NCERT Solutions Class 11 Physics Chapter 4 Laws of Motion

The Class 11 Physics NCERT Solutions Chapter 4 Laws of Motion will help students prepare for Boards, JEE Main, JEE Advanced, NEET, CUET and NDA in 2026-27. Every back-exercise question is solved with full working, so students can check each free-body diagram, force balance, friction calculation, and momentum step one line at a time.

This chapter turns the motion ideas from earlier chapters into cause-and-effect physics, and its methods are used in almost every mechanics problem that follows.

• CBSE Weightage: 6 to 8 marks, usually one numerical on friction or connected bodies plus a short answer on Newton's laws.
• Questions solved: all Exercise 4.1 onward, covering Newton's laws, momentum, impulse, friction, and circular motion.
• Key formulas: F = ma, impulse = change in momentum, limiting friction f = μN, and the banking-of-roads speed relation.

Each solution in this Class 11 Physics NCERT Solutions Chapter 4 Laws of Motion compilation is curated by subject experts, based on the 2026-27 NCERT textbook, and refined against the last five years of CBSE Board, JEE Main and NEET papers.

Newton's Three Laws of Motion and the Concept of Inertia

The whole chapter rests on Newton's three laws. They connect the forces on a body to how its motion changes. The NCERT questions ask students to state each law and then use it in a numerical, so the solved answers do both.

• First law (law of inertia): a body stays at rest or in uniform motion unless a net external force acts on it. Inertia is this tendency to resist a change in motion, and it grows with mass.
• Second law: the net force equals the rate of change of momentum, which for constant mass gives F = ma. This law gives the number in every force numerical.
• Third law: every action has an equal and opposite reaction, and the two forces act on different bodies.

A common error is to add an action and its reaction as if they cancel. Action and reaction act on two different bodies, so they never cancel on the same object. Questions like Exercise 4.1 test the first law with everyday examples, and the solutions explain the reasoning a student can reproduce in the exam.

Linear Momentum, Impulse and Newton's Second Law

Linear momentum is the product of mass and velocity, written p = mv. It measures how hard it is to stop a moving body. Newton's second law is really a statement about momentum, and the NCERT solutions use this form whenever a force acts for a short time.

• Second law in full form: F = dp/dt, the force equals the rate of change of momentum.
• Impulse: the product of force and the time it acts, J = F Δt.
• Impulse-momentum theorem: impulse equals the change in momentum, F Δt = Δp.

This theorem explains why a longer contact time means a smaller force for the same change in momentum, which is the idea behind airbags and a cricketer pulling the hands back while catching. Numericals such as Exercise 4.10 give a force and a time and ask for the change in velocity. Write the impulse first, set it equal to the change in momentum, then solve for the unknown. The Class 11 Physics NCERT Solutions Chapter 4 Laws of Motion show this order on separate lines so no step is skipped.

Conservation of Momentum and Equilibrium of Forces

When no external force acts on a system, its total momentum does not change. This law of conservation of momentum follows directly from Newton's third law, and it solves every collision and recoil problem in the chapter without needing the force itself.

• Conservation rule: total momentum before equals total momentum after, m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂.
• Recoil of a gun: the forward momentum of the bullet equals the backward momentum of the gun, so the gun kicks back.
• Equilibrium of forces: when the net force is zero, the body is in equilibrium and its acceleration is zero.

For a body in equilibrium under three forces, the forces form a closed triangle, and this idea is tested in the concurrent-force problems. If three forces keep a body in equilibrium, each force is proportional to the sine of the angle between the other two. Questions like Exercise 4.15 ask students to find an unknown tension or angle, and the solved answers resolve the forces into components before applying the equilibrium condition.

Free-Body Diagrams and Friction: Static, Kinetic and Rolling

A free-body diagram shows a single body with every force on it drawn as an arrow. It is the first step in almost every numerical, and the NCERT solutions draw one before writing any equation. Once the diagram is clear, the force equations follow directly.

Static friction adjusts itself up to a maximum called limiting friction, while kinetic friction stays roughly constant. Rolling friction is much smaller than sliding friction, which is why wheels and ball bearings are used. Problems on an inclined plane, such as Exercise 4.20, ask for the acceleration when friction acts. The solved answers resolve the weight along and across the slope, then subtract friction from the driving force, exactly as CBSE expects.

Dynamics of Circular Motion and Banking of Roads

A body moving in a circle at steady speed still accelerates, because its direction keeps changing. This centripetal acceleration points to the centre and needs a real force to supply it. The NCERT solutions identify that force in each case before writing the equation.

• Centripetal force: the net inward force, F = mv²/r, supplied by tension, gravity, or friction depending on the situation.
• Car on a level road: friction alone provides the centripetal force, so the safe speed is limited by v = √(μrg).
• Banking of roads: tilting the road lets a part of the normal force point inward, so a car can turn even without friction.

For a frictionless banked road, the safe speed is v = √(rg tan θ), where θ is the banking angle. Banking lets a vehicle take a turn safely without depending only on friction. The NCERT Solutions for Class 11 Physics Chapter 4 Laws of Motion work the banking and level-road cases side by side, so students see exactly which force supplies the turn in each. This comparison is a favourite in JEE Main and NEET objective papers.

Exercise-wise Breakdown for Class 11 Physics Chapter 4 Laws of Motion

The NCERT back-exercise splits into clear groups. Use this map to plan which answers to practise first for the 2026-27 boards. Every group links to the full solved set.

Solving the groups in this order builds the skills in the same sequence the chapter teaches them. Start with Newton's laws, then momentum, then friction and circular motion, because each group uses the one before it.

Common Mistakes Students Make in the Laws of Motion Chapter

These slips happen while drawing the diagram or setting up the equation, not because the concept is unclear. Each one costs 1 to 3 marks in the CBSE paper, so the solved answers point them out at the exact step.

Student Feedback on the Laws of Motion Chapter

Practice Questions for Class 11 Physics Chapter 4 Laws of Motion

Once the solved answers are clear, test yourself on the full question set. The practice page has every NCERT question with a step-by-step Solution and an Expert Solution behind a click.

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