The three laws stated by Isaac Newton that describe the relationship between the motion of a body and the forces acting on it, forming the foundation of classical mechanics.
- First law (law of inertia): a body continues at rest or in uniform straight-line motion unless acted upon by a net external force.
- Second law: rate of change of momentum is proportional to the applied force, giving force equals mass times acceleration (F = ma); the SI unit of force is the newton (N).
- Third law: for every action there is an equal and opposite reaction; the two forces act on different bodies.
- Momentum (mass times velocity) is conserved in the absence of an external force; this explains recoil of a gun and rocket propulsion.
- Everyday examples: a passenger lurching forward when a bus brakes (inertia), the kick of a fired rifle (third law), seat belts (inertia of motion).
Newton's laws are core physics facts that appear as definitions, units (the newton), and everyday-phenomena questions, and they underpin ballistics and recoil relevant to force training.
Inertia is a property (resistance to change in motion), not a force; action and reaction act on different bodies, so they never cancel each other.
Three laws: inertia, F = ma, and equal-opposite reaction; force is measured in newtons.