1. What is Friction?

In our daily life, we walk, write, hold objects, sit on chairs, use vehicles and many other things easily. All these activities are possible because of a force called friction.

Definition: Friction is a force that opposes the relative motion or the tendency of motion between two surfaces in contact.

When you try to slide a book on a table, it does not keep moving for a long distance. It slows down and finally stops because the surface of the table opposes its motion. This opposing force is friction.

Without friction, we would not be able to walk properly, hold things firmly or even stop moving vehicles safely. On the other hand, too much friction causes wastage of energy and wear and tear of parts. That is why friction is often called a necessary evil.

2. Cause of Friction

Surfaces that look smooth to the naked eye are actually rough when seen under a microscope. They have many tiny irregularities like hills and valleys.

When two surfaces are in contact, these irregularities interlock with each other. When one body tries to move over the other, these irregularities resist motion. This interlocking is the main cause of friction.

• Greater roughness ⇒ more interlocking ⇒ more friction.
• Smoother surfaces ⇒ less interlocking ⇒ less friction.

In addition to irregularities, the force with which the two surfaces are pressed together also affects friction. Larger normal force increases interlocking and therefore friction.

3. Types of Friction

3.1 Static Friction

Static friction is the frictional force that prevents an object from starting to move when a force is applied.

For example, when you try to push a heavy box, it does not move immediately. This is because static friction balances your applied force up to a certain limit.

• Exists when there is no relative motion between surfaces.
• Increases as applied force increases, up to a maximum value.
• The maximum value of static friction is called limiting friction.

3.2 Sliding (Kinetic) Friction

When an object actually starts moving over a surface, the friction acting on it is called sliding friction or kinetic friction.

Example: A book sliding on a table or a block sliding down a rough inclined plane.

• Acts when two surfaces are in relative motion.
• Sliding friction is usually less than static friction for the same surfaces.
• That is why it is easier to keep an object moving than to start moving it from rest.

3.3 Rolling Friction

When an object rolls over a surface, the friction acting between the surfaces is called rolling friction.

Example: Wheels, ball bearings, rollers etc. Rolling friction is much smaller than sliding friction.

• Less irregularities come in direct contact due to rolling.
• Hence, rolling motion reduces the interlocking between surfaces.

Order of friction (for same surfaces):

Static friction > Sliding friction > Rolling friction

3.4 Fluid Friction (Drag)

Liquids and gases also offer resistance to the motion of objects through them. This resistance is called fluid friction or drag.

Examples:

• Air resistance on a moving car or bicycle.
• Resistance offered by water to a swimmer or a boat.

Fluid friction depends on:

• Speed of the object through the fluid.
• Shape of the object.
• Nature (viscosity) of the fluid.

4. Friction – A Necessary Evil

Friction is helpful in many activities, but at the same time it also has harmful effects. That is why we call friction a necessary evil.

4.1 Advantages (Useful Effects) of Friction

• We are able to walk because of friction between our shoes and the ground.
• Friction between tyres and road helps vehicles to move and stop.
• We can write with a pen or pencil due to friction between the tip and paper.
• Friction helps us to hold and grip objects with our hands.
• Nails, screws and bolts stay fixed due to friction between their surfaces and the materials.
• Belts in machines (like conveyor belts, fans) work due to friction between belt and pulley.

4.2 Disadvantages (Harmful Effects) of Friction

• Friction produces heat, which can cause wastage of energy.
• It causes wear and tear of machine parts, tyres, shoe soles, etc.
• Extra energy is required to overcome friction, leading to more fuel consumption.
• Excessive friction can slow down motion and reduce efficiency of machines.

5. Increasing and Decreasing Friction

5.1 How to Increase Friction?

In some situations, we want more friction to get better grip and control.

• Roughening the surfaces: Roads and playgrounds are made rough to increase friction.
• Treading of tyres: Patterns on tyres (treads) increase friction and prevent slipping.
• Use of suitable materials: Athletes use special shoes; car tyres are made of rubber for better grip.
• Applying powder/resin: Gymnasts and weightlifters use chalk powder or resin to improve grip.

5.2 How to Reduce Friction?

In many machines, we want to reduce friction so that they run smoothly and use less energy.

• Lubrication: Applying oil, grease or graphite between moving surfaces fills the irregularities and makes them smoother. This reduces friction.
• Use of ball bearings: In machines like fans, cycles and cars, small metal balls are placed between moving parts. They reduce sliding friction and convert it into rolling friction, which is much smaller.
• Polishing surfaces: Polishing reduces roughness of surfaces and decreases friction.
• Streamlining: Giving bodies a streamlined shape reduces fluid friction (drag) in air and water.

6. Streamlined Shapes and Fluid Friction

Objects moving through fluids experience fluid friction (drag). To reduce this friction and move smoothly, bodies are designed in a special shape called streamlined shape.

A streamlined body is narrow in front and wider in the middle and tapers at the back, like the shape of a fish or an aeroplane.

• Birds, fishes and many animals have naturally streamlined bodies.
• Ships, submarines, aeroplanes and fast trains are designed with streamlined front to reduce fluid friction.
• Less fluid friction means higher speed, less fuel consumption and better efficiency.

7. Friction and Heat

When two surfaces rub against each other, friction converts part of mechanical energy into heat energy.

Examples:

• Rubbing hands together to get warmth in winter.
• Brakes of vehicles become hot after repeated use.
• Machine parts become hot if proper lubrication is not done.

This heating effect is useful sometimes (e.g., lighting a matchstick) but is mostly undesirable in machines because it causes loss of useful energy and can damage parts.

8. Summary of Chapter 12: Friction (Quick Revision)

• Friction is a contact force that opposes relative motion between surfaces in contact.
• It is caused by interlocking of microscopic irregularities on surfaces and depends on nature of surfaces and normal force.
• Types of friction: static friction (before motion), sliding friction (during sliding), rolling friction (during rolling) and fluid friction (drag in liquids and gases).
• For the same surfaces: static friction is highest, sliding friction is lower and rolling friction is the least.
• Friction is a necessary evil: it helps in walking, writing, driving and holding objects, but also causes heating and wear and tear.
• Friction can be increased by roughening surfaces, using treads on tyres and special grip materials.
• Friction can be reduced by using lubricants, ball bearings, polishing surfaces and giving bodies a streamlined shape.
• Fluid friction or drag acts on bodies moving through fluids and depends on speed, shape and nature of the fluid.
• Streamlined shapes of vehicles and animals help in reducing drag and moving smoothly through air or water.

These revision notes for CBSE Class 8 Science – Chapter 12: Friction cover all important NCERT concepts in a simple, exam-oriented manner. Students should read the notes carefully, relate them with examples from daily life and practice questions to score well in their Class 8 Science examinations.