Newton's Second Law

Newton's Second Law tells us how forces cause acceleration:

$$\sum \vec{F} = m \vec{a}$$

The net force $\sum \vec{F}$ is the vector sum of all forces acting on a system. It points in the same direction as the acceleration $\vec{a}$.

Choosing Your System: Before applying Newton's second law, you must decide what object (or group of objects) you're analyzing. This is your system. Draw a boundary around it.

Only external forces—forces from outside the boundary—go in your free-body diagram and contribute to $\sum \vec{F}$. Forces between parts inside the system are internal forces. They don't affect the system's overall acceleration because they cancel in pairs.

The FBD below shows two external forces acting on a system. The resultant $\sum \vec{F}$ is their vector sum.