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 = 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