How to Apply Newton’s Laws

Follow this systematic procedure:

1. Identify the object(s) of interest—either a single particle, rigid body, or a defined system.
2. Choose an inertial reference frame (e.g., Earth’s surface for most introductory problems).
3. Identify all external forces acting on the object.
   → Internal forces (e.g., molecular forces within a block, tension between two parts of a chosen system) do not appear in the net force.
4. Draw a free-body diagram (FBD):
   • Represent the object as a dot or simple shape.
   • Draw all external force vectors acting on it, labeled clearly (e.g., $\vec{W}$, $\vec{N}$, $\vec{T}$, $\vec{f}_k$).
   • Ensure vectors originate from the object and reflect correct direction.
5. Apply Newton’s Second Law:
   • Use vector form: $\vec{F}_{\text{net}} = m\vec{a}$, or
   • Resolve into components (e.g., $x$ and $y$):
     $$ \sum F_x = m a_x, \quad \sum F_y = m a_y $$
   • Solve the resulting equations algebraically, respecting signs and coordinate conventions.