the first of the three laws of motion and interaction formulated by Sir Isaac Newton. These laws are intended to describe how all moving bodies on the Earth react: (1) a body at rest remains at rest and a body in motion remains in motion unless acted on by an external force; (2) the motion of a body changes in proportion to the size of the force applied to it; (3) every action produces an equal but opposite reaction. See also Newton in this glossary.—AcademyLevel II Glossary