Newton's Laws Applied to Machines
First Law (Inertia): An object at rest stays at rest; a moving object continues at constant velocity — unless a net external force acts on it. More mass = more inertia = harder to start or stop.
Second Law: F = ma. Net force = mass × acceleration. Same force on a heavier object → less acceleration. Measured in Newtons (N).
Third Law: Every action has an equal and opposite reaction. A rocket expels gas backward; the reaction pushes the rocket forward.
Equilibrium: Net force = 0. A stationary bridge is in static equilibrium — gravity pulls down, supports push up, everything balances.