CBSE · Class 11 · Physics · Chapter 5

Laws of
Motion

Complete chapter resources for CBSE Class 11 Physics — topic breakdown, Newton's laws, friction and circular motion formulas, sample questions, previous year board questions, and instant AI question paper generation.

4Topics
8–10Board marks
8Sample questions
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Key Formulas — Chapter 5
  • Newton's 2nd law: F = ma (net force = mass × acceleration)
  • Impulse: J = F·Δt = Δp = m(v − u)
  • Static friction: f_s ≤ μ_s · N
  • Kinetic friction: f_k = μ_k · N (μ_k < μ_s)
  • Centripetal force: F_c = mv²/r = mω²r
  • Banking angle: tan θ = v²/(rg)
Chapter Overview

What this chapter covers

Chapter 5 of NCERT Class 11 Physics introduces the three fundamental Laws of Motion formulated by Isaac Newton. The First Law (Law of Inertia) states that a body continues in its state of rest or uniform motion unless acted upon by a net external force. It introduces the concept of inertia — the tendency of a body to resist any change in its state of motion — and establishes the idea of a force as the cause of acceleration.

The Second Law connects force, mass, and acceleration: F = ma, or more precisely, the rate of change of linear momentum equals the net external force. This leads directly to the impulse-momentum theorem (J = Δp), which explains phenomena like why a cricket ball stings less when you pull your hand back while catching. The Third Law states that every action has an equal and opposite reaction, with applications ranging from rocket propulsion to normal forces and tension in strings. The chapter also develops the method of drawing free-body diagrams (FBD) to systematically apply Newton's laws to systems of connected bodies.

A major applied section covers friction — static friction (up to a maximum threshold) and kinetic friction (constant during motion), with the laws of limiting friction and coefficients μ_s and μ_k. The chapter closes with circular motion dynamics: the centripetal force requirement (mv²/r), motion in a vertical circle, and the physics of banked roads. Board questions routinely involve FBD-based numericals on Atwood machines, inclined planes, and banked curves.

Topics

What's inside Chapter 5

As per NCERT Class 11 Physics (CBSE syllabus)

Topic 1
Newton's Three Laws of Motion
Law of Inertia, F = ma (rate of change of momentum), and action-reaction pairs. Concept of net force, free-body diagrams, and applying the laws to single-body and connected-body systems (Atwood machine, pulley systems).
Topic 2
Impulse and Linear Momentum
Linear momentum p = mv, impulse J = FΔt = Δp, and conservation of linear momentum (internal forces sum to zero). Applications: collision scenarios, rocket propulsion, recoil of a gun.
Topic 3
Friction
Static friction (f_s ≤ μ_s N), limiting friction, kinetic friction (f_k = μ_k N). Advantages and disadvantages of friction. Motion on inclined planes with and without friction. Methods to reduce friction: lubrication, ball bearings.
Topic 4
Circular Motion Dynamics
Centripetal acceleration (v²/r) and centripetal force (mv²/r). Circular motion in a horizontal plane (conical pendulum). Motion in a vertical circle (minimum speed at the top). Banked roads: tan θ = v²/(rg).
Connections

How this chapter fits in

Useful for setting question difficulty and cross-chapter papers.

Builds on
Ch 3 · Motion in a Straight Line
Kinematics equations (v = u + at, s = ut + ½at²) underpin F = ma problems
Ch 4 · Motion in a Plane
Projectile motion and circular kinematics — prerequisite for centripetal force
Chapter 5 Laws of
Motion
Leads to
Ch 6 · Work, Energy & Power
Work-energy theorem and power are direct applications of Newton's second law
Ch 7 · System of Particles & Rotational Motion
Torque and angular momentum extend Newton's laws to rotational dynamics
Board Blueprint

Marks & question-type breakdown

Typical pattern based on CBSE Class 11 Physics board papers from the last five years.

Question type Marks Typical count What's usually tested
MCQ / Assertion-Reason 1 1–2 Identification of Newton's law, direction of friction, or nature of centripetal force
Very Short Answer 2 1 Impulse calculation, state laws of friction, or define inertia with an example
Short Answer 3 1 FBD + equations for connected bodies on an inclined plane or Atwood machine
Long Answer / Numerical 4–5 1 Circular motion (banked road / vertical circle), multi-body friction problems
Total (approximate) 8–10 4–5 Weightage varies across paper sets and years
Sample Questions

8 sample questions — generated by MarksZen AI

Aligned to CBSE Class 11 Physics Chapter 5. Covers all question types across Easy, Medium, and Hard difficulty.

Q1 Easy 1 mark MCQ
A body of mass 5 kg is acted upon by a net force of 20 N. The acceleration produced is: (a) 0.25 m/s² (b) 4 m/s² (c) 100 m/s² (d) 15 m/s²
Q2 Easy 2 marks Short Answer
A force of 500 N acts on a body for 0.02 s. Calculate the impulse of the force and the change in momentum of the body.
Q3 Medium 2 marks Short Answer
A block of mass 10 kg rests on a horizontal surface. The coefficient of static friction is 0.4 and kinetic friction is 0.3. Find (i) the maximum static friction force, and (ii) the kinetic friction force when the block is moving. (Take g = 10 m/s²)
Q4 Medium 3 marks Short Answer
Two blocks of masses 3 kg and 5 kg are connected by a light string over a frictionless pulley (Atwood machine). Draw a free-body diagram for each block and find the acceleration of the system and the tension in the string. (Take g = 10 m/s²)
Q5 Medium 3 marks Word Problem
A 4 kg block is pushed up a smooth inclined plane of angle 30° with a force F applied parallel to the incline. If the block moves up with a constant velocity, find the magnitude of F. (Take g = 10 m/s²) Draw the FBD and write all force equations.
Q6 Hard 4 marks Numerical
A car of mass 1500 kg moves on a level circular road of radius 200 m. The coefficient of static friction between the tyres and the road is 0.6. (i) Find the maximum safe speed for the car. (ii) If the road is banked at an angle such that no friction is needed at 60 km/h, find the banking angle. (Take g = 10 m/s²)
Q7 Hard 5 marks Numerical
A small ball of mass 0.2 kg is attached to a string of length 0.5 m and is revolved in a vertical circle. (i) Find the minimum speed at the top of the circle for the string to remain taut. (ii) Find the tension in the string at the bottom of the circle if the speed at the bottom is 5 m/s. (iii) Calculate the difference in tension between the bottom and top of the circle. (Take g = 10 m/s²)
Q8 Hard 5 marks Case-Based
A box of mass 20 kg is placed on a rough inclined plane of inclination 37° (sin 37° = 0.6, cos 37° = 0.8). The coefficient of static friction is 0.5. (i) Find the component of weight along and perpendicular to the incline. (ii) Determine whether the block will slide or remain stationary. (iii) If a force of 80 N is applied up the incline, find the net force on the block and its acceleration. (Take g = 10 m/s²)
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Previous Year Questions

From CBSE board examinations

Actual questions from past Class 11 Physics board papers — Laws of Motion chapter.

Board 20223 marks
Two masses 8 kg and 12 kg are connected at the two ends of a light inextensible string that goes over a frictionless pulley. Find the acceleration of the masses and the tension in the string when the masses are released. (All India 2022, take g = 10 m/s²)
Board 20232 marks
A batsman deflects a ball by an angle of 45° without changing its initial speed which is equal to 54 km/h. What is the impulse imparted to the ball? (Mass of the ball is 0.15 kg.) (Delhi 2023)
Board 20205 marks
A block of mass 10 kg is placed on a rough inclined surface (angle of inclination = 30°, μ = 0.5). (i) Draw the free-body diagram. (ii) Find whether the block slides down or remains stationary. (iii) Calculate the minimum force required to push the block up the incline at constant velocity. (CBSE 2020, g = 9.8 m/s²)
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FAQ

Questions teachers ask

How many marks does Laws of Motion carry in the CBSE Class 11 Physics board exam? +
Laws of Motion typically carries 8–10 marks in the CBSE Class 11 Physics board exam. Questions span all types — one 1-mark MCQ on Newton's laws or friction, one 2-mark short answer on free-body diagrams or the laws of friction, and one 3–5 mark numerical on connected-body systems or circular motion. The chapter forms part of Unit II (Laws of Motion) which has a defined weightage in the CBSE marking scheme.
What is the difference between static friction and kinetic friction, and how are they tested in board exams? +
Static friction acts on a body at rest and can vary up to a maximum value (f_s ≤ μ_s N). Kinetic (dynamic) friction acts on a moving body and has a constant value f_k = μ_k N, where μ_k < μ_s. Board questions typically ask students to find the minimum force needed to set a body in motion (using μ_s) or the force needed to keep it moving at constant velocity (using μ_k). Problems combining both on inclined planes are common 3-mark questions.
How should students draw free-body diagrams (FBD) to score full marks in CBSE board exams? +
CBSE examiners award separate marks for a correct FBD. Students should isolate the body, draw all forces as arrows from the centre with correct direction — weight (mg) downward, normal reaction (N) perpendicular to the surface, tension (T) along the string away from the body, and friction along the surface opposing motion. Every force must be labelled. Then apply Newton's second law (ΣF = ma) along chosen axes. A missing or incorrect FBD typically costs 1 mark out of 3–5.
What is the impulse-momentum theorem and how does it appear in board questions? +
The impulse-momentum theorem states that the impulse of a force equals the change in momentum of the body: J = F·Δt = Δp = m(v − u). Board questions use it in two ways: (1) given average force and contact time, find the change in momentum; (2) given initial and final velocities, find the average force. Cricket/ball-impact word problems are a recurring PYQ pattern and are usually 2–3 mark questions.
How do I generate a custom question paper for Laws of Motion using MarksZen? +
Sign up for a free MarksZen account, choose CBSE Class 11 Physics, select Chapter 5 (Laws of Motion), set your preferred question-type mix (MCQ, short answer, numericals) and total marks — the AI generates a complete board-aligned paper with answer key in under 2 minutes, ready for PDF export.