Friction is defined as the force that opposes the relative motion or the tendency of motion between two surfaces in contact. This opposition always acts along the surface of contact and in a direction opposite to the motion or attempted motion. Friction does not act in the absence of contact. If two objects are not touching each other, friction cannot exist between them.
Due to this reason, friction is classified as a contact force. Friction is not a fundamental force like gravity. It arises due to interaction between surfaces and depends on the nature of those surfaces and the force pressing them together.
Table of Contents
Why Friction Exists
No surface is perfectly smooth, even if it appears smooth to the naked eye. When examined under a microscope, every surface shows irregularities such as small bumps and depressions.
When two surfaces come into contact, these irregularities interlock with each other. When one surface tries to move over the other, this interlocking resists motion and gives rise to frictional force.
Apart from surface roughness, molecular attraction between surfaces also contributes to friction. Therefore, friction exists due to a combination of surface roughness and intermolecular forces.
Direction of Contact Force
Friction is classified as a contact force because physical contact between surfaces is essential for its action. Without contact, friction cannot exist. Air resistance and fluid resistance are also forms of friction because they involve contact between a moving object and fluid particles. In examinations, friction is always categorised under contact forces and never under non-contact forces.
Types of Friction
Friction is classified into different types based on the nature of motion between the surfaces.
Static Friction
Static friction is the frictional force that acts when an object is at rest relative to a surface and an external force tries to move it. It prevents the object from starting to move.
The magnitude of static friction is not fixed. It adjusts itself to match the applied force up to a maximum value. As the applied force increases, static friction also increases.
Static friction is responsible for keeping objects at rest even when forces act on them, such as a book remaining on an inclined surface.
Limiting Friction
Limiting friction is the maximum value of static friction that acts just before the object starts moving. At this stage, the object is on the verge of motion. Limiting friction depends on the nature of surfaces in contact and the normal reaction between them. Rougher surfaces produce higher limiting friction. In exams, limiting friction is identified as the frictional force acting when motion is about to begin.
Formula for Limiting Friction
Fmax = μs N
Where:
μs = coefficient of static friction
N = normal reaction
Exam condition: This formula applies only at the point of impending motion.
Sliding Friction
Sliding friction acts when one surface actually slides over another surface. It always acts opposite to the direction of sliding motion. Sliding friction is less than limiting friction. This is why it is easier to keep an object moving once it has started moving than to start it from rest.
Examples include a book sliding on a table or a box being dragged across the floor.
Formula for Sliding Friction
Fk = μk N
Where:
μk = coefficient of kinetic (sliding) friction
Exam fact: μk is always less than μs.
Rolling Friction
Rolling friction occurs when an object rolls over a surface, such as a wheel or a ball. It is much smaller than sliding friction. Rolling reduces surface interlocking and deformation, which reduces frictional resistance. This is why wheels and ball bearings are used in vehicles and machines.
Order of Friction Forces (Very Important)
Rolling < Sliding < Limiting
This order is directly tested in NDA and CDS MCQs.
Factors Affecting Friction
The frictional force depends mainly on the nature of surfaces in contact. Rough surfaces produce more friction than smooth surfaces. Friction also depends on the normal reaction between surfaces. Greater the normal reaction, greater the friction. Contrary to common belief, friction does not depend on the area of contact for solid surfaces. This is a frequent exam trap.
Normal Reaction and Friction
Normal reaction is the force exerted by a surface perpendicular to the object in contact with it. Friction is directly proportional to this normal reaction. If the normal reaction increases, friction increases. This is why it is harder to push a heavy object than a light object on the same surface. Normal reaction plays a central role in friction-related numericals.
Numerical Example 1 (NDA / AFCAT Level)
A block of mass 10 kg rests on a horizontal surface. Coefficient of static friction μs = 0.2. Take g = 10 m/s². Normal reaction,N = mg = 10 × 10 = 100 N
Limiting friction,F = μs N = 0.2 × 100 = 20 N
This is the maximum frictional force that can act before motion starts.Numerical Example 2 (Concept + Formula)
If μ = 0, then frictional force is zero.
This represents an ideal smooth surface.
Such questions are used to eliminate incorrect options in MCQs.
Friction and Motion
Friction plays a crucial role in controlling motion. It slows down moving objects and can eventually bring them to rest. When brakes are applied to a vehicle, friction between brake pads and wheels reduces the speed of the vehicle. Without friction, objects once set in motion would continue moving indefinitely, making control of motion impossible.
Friction and Energy
Friction converts mechanical energy into heat energy. This is why rubbing hands together produces warmth.
Due to friction, some useful energy is always lost as heat, reducing efficiency of machines. In some cases, this heat is useful, such as lighting a matchstick.
Advantages of Friction
- Friction enables walking without slipping.
- Friction allows writing on paper and gripping objects.
- Friction makes braking and motion control possible.
- Without friction, practical life would not function normally.
Disadvantages of Friction
- Friction causes wear and tear of machine parts.
- Friction leads to loss of energy and reduces efficiency.
- Friction produces unwanted heat and noise.
- This is why friction is reduced wherever possible.
Methods of Increasing Friction
- Friction can be increased by roughening surfaces, such as grooves on tyres.
- Increasing normal reaction also increases friction.
- Using materials with higher coefficients of friction increases grip.
Methods of Reducing Friction
- Friction can be reduced by polishing surfaces.
- Lubricants like oil and grease reduce surface interlocking.
- Using wheels and ball bearings converts sliding friction into rolling friction.
Friction in Fluids
- Friction also exists in liquids and gases and is called fluid friction or drag.
- Air resistance opposes the motion of objects through air.
- Streamlined shapes reduce fluid friction and improve efficiency.
Common Misconceptions About Friction
- Friction does not always oppose applied force; it opposes motion.
- Friction is not always harmful; it is often necessary.
- Greater surface area does not mean greater friction for solid surfaces.
Boundary Conditions
- Classical friction laws apply to everyday objects and speeds.
- At microscopic levels, friction behaves differently and is not part of basic mechanics.
- For NDA, CDS, AFCAT, and CAPF, only classical friction concepts are required.
PYQs – Friction
Q1. Which of the following is the cause of friction? [NDA 2016]
(a) Smoothness of surfaces
(b) Roughness of surfaces
(c) Area of contact
(d) Shape of objects
Show Answer
(b) Roughness of surfaces
Explanation: Friction arises mainly due to interlocking of surface irregularities.
Q2. Rolling friction is [CDS 2018]
(a) Greater than sliding friction
(b) Equal to sliding friction
(c) Less than sliding friction
(d) Independent of surfaces
Show Answer
(c) Less than sliding friction
Explanation: Rolling reduces surface interlocking.
Q3. Friction always acts [AFCAT 2017]
(a) In the direction of motion
(b) Opposite to motion
(c) Perpendicular to motion
(d) Along applied force
Show Answer
(b) Opposite to motion
Explanation: Friction always opposes motion or tendency of motion.
Q4. Which method reduces friction in machines? [CAPF 2016]
(a) Roughening surfaces
(b) Increasing load
(c) Using lubricants
(d) Increasing area of contact
Show Answer
(c) Using lubricants
Explanation: Lubricants reduce interlocking of surfaces.
FAQs – Friction
Is friction a contact force?
Yes, friction requires physical contact.
Which friction is maximum?
Limiting friction.
Does friction depend on area of contact?
No, for solid surfaces it does not.
Why are wheels used in vehicles?
To reduce friction.
Is air resistance friction?
Yes, it is fluid friction.
Can friction be eliminated completely?
No, it can only be reduced.
Why do tyres have grooves?
To increase friction.
Why do machines heat up?
Due to friction.
What happens if μ = 0?
Surface becomes ideal smooth.
Last Moment Notes (Cheat Sheet)
- Friction opposes relative motion or tendency of motion between contacting surfaces.
- Friction is a contact force and depends on surface nature and normal reaction.
- Static friction adjusts its value up to a maximum limit.
- Limiting friction acts at the verge of motion and equals μN.
- Sliding friction is less than limiting friction.
- Rolling friction is the least among all types.
- Friction opposes motion, not applied force.
- Friction converts mechanical energy into heat.
- Lubrication and rolling reduce friction.
- μ depends on surface nature, not mass or area of contact.