Newton’s first law states that an object maintains constant velocity unless acted upon by a net external force. This defines inertial frames: reference frames in which the law holds.

Newton’s second law is $\sum \vec{F} = m\vec{a}$. The net force is the vector sum of all external forces. If the net force is zero, acceleration is zero.

For every force exerted by object A on object B, object B exerts an equal and opposite force on object A. These two forces act on different bodies, so they do not cancel on a single free-body diagram.

A free-body diagram isolates one object and draws only the external forces acting on it. Common forces include weight $mg$, normal force, tension, friction, spring force, and drag. Correct diagrams are the foundation of almost every mechanics problem.