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Friction: Core Concepts & Formulas
Class 11 Physics • Chapter: Laws of Motion • Comprehensive Study & Revision Sheet
1. Introduction to Friction
Definition: Friction is an opposing contact force that comes into play whenever two surfaces are in contact
and there is relative motion (or a tendency of relative motion) between them.
• Direction: It always acts tangentially to the contact surface and opposite to the direction of motion (or
impending motion).
• Origin: Arises due to microscopic roughness, surface irregularities, and molecular adhesive bonds formed
at contact points.
SI Unit: Newton (N) Dimensional Formula: [M1L1T-2]
2. Types of Friction
A. Static Friction (fs)
The frictional force that acts between two surfaces when an applied external force tends to move a body, but
the body remains at rest.
• Nature: Self-adjusting force. It balances the applied force exactly (f = F
s applied) up to its threshold value.
• Example: A heavy book placed on a wooden table remaining stationary when pushed gently.
B. Limiting Friction (fL or fs,max)
The maximum value of static friction just before the body starts moving over the surface. It represents the
breaking point of static equilibrium.
fL = μs × N
Where μs = coefficient of static friction, and N = normal reaction force (N = mg on a horizontal plane).
C. Kinetic (Dynamic) Friction (fk)
The opposing force that acts between the contact surfaces when a body is actually in relative motion over
another surface.
fk = μk × N
Class 11 Physics • Revision Notes Series Page 1 of 2
Friction: Core Concepts & Formulas
Class 11 Physics • Chapter: Laws of Motion • Comprehensive Study & Revision Sheet
1. Introduction to Friction
Definition: Friction is an opposing contact force that comes into play whenever two surfaces are in contact
and there is relative motion (or a tendency of relative motion) between them.
• Direction: It always acts tangentially to the contact surface and opposite to the direction of motion (or
impending motion).
• Origin: Arises due to microscopic roughness, surface irregularities, and molecular adhesive bonds formed
at contact points.
SI Unit: Newton (N) Dimensional Formula: [M1L1T-2]
2. Types of Friction
A. Static Friction (fs)
The frictional force that acts between two surfaces when an applied external force tends to move a body, but
the body remains at rest.
• Nature: Self-adjusting force. It balances the applied force exactly (f = F
s applied) up to its threshold value.
• Example: A heavy book placed on a wooden table remaining stationary when pushed gently.
B. Limiting Friction (fL or fs,max)
The maximum value of static friction just before the body starts moving over the surface. It represents the
breaking point of static equilibrium.
fL = μs × N
Where μs = coefficient of static friction, and N = normal reaction force (N = mg on a horizontal plane).
C. Kinetic (Dynamic) Friction (fk)
The opposing force that acts between the contact surfaces when a body is actually in relative motion over
another surface.
fk = μk × N
Class 11 Physics • Revision Notes Series Page 1 of 2
