Newton's three laws of motion describe how objects move and interact under forces. The first law (inertia) states that an object maintains uniform motion unless acted on by a net external force. The second law relates net force, mass, and acceleration (F = m·a). The third law states that for every action there is an equal and opposite reaction.
First law — Inertia
In the absence of net external force, velocity remains constant. This explains why seatbelts matter.
Second law — Dynamics
Acceleration is proportional to net force and inversely proportional to mass. Double the force, double the acceleration.
Third law — Action/Reaction
Forces occur in pairs on different bodies. Rockets accelerate because exhaust gases push back with equal magnitude.