Newton's Laws of Motion

Newton's First Law (Inertia): A body at rest stays at rest, and a body in motion continues in uniform motion, unless acted on by a net external force.

Newton's Second Law: F = ma — the net force on a body equals its mass times acceleration. This is the workhorse of JEE mechanics.

Newton's Third Law: For every action, there is an equal and opposite reaction. The forces act on DIFFERENT bodies.

The concept of inertial frames is fundamental — Newton's laws hold only in non-accelerating reference frames.

Free Body Diagrams (FBDs) are the most important tool. Always isolate the body and draw ALL forces acting on it.

In connected body problems (Atwood machine, pulleys), use constraint equations: acceleration of connected bodies is related.

Pseudo forces appear in non-inertial frames. In a frame accelerating at 'a', a pseudo force = −ma acts on every body.

The normal force is NOT always equal to mg. It depends on the situation (inclines, lifts, circular motion).