First law: inertia
An object stays at rest or in uniform motion unless an external force acts on it. That is why passengers lean forward when a bus brakes suddenly — their bodies keep moving while the bus slows down.
In MCQs, look for keywords like ‘no net force’, ‘constant velocity’, and ‘remains at rest’.
Second law: F = ma
Force equals mass times acceleration. Doubling the force doubles acceleration if mass is constant. Doubling mass halves acceleration for the same force.
Always check units: force in newtons, mass in kilograms, acceleration in m/s².
Third law: action and reaction
Forces come in pairs. When you push a wall, the wall pushes back with equal magnitude. Rockets accelerate because expelled gas pushes the rocket forward.
Remember: action and reaction act on different bodies, so they never cancel each other on the same object.