Course Content
Class 11 Physics Chapter 4 Motion In A Plane
4 Motion in a plane 4.1 Introduction 4.2 Scalars and vectors 4.3 Multiplication of vectors by real numbers 4.4 Addition and subtraction of vectors – graphical method 4.5 Resolution of vectors 4.6 Vector addition – analytical method 4.7 Motion in a plane 4.8 Motion in a plane with constant acceleration 4.9 Relative velocity in two dimensions 4.10 Projectile motion 4.11 Uniform circular motion
Class 11 Physics Chapter 5 Laws of motion
Section Name Topic Name 5 Laws of motion 5.1 Introduction 5.2 Aristotle’s fallacy 5.3 The law of inertia 5.4 Newton’s first law of motion 5.5 Newton’s second law of motion 5.6 Newton’s third law of motion 5.7 Conservation of momentum 5.8 Equilibrium of a particle 5.9 Common forces in mechanics 5.10 Circular motion 5.11 Solving problems in mechanics
Class 11 Physics Chapter 6 Work Energy and Power
Section Name Topic Name 6 Work Energy and power 6.1 Introduction 6.2 Notions of work and kinetic energy : The work-energy theorem 6.3 Work 6.4 Kinetic energy 6.5 Work done by a variable force 6.6 The work-energy theorem for a variable force 6.7 The concept of potential energy 6.8 The conservation of mechanical energy 6.9 The potential energy of a spring 6.10 Various forms of energy : the law of conservation of energy 6.11 Power 6.12 Collisions
Class 11 Physics Chapter 7 Rotation motion
Topics Introduction Centre of mass Motion of COM Linear Momentum of System of Particles Vector Product Angular velocity Torque & Angular Momentum Conservation of Angular Momentum Equilibrium of Rigid Body Centre of Gravity Moment of Inertia Theorem of perpendicular axis Theorem of parallel axis Moment of Inertia of Objects Kinematics of Rotational Motion about a Fixed Axis Dynamics of Rotational Motion about a Fixed Axis Angular Momentum In Case of Rotation about a Fixed Axis Rolling motion
Class 11 Physics Chapter 9 mechanics properties of solid
Section Name Topic Name 9 Mechanical Properties Of Solids 9.1 Introduction 9.2 Elastic behaviour of solids 9.3 Stress and strain 9.4 Hooke’s law 9.5 Stress-strain curve 9.6 Elastic moduli 9.7 Applications of elastic behaviour of materials
Class 11 Physics Chapter 11 Thermal Properties of matter
Section Name Topic Name 11 Thermal Properties of matter 11.1 Introduction 11.2 Temperature and heat 11.3 Measurement of temperature 11.4 Ideal-gas equation and absolute temperature 11.5 Thermal expansion 11.6 Specific heat capacity 11.7 Calorimetry 11.8 Change of state 11.9 Heat transfer 11.10 Newton’s law of cooling
Class 11 Physics Chapter 14 Oscillations
Section Name Topic Name 14 Oscillations 14.1 Introduction 14.2 Periodic and oscilatory motions 14.3 Simple harmonic motion 14.4 Simple harmonic motion and uniform circular motion 14.5 Velocity and acceleration in simple harmonic motion 14.6 Force law for simple harmonic motion 14.7 Energy in simple harmonic motion 14.8 Some systems executing Simple Harmonic Motion 14.9 Damped simple harmonic motion 14.10 Forced oscillations and resonance
Class 11th Physics Online Class For 100% Result
About Lesson


  • Friction is a contact force that opposes relative motion.
  • No friction exists till an external force is applied.

There are 3 types of Friction:

  • Static friction
    • Force that resists initiation of motion of one body over another with which it is in contact
    • Opposes Impending motion
    • Denoted by fs  

Let the ball be at rest initially

Applied force, Fa = 0; Static friction, fs = 0

Law of Motion Class 11 Physics Notes and Solution

fs acts when a body is at rest. Hence called Static friction

  • Limiting value of fs depends on Normal reaction, and is independent of the area of contact

fmax ∝ N

fmax = constant * N

fmax = μs N

where μs is the coefficient of static friction

This coefficient depends on the nature of surfaces in contact.

  • According to the Law of Static friction, Static friction is always less than or equal to the limiting value of fs

  fs  =< fmax 

  fmax = μs N 

  f =< μs N

Kinetic friction

  • Force that resists motion of one body over another with which it is in contact.
  • Denoted by fk
  • As motion starts, fs vanishes and fk appears
Law of Motion Class 11 Physics Notes and Solution

(Fa – fk) = ma

a = (Fa – fk)/m

  1. When applied force = fk

Fa = fk

Therefore, a = 0 i.e. the body moves with uniform velocity

  1. When applied force = 0

Fa = 0

A = -fk/m

No motion occurs, the body stops

Relation between Coefficient of Static & Kinetic friction

fk = μk N

fs =< μs N

fs > fk (to keep the body moving)

μs N > μk N

Or,   μk  <  μs

Coefficient of kinetic friction is smaller than the coefficient of static friction.

Law of Motion Class 11 Physics Notes and Solution

Problem: Calculate the force required for pushing a  30 kg wooden bar  over a wooden floor at a constant speed. Coefficient of friction of wood over wood = 0.25


M = 30 kg

μ = 0.25

(Fa – f) = ma

For constant speed, a = 0

So, Fa = f= μN

Law of Motion Class 11 Physics Notes and Solution

(i)Divided by (ii) gives

1/μ = (L-l)/l

l = μL – μl

l + μl = μL

Therefore, l = μL/( μ + 1)

Law of Motion Class 11 Physics Notes and Solution
Law of Motion Class 11 Physics Notes and Solution
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