NCERT Notes Class 11 Physics Chapter 4 Laws of Motion

These Class 11 Physics Notes Chapter 4 Laws of Motion pull together every law, formula and free-body method that the Boards, JEE Main, JEE Advanced, NEET, CUET and NDA papers actually test in 2026-27. Use them to revise Newton's laws, momentum, friction and circular motion fast, with definitions and diagrams in one place.

This chapter turns the motion graphs of the earlier chapters into cause and effect, so force and acceleration finally connect.

• CBSE Weightage: 6 to 8 marks, usually one short answer plus one numerical on friction or connected bodies.
• Topics covered: Newton's three laws, inertia, momentum and impulse, conservation of momentum, friction, and circular motion with banking.
• Key formulas: F = ma, impulse-momentum theorem, and the equations for friction and safe speed on a bend.

These Class 11 Physics Notes Chapter 4 Laws of Motion are curated by subject experts, based on the 2026-27 NCERT textbook, and checked against the last five years of CBSE Board, JEE Main and NEET papers.

Topic-by-Topic Summary of Laws of Motion

The chapter explains why objects speed up, slow down or turn. It starts with the idea of force, states Newton's three laws, then applies them to momentum, friction and motion on a curved path. Here is the quick map of what each topic gives you.

• Newton's laws: the first law defines inertia, the second law gives F = ma, and the third law pairs every force with a reaction.
• Momentum and impulse: linear momentum p = mv, and the impulse that changes it during a collision.
• Conservation of momentum: total momentum stays fixed when no outside force acts.
• Friction: static, kinetic and rolling friction, and how each one opposes motion.
• Circular motion: the centripetal force needed to turn, and the banking of roads.

Revise the topics in this order, because each one uses the one before it. Master the second law first, and every numerical in the chapter opens up. These Class 11 Physics Notes Chapter 4 Laws of Motion follow the same sequence as the NCERT textbook.

Newton's Three Laws of Motion and Inertia

The three laws are the heart of the chapter. Together they tell you what a force does and why an object keeps its state of motion until a force changes it. Learn each statement in one clean line, because Boards ask for the exact wording.

• First law: a body stays at rest or in uniform motion unless a net external force acts on it. This property is called inertia.
• Second law: the rate of change of momentum equals the applied force, which gives F = ma for constant mass.
• Third law: for every action there is an equal and opposite reaction, and the two act on different bodies.

Inertia comes in three forms: inertia of rest, inertia of motion, and inertia of direction. A heavier body has more inertia, so mass is the measure of inertia. Action and reaction always act on two different objects, so they never cancel. This single point clears up most third-law confusion in the exam.

Linear Momentum, Impulse and the Impulse-Momentum Theorem

Momentum links mass and velocity into one quantity that the second law is really about. Linear momentum is p = mv, and it points in the direction of the velocity. When a force acts for a short time, it delivers an impulse that changes this momentum.

• Linear momentum: p = mv, a vector with SI unit kg m s^-1.
• Impulse: J = F Δt, the product of force and the time it acts.
• Impulse-momentum theorem: impulse equals the change in momentum, F Δt = Δp.

A large force over a tiny time can give the same impulse as a small force over a long time. This is why a cricketer pulls his hands back while catching a ball, spreading the same momentum change over more time to cut the force. Impulse is measured in newton second, the same unit as momentum. Remember this equality, since it is a favourite one-mark check in the paper.

Conservation of Linear Momentum

When no net external force acts on a system, its total linear momentum does not change. This follows straight from the second and third laws, and it is the tool for every collision and recoil numerical in the chapter.

• Statement: if F_ext = 0, then total momentum before equals total momentum after.
• Recoil of a gun: the forward momentum of the bullet equals the backward momentum of the gun.
• Collisions: the momentum of the two bodies together stays the same, even when kinetic energy is lost.

Write the momentum equation with correct signs for direction, since momentum is a vector. Momentum is conserved in every collision, but kinetic energy is conserved only in an elastic one. Keeping these two ideas separate stops the most common slip in collision problems. These Class 11 Physics Notes Chapter 4 Laws of Motion show the sign convention step by step.

Free-Body Diagrams and Equilibrium of Forces

A free-body diagram shows a single object with every force acting on it drawn as an arrow. It is the first step in any force numerical, because it turns a wordy problem into a clean set of equations. Draw it before you write a single line of maths.

• Isolate one body and mark weight, normal reaction, tension and friction on it.
• Choose axes, usually along and across the surface or the incline.
• Resolve each slanted force into components along those axes.
• Apply F = ma along each axis to get the equations.

Equilibrium means the net force is zero, so the body is at rest or moving with constant velocity. For a body under three forces in equilibrium, Lami's theorem relates each force to the sine of the angle between the other two. A body in equilibrium has zero acceleration, not always zero velocity. This distinction earns marks in the reasoning questions.

Friction: Static, Kinetic and Rolling

Friction is the force that opposes relative motion between two surfaces in contact. The chapter splits it into three kinds, and each one is tested in both Boards and entrance papers. Getting the limiting case right is where most marks are won or lost.

Static friction is self-adjusting, so it grows to match the applied force until it reaches its limiting value. Once the body moves, kinetic friction takes over and is slightly less than limiting friction. Rolling friction is the smallest, which is why wheels replaced dragging. The angle of friction and the angle of repose both come from these same coefficients.

Dynamics of Circular Motion and Banking of Roads

To move in a circle, a body needs a force pulling it toward the centre. This centripetal force is not a new force; it is provided by tension, gravity, friction or the normal reaction. The chapter applies this to a car on a bend and to the banking of roads.

• Centripetal force: F = mv²/r, always directed toward the centre.
• Level road: friction alone supplies the turning force, so safe speed is v = √(μ_s rg).
• Banked road: tilting the road lets the normal reaction help, raising the safe speed.

On a banked road, the maximum safe speed is v = √(rg (tanθ + μ_s) / (1 − μ_s tanθ)). Banking lets a vehicle turn safely even if the road is icy and friction is zero. Remember that the centripetal force points to the centre, while the outward centrifugal feel is only felt in a rotating frame, not a real force.

All Formulas for Laws of Motion

Every formula you need for the chapter sits in one table below, with its meaning and its SI unit. Learn the second-law and friction rows first, since those carry the most marks in both Boards and entrance papers.

Carry the SI unit on every line of your working. Losing the unit is a silent way to drop the final mark even when the number is right. Keep this table open while you solve the back-exercise numericals.

Key Definitions and Derivations for Laws of Motion

Boards short-answer questions often ask for a clean definition in one or two lines. Learn these word-for-word, because a vague definition loses easy marks. Each one also sets up a derivation you can be asked to show.

A common derivation asks you to obtain F = ma from the second law by writing force as the rate of change of momentum. Start from F = dp/dt and keep mass constant to reach F = ma. The banking derivation and the connected-bodies derivation are the other two that appear most often.

Common Mistakes Students Make in Laws of Motion

These slips happen while drawing or calculating, not because the concept is unclear. Each one costs 1 to 3 marks in the paper, so watch for them at the exact step.

Laws of Motion Weightage in CBSE Boards, JEE and NEET

This chapter is a scoring one and shows up every year. It usually carries a short answer plus a numerical on friction, connected bodies or circular motion. Here is how the marks split across the main exams for 2026-27.

Friction and connected-body numericals are the single most tested ideas from this chapter across all four exams. Master the free-body method first, then friction, then circular motion, in that order of return on effort.

How to Revise Laws of Motion Quickly

Use these Class 11 Physics Notes Chapter 4 Laws of Motion for a fast, ordered recap the night before a test. The checklist below takes about 30 minutes and hits every marks-heavy idea.

• First 10 minutes: write the three laws in exact words and the formulas for momentum, impulse and centripetal force.
• Next 10 minutes: draw the free-body diagram for a block on an incline and one for a two-block pulley.
• Last 10 minutes: redo one friction numerical and one banking-of-roads numerical from the back exercise.

Close the loop by stating why action and reaction do not cancel. If you can do all three blocks without notes, the chapter is exam-ready. Keep the All Formulas table beside you for the first pass only, then try it closed-book.

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