NCERT Solutions for Class 11 Physics Chapter 5 – Laws of Motion
Newton's three laws, friction, free body diagrams, impulse, and free PDF for Chapter 5 Laws of Motion.
About Chapter 5 – Laws of Motion
Chapter 5, Laws of Motion, is the heart of classical mechanics and arguably the most important chapter in Class 11 Physics. It introduces Newton's three fundamental laws of motion, which were first published in his landmark work Principia Mathematica in 1687 and have since shaped our understanding of the physical universe for over 300 years. These laws connect force, mass, and motion in a precise mathematical framework that applies to everything from a falling apple to a rocket launching into space.
The First Law of Motion (Law of Inertia) states that every object continues in its state of rest or uniform motion unless acted upon by an external force. This introduces the important concept of inertia – the resistance of an object to changes in its state of motion. The Second Law defines force quantitatively: F = ma, linking the net force on an object to its mass and the resulting acceleration. It also generalises to F = dp/dt (rate of change of momentum), which is more broadly applicable. The Third Law states that every action has an equal and opposite reaction.
The chapter also deals extensively with friction (static and kinetic), free body diagrams, and the analysis of systems of connected bodies. Students learn to solve problems involving ropes, pulleys, inclined planes, and circular motion. For CBSE board exams, this chapter carries significant weightage. JEE and NEET aspirants must be very thorough with free body diagram techniques, pseudo forces in non-inertial frames, and friction laws, as problems from this chapter are regularly featured in competitive examinations.
NCERT Solutions for Class 11 Physics Chapter 5 – Free PDF Download
NCERT Solutions – Chapter 5: Laws of Motion (All Exercises)
Download the complete solved PDF including free body diagram problems, friction numericals, and connected body systems.
Important Formulas – Chapter 5: Laws of Motion
| Concept / Law | Formula | Description |
|---|---|---|
| Newton's Second Law | F⃗ = ma⃗ (or F = dp/dt) | Net force = mass × acceleration; rate of change of momentum |
| Linear Momentum | p⃗ = mv⃗ | Product of mass and velocity; vector quantity |
| Impulse | J = F·Δt = Δp | Change in momentum; area under F-t graph |
| Law of Conservation of Momentum | Σp⃗ = constant (if F_ext = 0) | Total momentum of isolated system is conserved |
| Newton's Third Law | F_AB = −F_BA | Forces are equal, opposite, and act on different bodies |
| Weight | W = mg | Gravitational force on a body; g = 9.8 m/s² |
| Normal Force (horizontal surface) | N = mg | Reaction of surface on body; perpendicular to surface |
| Static Friction (max) | f_s = μ_s N | Maximum static friction before object starts to slide |
| Kinetic Friction | f_k = μ_k N | Friction during motion; μ_k < μ_s always |
| Acceleration on incline (no friction) | a = g sinθ | Component of gravity along the slope |
| Acceleration on incline (with friction) | a = g(sinθ − μ_k cosθ) | Net force along slope reduced by friction |
| Pseudo Force (non-inertial frame) | F_pseudo = −ma₀ | Fictitious force in accelerating reference frame |
Subtopics of Chapter 5 – Laws of Motion
5.1 Aristotle's Fallacy and Galileo's Insight
Aristotle wrongly claimed force is needed to keep objects moving. Galileo showed inertia: objects continue moving without external force.
5.2 Newton's First Law – Law of Inertia
A body remains at rest or in uniform motion unless a net external force acts on it. Defines inertial frames of reference.
5.3 Newton's Second Law
F = ma (for constant mass). More general form: F = dp/dt. Allows calculation of force, acceleration, or mass when two are known.
5.4 Newton's Third Law
For every action there is an equal and opposite reaction. Action-reaction pairs act on different bodies and never cancel each other.
5.5 Conservation of Momentum
When no external force acts on a system, its total momentum remains constant. Foundation of collision analysis and rocket propulsion.
5.6 Equilibrium of a Particle
A particle is in equilibrium if the net force on it is zero. Used in problems with concurrent forces, strings, and inclined planes.
5.7 Friction – Static and Kinetic
Static friction prevents relative motion; kinetic friction opposes it. μ_s > μ_k. Friction is independent of contact area (Amontons' law).
5.8 Circular Motion and Newton's Laws
For circular motion, net inward force = mv²/r (centripetal). Provided by friction, normal force, tension, or gravity depending on context.
| Resource Name | Description | Best For |
|---|---|---|
| NCERT Solutions | Detailed answers and explanations for NCERT textbook questions across all classes and subjects. | Homework, assignments, and exam preparation |
| NCERT Solutions for Class 11 | Chapter-wise solutions for all Class 11 subjects including Physics, Chemistry, Mathematics, Biology, and English. | Class 11 board exam preparation |
| NCERT Solutions for Class 11 Physics | Step-by-step solutions covering all chapters such as Motion, Laws of Motion, Work Energy and Power, Thermodynamics, and Waves. | Concept building and numerical problem-solving |
| NCERT Exemplar Class 11 Physics | Advanced and application-based questions designed to strengthen conceptual understanding and analytical skills. | JEE, NEET, Olympiads, and higher-order practice |
| Physics Formula | Chapter-wise collection of important formulas, equations, and derivations for quick revision. | Last-minute revision and numerical practice |
Quick Reference – Types of Forces and Applications
| Situation | Forces Involved | Key Relation |
|---|---|---|
| Object on flat surface at rest | Weight (down), Normal (up) | N = mg |
| Object pushed horizontally (friction) | Applied, Normal, Friction, Weight | a = (F − μ_k mg)/m |
| Block on incline (sliding down) | mg sinθ (down), μ_k mg cosθ (up slope) | a = g(sinθ − μ_k cosθ) |
| Atwood Machine | T same for both; net force = (m₁−m₂)g | a = (m₁−m₂)g/(m₁+m₂) |
| Lift accelerating up | N > mg | N = m(g + a) |
| Lift accelerating down | N < mg | N = m(g − a) |
| Car on curved road | Friction provides centripetal force | f = mv²/r ≤ μ_s mg |
| Rocket propulsion | Thrust (reaction to exhaust) | F = −v_exhaust · (dm/dt) |
NCERT Solutions for Class 11 Physics Chapter 5: Laws of Motion – FAQs
Newton’s First Law states that an object remains at rest or in uniform motion unless acted upon by an external force. This law introduces the concept of inertia. Students learn why passengers move forward when a vehicle stops suddenly and why seat belts are necessary. Understanding this law helps explain everyday observations and forms the basis of mechanics. Many conceptual questions in examinations are based on inertia and its applications.
Newton’s Second Law establishes a quantitative relationship between force, mass, and acceleration. It explains how forces influence motion and provides the foundation for solving most mechanics problems. Students apply this law extensively in NCERT exercises involving connected bodies, elevators, and inclined planes. Understanding this law helps students solve numerical questions logically and accurately. It is one of the most important concepts in Class 11 Physics.
Newton’s Third Law states that every action has an equal and opposite reaction. These forces act on different bodies and occur simultaneously. Examples include walking, swimming, and rocket propulsion. Students often misunderstand action-reaction pairs, making this a frequently asked examination topic. A clear understanding helps explain various natural and technological phenomena. This law also strengthens conceptual understanding of force interactions.
Friction is the force that opposes relative motion between surfaces in contact. It arises due to surface irregularities and molecular interactions. The chapter discusses static friction, kinetic friction, and rolling friction. Friction can be beneficial or harmful depending on the situation. Students solve numerical problems involving frictional forces and learn methods of reducing friction through lubrication. Understanding friction is essential for real-world applications and engineering concepts.
The law of conservation of momentum states that the total momentum of an isolated system remains constant if no external force acts on it. This principle explains collisions, explosions, and recoil phenomena. Students frequently encounter numerical and conceptual questions based on momentum conservation. Understanding this law simplifies the analysis of interacting bodies and provides a powerful problem-solving tool in mechanics.