Newton's Laws of Motion describe only the motion of a body as a whole and are valid only for motions relative to a reference frame. The following are brief modern formulations of Newton's three laws of motion:

First law

An object at rest tends to stay in rest and an object in motion tends to stay in motion in a straight line at constant speed unless acted upon by an external, unbalanced force.

Second law

The rate of change of momentum of a body is proportional to the resultant force acting on the body and is in the same direction.

Third law

To every action (force applied) there is an equal and opposite reaction (equal force applied in the opposite direction).

Another way of stating Newton's third law, an interaction between two objects, is that, if object A exerts a force on object B, object B will exert the same magnitude force on A, but in the opposite direction.

It is important to note that these three laws together with his law of gravitation provide a satisfactory basis for the explanation of motion of everyday macroscopic objects under everyday conditions. However, when applied to extremely high speeds or extremely small objects, Newton's laws break down; this was remedied by Albert Einstein's Special Theory of Relativity for high speeds and by quantum mechanics for small objects.