Chapter 11: Friction 🔥
A Comprehensive Guide for PSTET Paper-2 (Science)
Chapter Overview
| Section | Topic | PSTET Weightage | Page No. |
|:---:|:---|::---:|:---:|
| 11.1 | What is Friction? - A Force | High | 2 |
| 11.2 | Factors Affecting Friction | High | 7 |
| 11.3 | Friction: A Necessary Evil (Advantages and Disadvantages) | High | 12 |
| 11.4 | Increasing and Reducing Friction | High | 17 |
| Practice Zone | MCQs & Pedagogical Questions | - | 23 |
Learning Objectives 🎯
After studying this chapter, you will be able to:
✅ Define friction as a force that opposes relative motion between surfaces in contact
✅ Distinguish between static, sliding, rolling, and fluid friction with examples
✅ Explain the factors that affect the magnitude of friction—nature of surfaces, pressing force, and material type
✅ Analyze friction as a "necessary evil" by evaluating its advantages and disadvantages
✅ Describe various methods to increase or decrease friction in daily life situations
✅ Apply pedagogical strategies to teach friction concepts effectively to upper primary students
Pedagogical Link 🔗
For Teachers: This chapter directly aligns with:
Class 8 Science NCERT Chapter 12: "Friction"
Teaching Tips:
Begin with kinesthetic activities—students rub hands together, slide objects on different surfaces
Use simple experiments like rolling a ball on carpet vs. tile floor to demonstrate friction variation
Create a "Friction is a Necessary Evil" debate in class—students argue for advantages vs. disadvantages
Use real-life examples from sports, transportation, and daily chores to make concepts relatable
Conduct the chopstick-rice bottle experiment to demonstrate friction's gripping power
Section 11.1: What is Friction? - A Force 🧠
Introduction
Have you ever wondered why a rolling ball eventually stops, why we don't slip while walking, or why we can light a matchstick by striking it? The answer to all these questions lies in a force that is everywhere around us—friction. It is a force we experience constantly, yet often take for granted .
11.1.1 Definition of Friction
Definition: Friction is a force that opposes the relative motion between two surfaces that are in contact with each other .
Key Characteristics:
Friction always acts in the direction opposite to the motion or attempted motion
It is a contact force—it exists only when two surfaces are touching
Friction can be both helpful (allowing us to walk) and harmful (causing wear and tear)
Simple Understanding:
If you push a book across a table to the right, the force of friction acts toward the left, opposing its motion and causing it to slow down .
11.1.2 Why Does Friction Occur?
The Microscopic Explanation:
No surface is perfectly smooth. Even surfaces that look smooth to the naked eye appear rough and bumpy when viewed under a microscope .
Visualize This:
A polished metal surface that looks mirror-smooth actually has mountains and valleys when magnified
When two surfaces come in contact, these microscopic irregularities interlock with each other
These interlocking "catch and grab" each other, creating resistance to motion
The Adhesion Explanation:
Beyond mechanical interlocking, there is also adhesion—the force of attraction between molecules of different materials . When two surfaces are in close contact, atoms and molecules at the contact points attract each other, further opposing motion .
Table 11.1: Why Friction Occurs—Two Perspectives
| Perspective | Explanation | Analogy |
|---|---|---|
| Mechanical Interlocking | Microscopic bumps and valleys on surfaces catch each other | Like two brushes with bristles interlocking |
| Adhesive Forces | Molecules of different materials attract each other at contact points | Like tiny magnets pulling surfaces together |
11.1.3 Types of Friction
Friction is not a single force—it manifests in different ways depending on the situation. There are four main types of friction :
A. Static Friction
Definition: Static friction is the force of friction that acts on objects when they are at rest relative to each other . It prevents a stationary object from starting to move.
Characteristics:
It opposes the initiation of motion
It adjusts itself to match the applied force up to a maximum limit
No energy is involved in static friction because nothing has moved
How It Works:
When you try to push a heavy desk, static friction pushes back with exactly the force you apply—up to a point. If you push harder and harder, eventually the desk will move. The force at which it just starts to move is the maximum static friction .
Examples:
A car parked on a hill without rolling down
The girls climbing a rock wall—static friction keeps their hands and feet from slipping
B. Sliding (Kinetic) Friction
Definition: Sliding friction (also called kinetic friction) is the force that acts when an object is sliding over a surface .
Characteristics:
It opposes ongoing motion
This explains why it's easier to keep an object moving than to start it moving
Its size depends on the nature of the surfaces and how hard they are pressed together
Examples:
C. Rolling Friction
Definition: Rolling friction acts when an object rolls over a surface .
Characteristics:
This is why wheels are such an important invention—they convert sliding friction into much smaller rolling friction
Comparison:
Rolling friction < Sliding friction < Static friction [citation:2]
Examples:
Ball bearings in machines—they let parts roll rather than slide
Bicycles, cars, skateboards, and roller skates all use wheels to take advantage of low rolling friction
Moving heavy machinery by placing logs underneath (ancient method of reducing friction)
D. Fluid Friction
Definition: Fluid friction (also called drag) is the friction that acts on objects moving through a fluid—either a liquid or a gas .
Characteristics:
It opposes motion through air, water, or any fluid
It depends on the nature of the fluid (more viscous fluids create more friction)
Examples:
Pushing your open hand through water in a pool—you feel resistance
A flat piece of paper fluttering down slowly vs. a wadded paper ball falling straight down
Aerodynamic design of cars and airplanes to reduce fluid friction
Table 11.2: Comparison of Friction Types
| Type | When It Acts | Strength | Example |
|---|---|---|---|
| Static Friction | Object at rest | Strongest | Pushing a heavy desk that won't move |
| Sliding Friction | Object sliding | Medium | Box sliding across floor |
| Rolling Friction | Object rolling | Weakest | Bicycle moving on road |
| Fluid Friction | Object in fluid | Varies | Hand moving through water |
11.1.4 Friction Produces Heat
One of the most important characteristics of friction is that it produces heat .
Why Does Friction Produce Heat?
When surfaces rub against each other, the friction causes the molecules on the rubbing surfaces to move faster. This increased molecular motion means they have more energy, which translates to a higher temperature—they feel warmer .
Useful Heat from Friction:
Harmful Heat from Friction:
11.1.5 Fun with Friction: Classroom Activity 🧪
Activity: Dropping Paper
11.1.6 Pedagogical Implications
| Teaching Strategy | Description | PSTET Focus |
|---|---|---|
| Hands-on Activities | Students rub hands, slide objects, drop paper—experience friction directly | Experiential learning |
| Microscope Observation | View supposedly smooth surfaces under microscope to see irregularities | Scientific observation |
| Classification Activity | Sort given situations into types of friction | Analytical thinking |
| "Feel the Heat" | Students rub hands vigorously and discuss why they feel warm | Connecting cause and effect |
Section 11.2: Factors Affecting Friction 📊
Introduction
Have you noticed that it's easier to slide on a polished floor than on a rough carpet? That heavy boxes are harder to move than light ones? These observations tell us that friction is not constant—it varies depending on several factors .
11.2.1 Major Factors Affecting Friction
Table 11.3: Factors Affecting Friction
11.2.2 Detailed Explanation of Each Factor
A. Nature of Surfaces (Roughness)
Key Principle: Rougher surfaces have more friction between them than smoother surfaces .
Why?
Rough surfaces have more microscopic irregularities (bumps and dips)
These irregularities catch and interlock with each other
More interlocking means more resistance to motion
Real-life Examples:
Concrete road vs. ice: Walking on concrete is easy (high friction); walking on ice is slippery (low friction)
Sandpaper vs. polished wood: Sandpaper creates much more friction
Tyre treads: Rough patterns on tyres increase friction with the road
📝 PSTET Note: This is why we put sand on icy sidewalks—it increases roughness and therefore friction, making walking safer .
B. Force Pressing Surfaces Together (Normal Force)
Key Principle: Heavier objects have more friction. The greater the force pressing two surfaces together, the greater the friction between them .
Why?
When objects are pressed together harder, the irregularities are forced into each other more deeply
This creates stronger interlocking
More force is needed to overcome this interlocking
Mathematical Relationship:
Friction ∝ Normal Force Friction = μ × Normal Force [citation:4][citation:9]
Where μ (mu) is the coefficient of friction—a number that depends on the materials.
Real-life Examples:
A heavy box is harder to push than an empty box of the same size
Adding weight to a sled increases friction with the ground
A heavily loaded truck has more tire friction with the road than an empty truck
C. Type of Material
Key Principle: Different materials have different amounts of friction, even if they are equally smooth .
Why?
Soft materials (like rubber) can deform and fit into the irregularities of the other surface
This creates more contact points and more interlocking
Hard materials (like steel) deform less, so fewer contact points
Real-life Examples:
Rubber soles on shoes provide good grip (high friction)
Steel on ice has very low friction
Car tyres are made of rubber precisely because of its high friction with roads
D. Area of Contact
Key Principle: Increasing the area of surfaces that are touching increases friction between them .
Why?
Larger area means more points where irregularities can interlock
However, this effect is smaller than the effect of roughness or pressing force
Real-life Examples:
You can't slide as far on ice with shoes (larger area) as you can on thin blades of ice skates (smaller area)
Wide tyres provide more grip than narrow tyres
E. Relative Speed
For Solids: The sliding friction force doesn't depend very much on the relative speed . It remains roughly constant.
For Fluids: Fluid friction (drag) increases with speed. At high speeds, air resistance (which is proportional to the square of the speed) becomes a major factor limiting how fast a car can go .
11.2.3 Summary: Factors Affecting Friction
Table 11.4: How to Change Friction by Changing Factors
| To INCREASE Friction | To DECREASE Friction |
|---|---|
| Make surfaces rougher | Make surfaces smoother |
| Increase pressing force (add weight) | Decrease pressing force (reduce weight) |
| Use softer materials | Use harder materials |
| Increase contact area | Decrease contact area |
11.2.4 Fun with Friction: Classroom Activity 🧪
Activity: The Chopstick-Rice Bottle Trick
11.2.5 Pedagogical Implications
| Teaching Strategy | Description | PSTET Focus |
|---|---|---|
| Comparative Experiments | Slide objects on different surfaces (carpet, tile, sandpaper) | Scientific investigation |
| Weight Addition Activity | Add weights to a box and measure force needed to move it | Quantitative thinking |
| Material Testing Station | Test friction of different materials (rubber, wood, metal, plastic) | Hands-on learning |
| Prediction-Observation-Explanation | Predict which surface will have more friction, then test | Scientific method |
Section 11.3: Friction: A Necessary Evil ⚖️
Introduction
Friction is often described as a "necessary evil" . This means that while friction causes many problems (it is an "evil"), it is also essential for our daily life (it is "necessary"). Without friction, life as we know it would be impossible .
11.3.1 Why is Friction "Necessary"? (Advantages)
Friction plays a crucial role in countless everyday activities. Without it, we couldn't walk, hold objects, or even stop moving vehicles .
Table 11.5: Advantages of Friction
📝 PSTET Note: Friction between our feet and the ground allows our movements like standing, walking, and running. Without friction, we would be like someone trying to walk on smooth ice—completely helpless .
11.3.2 Why is Friction an "Evil"? (Disadvantages)
While friction is essential, it also creates significant problems, especially in machines and moving parts .
Table 11.6: Disadvantages of Friction
📝 PSTET Note: Friction causes mechanical parts to seize and wear out. It results in wastage of energy in the form of heat, which is why machines need lubrication and cooling systems .
11.3.3 The "Necessary Evil" Concept Explained
Friction is unique because it plays two opposing roles in our lives:
| Aspect | Friction as Friend | Friction as Foe |
|---|---|---|
| Walking | Essential for grip | Causes wear of shoe soles |
| Driving | Enables motion and stopping | Causes tyre wear, wastes fuel |
| Machines | Belts and clutches need friction to work | Causes overheating, part wear |
| Sports | Ball grip, player traction | Causes equipment wear |
Classroom Discussion Point:
Ask students: "If you could magically turn off ALL friction for one minute, what would happen?"
You would fall down and couldn't get up
Your clothes would fall off (no friction holding knots)
Cars would slide uncontrollably
You couldn't hold anything
Nails would fall out of walls
This helps students appreciate that despite its disadvantages, friction is absolutely necessary .
11.3.4 The Balance: Managing Friction
Since friction is both essential and problematic, we don't want to eliminate it completely—we want to manage it appropriately for each situation .
| Situation | Need | Action |
|---|---|---|
| Car tyres on road | High friction (safety) | Increase friction (tread patterns) |
| Car engine parts | Low friction (efficiency) | Decrease friction (oil lubrication) |
| Shoes for hiking | High friction (grip) | Rough soles with deep treads |
| Ballet shoes | Low friction (smooth movement) | Smooth soles |
| Brake pads | High friction (stopping) | Rough, heat-resistant materials |
| Bearings | Low friction (smooth rotation) | Ball bearings, lubrication |
11.3.5 Pedagogical Implications
| Teaching Strategy | Description | PSTET Focus |
|---|---|---|
| Debate Activity | Divide class into "Advantages" and "Disadvantages" teams | Critical thinking, articulation |
| "No Friction Day" Imagination | Students imagine and describe a day without friction | Creative thinking, application |
| Real-life Case Studies | Examine situations where friction helps/hinders | Analytical thinking |
| T-chart Creation | Students create T-charts of advantages/disadvantages | Visual organization |
Section 11.4: Increasing and Reducing Friction 🔧
Introduction
In real life, we often need to control friction—sometimes we want more of it, and sometimes we want less. Understanding how to increase or decrease friction allows us to design better machines, safer roads, and more effective tools .
11.4.1 Methods of Increasing Friction
When we need better grip, more traction, or stronger holding power, we want to increase friction .
Table 11.7: Methods to Increase Friction
Real-life Applications:
11.4.2 Methods of Reducing Friction
When we want machines to run smoothly, objects to slide easily, or energy to be conserved, we need to reduce friction .
Table 11.8: Methods to Reduce Friction
Real-life Applications:
11.4.3 Ball Bearings: A Special Case
Ball bearings are one of the most important inventions for reducing friction .
How Ball Bearings Work:
Small metal balls are placed between two moving surfaces
When one surface moves, the balls roll rather than slide
Rolling friction is much smaller than sliding friction
Applications of Ball Bearings:
Bicycle wheels and pedals
Skateboard wheels
Electric motors
Fans
Hard disk drives
Almost any rotating machinery
🧪 Classroom Activity: Moving Heavy Books
11.4.4 Comparison: Increasing vs. Decreasing Friction
Table 11.9: When to Increase vs. Decrease Friction
| Situation | Friction Need | Method Used |
|---|---|---|
| Shoes for trekking | Increase | Rough soles, deep treads |
| Shoes for dancing | Decrease | Smooth soles |
| Car tyres | Increase | Rubber material, tread patterns |
| Car engine | Decrease | Engine oil, ball bearings |
| Road surface | Increase (for safety) | Rough asphalt, not polished |
| Ski slope | Decrease (for speed) | Smooth ice/snow |
| Writing with pencil | Moderate | Graphite provides controlled friction |
| Writing on whiteboard | Low | Smooth marker slides easily |
11.4.5 Special Cases: Fluid Friction Reduction
For objects moving through fluids (air or water), special design is needed:
Streamlining:
Shaping objects so that fluids flow smoothly around them
Reduces drag (fluid friction)
Comparison:
A flat sheet of paper falls slowly (high air resistance)
This demonstrates how shape affects fluid friction
11.4.6 Pedagogical Implications
| Teaching Strategy | Description | PSTET Focus |
|---|---|---|
| "Fix the Problem" Scenarios | Present situations (squeaky door, slippery floor) and ask students to suggest friction solutions | Problem-solving, application |
| Ball Bearing Exploration | Take apart an old fan or toy to show ball bearings | Hands-on discovery |
| Lubrication Station | Test how oil affects sliding of objects | Experimental learning |
| Design Challenge | Students design shoes for different purposes (hiking, dancing, basketball) | Creative application |
| Streamlining Activity | Drop different shapes through water/air and compare | Scientific investigation |
Chapter Summary: Key Points for Revision 📝
Quick Revision Table
Practice Zone: PSTET-Style Questions 🎯
Content-Based MCQs
Q1. Friction is a force that:
a) Helps motion
b) Opposes motion
c) Has no effect on motion
d) Always increases speed
Q2. Which type of friction is the strongest?
a) Sliding friction
b) Rolling friction
c) Static friction
d) Fluid friction
Q3. Friction occurs because:
a) Surfaces are perfectly smooth
b) Surfaces have microscopic irregularities that interlock
c) Objects are heavy
d) Air is present
Q4. Which of the following is a method to INCREASE friction?
a) Adding oil to surfaces
b) Using ball bearings
c) Making grooves on tyres
d) Polishing the surface
Q5. Why is it easier to keep a heavy box moving than to start it moving?
a) Static friction > Sliding friction
b) Sliding friction > Static friction
c) Rolling friction > Static friction
d) All frictions are equal
Q6. Which factor does NOT affect the amount of friction?
a) Roughness of surfaces
b) Force pressing surfaces together
c) Colour of the object
d) Type of material
Q7. Friction is called a "necessary evil" because:
a) It is always harmful
b) It is always helpful
c) It has both advantages and disadvantages
d) It doesn't exist in reality
Q8. Ball bearings are used in machines to:
a) Increase friction
b) Decrease friction by converting sliding to rolling
c) Make machines heavier
d) Absorb heat
Q9. When you rub your hands together, they become warm because:
a) Hands contain heat naturally
b) Friction produces heat
c) Air is warm
d) Blood flow increases
Q10. Which of the following shows rolling friction?
a) A book sliding on a table
b) A bicycle moving on road
c) A box being pushed
d) A pencil writing on paper
Pedagogical MCQs
Q11. A teacher wants to demonstrate that friction produces heat. The best activity would be:
a) Show a video of a car braking
b) Ask students to rub their hands together vigorously and feel the warmth
c) Read from textbook
d) Draw diagrams on board
Q12. To teach that rough surfaces have more friction than smooth surfaces, a teacher should:
a) Give a lecture on surface roughness
b) Have students slide objects on sandpaper, then on polished wood, and compare
c) Show pictures of different surfaces
d) Write definitions on board
Q13. A student asks, "Why do we put oil in car engines?" The correct explanation is:
a) Oil makes the engine look clean
b) Oil reduces friction between moving parts, preventing wear and overheating
c) Oil helps the engine burn fuel better
d) Oil is just for tradition
Q14. While teaching the "necessary evil" concept, the best approach is:
a) Only list advantages
b) Only list disadvantages
c) Create a T-chart with advantages and disadvantages, then discuss
d) Tell students to memorize both lists
Q15. The most effective way to demonstrate the advantage of ball bearings is:
a) Show a diagram
b) Push a heavy book on rollers (pencils) vs. sliding it directly
c) Read about ball bearings from textbook
d) Show a video of a factory
Answer Key with Explanations
Pedagogical Reflection for Teachers 🤔
Think-Pair-Share Activity:
Think: How would you explain to your students that without friction, they couldn't walk to school, but with too much friction, machines would waste enormous energy?
Pair: Discuss with a colleague how you would set up a "Friction Discovery Corner" with different surfaces and objects for students to explore.
Share: Design a 15-minute activity to teach the difference between static and sliding friction using a heavy book and a spring balance.
NCERT Textbook Linkages 📚
| Class | Chapter | Topic |
|---|---|---|
| Class 8 | Chapter 12 | Friction |
| Class 9 | Chapter 9 | Force and Laws of Motion |
| Class 11 | Chapter 5 | Laws of Motion |
Chapter End Notes
Key Terminology Glossary
Quick Tips for PSTET Aspirants ⚡
✅ Memorize with Mnemonics:
Types of Friction: "Some Students Really Fail" = Static, Sliding, Rolling, Fluid
Static vs. Sliding: "Starting Strong, Sliding Simple" = Static is Stronger, Sliding is Smaller
Increase Friction: "Rough, Press, Soften" = Roughness, Pressing force, Soft materials
Decrease Friction: "Lube, Smooth, Roll, Streamline" = Lubricants, Smooth surfaces, Rolling, Streamlining
✅ Common Exam Traps:
Static vs. Sliding: Static friction is GREATER than sliding friction—it's harder to start motion than to maintain it
Rolling is smallest: Rolling friction is much smaller than sliding friction—this is why wheels are so important
Friction always opposes MOTION, not necessarily FORCE: It opposes the direction of motion or attempted motion
Lubricants don't eliminate friction: They reduce it by creating a thin layer between surfaces
Smooth doesn't mean no friction: Even very smooth surfaces have some friction
✅ Important Facts:
Friction produces heat—rubbing hands, matchsticks, engine overheating
Tyre treads increase friction by channeling water away and increasing roughness
Normal force is directly proportional to friction—more weight = more friction
✅ Application-Based Questions:
"Why are racing cars designed with very smooth, streamlined shapes?"
"Why do we sprinkle sand on icy roads in winter?"
"Why is it difficult to walk on a wet marble floor?"
"Why do gymnasts apply powder on their hands before performing?"
Answer: Powder absorbs moisture and increases friction, providing better grip on apparatus
Answers to "Check Your Understanding"
[To be filled by student]
📝 Note for Self-Study: After completing this chapter, ensure you can:
Define friction and explain why it occurs (microscopic irregularities)
Differentiate between static, sliding, rolling, and fluid friction with examples
List and explain 3 factors that affect the amount of friction
Explain why friction is called a "necessary evil" with 3 advantages and 3 disadvantages
Describe 3 methods to increase friction with real-life examples
Describe 3 methods to decrease friction with real-life examples
Explain how ball bearings work and why they are important
Explain streamlining and its purpose
Answer "Why?" questions about friction in daily life situations
