's law of inertia a reference to the three laws of motion and interaction formulated by Sir Isaac Newton (1642-1727), English scientist and mathematician. These laws are intended to describe how all moving bodies on the earth react: a body at rest remains at rest and a body in motion remains in motion unless acted on by an external force; the motion of a body changes in proportion to the size of the force applied to it; every action produces an equal but opposite reaction.