Relier Pairs - newton's laws of motionVersion en ligne - newton's laws of motion par Lindsay Saravia 1 The principle of inertia. 2 Law of action and reaction. 3 Force equals mass times acceleration. Whenever one body exerts a force on another, this second exerts an opposite of equal intensity and direction but opposite sense on the first. This is the fundamental formula of dynamics, and it comes from assuming a body at rest on a horizontal surface, which is subject to a force parallel to that surface, being able to dispense with friction: we will see that the body starts moving at a constant acceleration. It indicates that when a body is at rest, or describes a movement with the characteristics of MRU, the forces applied to it have a null result.