Friction
📚 Topic Overview: Friction – Types, Factors & Everyday Applications
This section explains what friction is, why it appears whenever surfaces are in contact,
how it can be both helpful and harmful, and how we can increase or reduce it when needed.
Students learn about types of friction (static, sliding, rolling, fluid), factors affecting
friction, and the idea of friction as a “necessary evil”, supported with classroom, transport,
and daily-life examples frequently used in exam questions.
1. Friction – Basic Idea & Everyday Observations
Concept / Theory
Friction is a force that opposes the relative motion or
tendency of motion between two surfaces in contact.
It always acts along the surface, in a direction opposite to the motion (or attempted motion)
of one surface over the other.
Friction arises because surfaces, even if they appear smooth, have tiny irregularities
that interlock with each other.
Everyday Observations of Friction
- We are able to walk without slipping because of friction between our feet and the ground.
- A moving ball on the floor gradually slows down and stops due to friction.
- We can hold a pen or pencil firmly because of friction between fingers and the object.
- Brakes in vehicles use friction to reduce speed and stop motion safely.
| English | Telugu |
|---|---|
| Friction | ఘర్షణ |
| Rough surface | పొరలుగల ఉపరితలం |
| Smooth surface | మృదువైన ఉపరితలం |
When students write with a chalk on a blackboard, friction between the chalk and board
surface leaves marks. If the board is made too smooth, chalk would not leave clear writing.
This simple classroom activity shows that friction is necessary for some actions.
Whenever a question mentions “slowing down”, “stopping”, “grip”, “slipping”, or “rubbing”
of surfaces, it is almost certainly related to friction. Identify whether friction is helping
(advantage) or causing loss of energy (disadvantage).
2. Factors Affecting Friction & Types of Friction
Factors Affecting Friction
Frictional force between two surfaces depends mainly on:
- Nature of surfaces in contact (rough or smooth).
- Normal force or how hard the surfaces are pressed together.
- Relative motion (whether the object is at rest, starting to move, or already moving).
| Factor | Effect on Friction | Example |
|---|---|---|
| Rough vs smooth surfaces | Rough surfaces → more friction; smooth → less | Carrom board with powder vs without powder |
| Normal force | Greater normal force → more friction | Heavier load is harder to slide than lighter load |
Types of Friction
| Type | Description | Example |
|---|---|---|
| Static friction | Friction that prevents the start of motion between surfaces at rest | Pushing a heavy box that does not move initially |
| Sliding (kinetic) friction | Friction when one surface slides over another | Book sliding on table |
| Rolling friction | Friction when an object rolls over a surface | Wheel of a bicycle moving on road |
| Fluid friction (drag) | Friction experienced by objects moving through fluids (liquids or gases) | Air resistance on a moving car, water resistance on a boat |
Generally, static friction > sliding friction > rolling friction for the same pair of surfaces.
| English | Telugu |
|---|---|
| Static friction | స్థిర ఘర్షణ |
| Sliding friction | స్లైడింగ్ ఘర్షణ |
| Rolling friction | రోలింగ్ ఘర్షణ |
A shopkeeper first pushes a heavy box and it does not move (static friction). When he
pushes harder, the box starts sliding (sliding friction). To move it easily, he later
keeps the box on small rollers or wheels (rolling friction). In this single example,
we can see three types of friction in sequence.
If the object is just about to move or resisting motion, think of static friction.
If it is sliding, think of sliding friction. If it is on wheels or ball bearings,
rolling friction is important. Many questions simply test identification of the type.
3. Friction – A Necessary Evil, Increasing & Reducing Friction
Friction as a “Necessary Evil”
Friction is called a necessary evil because it has both
advantages and disadvantages.
- Helpful (Necessary): Helps us walk, hold objects, write, drive vehicles safely.
- Harmful (Evil): Causes wear and tear of machine parts, produces unwanted heat, wastes energy.
When We Want to Increase Friction
- Tyres of vehicles have treads (patterns) to increase friction with road and prevent slipping.
- Sports shoes and kabaddi players’ shoes have rough soles for better grip.
- Chalk powder or resin is used by players for good grip in games like gymnastics or weightlifting.
When We Want to Reduce Friction
- Oil or grease is applied to the moving parts of machines to reduce wear and smoothen motion.
- Powder is sprinkled on carrom board so that coins move easily.
- Ball bearings are used in fans, bicycles and machines to reduce sliding friction and make rolling easier.
| Situation | We Want to… | Method Used |
|---|---|---|
| Vehicle on wet road | Increase friction | Tyre treads, good rubber quality |
| Machine gears | Reduce friction | Lubrication with oil or grease |
| Sliding cupboard on floor | Reduce friction | Using rollers or small wheeled trolleys |
| Climbing a hill | Increase friction | Using shoes with good grip |
| English | Telugu |
|---|---|
| Lubricant | స్నిగ్ధ పదార్థం (నూనె/గ్రీస్) |
| Grip | పట్టు |
| Wear and tear | అరిగిపోవడం / ఘర్షణ దెబ్బ |
In a busy city, buses and two-wheelers rely on friction between tyres and road to move
and stop safely. During rainy days, if the road becomes very smooth or oily, friction
decreases and vehicles may skid. That is why good tyre treads and proper road conditions
are important. Here friction is clearly “necessary” for safety.
When a question asks “Is friction useful or harmful in this situation?”, check the purpose:
do we want more grip or do we want smoother motion? This helps you decide whether the
required action is to increase friction (rough surfaces, treads) or reduce friction
(lubricants, smooth surfaces, ball bearings).
4. Exam Tips, Concept Highlights & Memory Sheet
Concept Highlights
- Friction opposes relative motion between surfaces in contact and always acts along the surface.
- Friction depends on nature of surfaces and how strongly they are pressed together, not directly on area of contact in simple models.
- Static friction prevents motion from starting; sliding friction acts during sliding; rolling friction is usually the smallest.
- Friction is useful for grip and control, but causes energy loss and wear in machines.
Quick Memory Sheet
| Idea | Short Memory Line |
|---|---|
| Definition of friction | Friction opposes motion between touching surfaces |
| Order of friction | Static > sliding > rolling (usually) |
| Necessary evil | We need friction for grip, but it wastes energy |
| Increasing friction | Make surfaces rough, treads, special shoes |
| Reducing friction | Use oil, grease, polish, ball bearings |
Sample Question Ideas (for Practice Sets)
- Give two examples each where friction is helpful and where it is harmful.
- Explain why it is difficult to walk on a smooth, wet floor.
- Why do vehicles have tyre treads? What would happen if tyres were completely smooth?
- How do ball bearings help in reducing friction in machines?
- Explain with an example why friction is called a “necessary evil”.
Many questions on friction are reasoning-type: “assertion–reason” or “match the following”.
Always think in pairs: situation ↔ type of friction ↔ whether we want to increase or reduce it.
This three-step thinking helps in eliminating wrong options quickly.