Chapter 4 — Laws of Motion

Chapter 4 of the Class 11 Physics NCERT textbook, "Laws of Motion", comprises Newton's three fundamental laws that govern how forces affect the motion of bodies: the law of inertia, the relationship between force and acceleration, and the principle that forces always occur in equal and opposite pairs.

• From inertia to Newton's laws — Starting from Galileo's insight about inertia, the chapter builds Newton's three laws into a single framework describing how forces change motion—when a body keeps its state, how force produces acceleration, and how forces always act in pairs.
• Momentum, impulse, and equilibrium — Force is tied to the rate of change of momentum, and impulse links force acting over time to a change in momentum. These ideas, together with equilibrium conditions, form the working core of mechanics.
• Everyday forces and applications — Common forces such as static and kinetic friction, along with applications to circular motion, show the laws at work. Momentum conservation for isolated systems ties the chapter together as a foundation for mechanics.

1. 01Newton's First Law: A body at rest or in uniform motion remains so unless acted upon by net external force; this is inertia, not the absence of forces
2. 02Newton's Second Law: F = dp/dt = ma expresses that force is proportional to rate of change of momentum and acts in the direction of acceleration
3. 03Newton's Third Law: Forces always occur in pairs between two bodies; action and reaction are simultaneous and equal-opposite but act on different bodies
4. 04Momentum p = mv is the product of mass and velocity; impulse equals force × time interval and equals change in momentum
5. 05Static friction opposes impending motion (fs ≤ μsN); kinetic friction opposes actual relative motion (fk = μkN), with μk < μs
6. 06Conservation of momentum: total momentum of an isolated system remains unchanged regardless of internal forces; collision example shows pb + pg = 0