First Law (Inertia)

A body continues in its state of rest or uniform motion unless acted upon by an external net force. Defines inertial reference frames.

Second Law (F = ma)

$F_{net}$ = $\frac{dp}{dt}$ = ma (for constant mass)

• F in Newtons (N) = kg*m/$s^2$
• Dimension: [$MLT^{-2}$]
• Vector equation: $F_x$ = $ma_x$, $F_y$ = $ma_y$ (resolve along axes)

Third Law (Action-Reaction)

$F_{A on B}$ = -$F_{B on A}$

• Same magnitude, opposite direction
• Same type of force (both gravitational, both contact, etc.)
• Act on DIFFERENT bodies (never cancel in FBD)
• Valid even when bodies are accelerating

Key Distinctions