By Subhash Kumar | Science Educator | UREducator
Published: | Updated:
Generate Customized PDF
Get a printer-friendly version of this complete lesson plan
Friction
Complete Teaching Resource for CBSE Class 8 Science Chapter 12
Class: VIII
Subject: Science
Chapter: 12
Duration: 4 Hours
What Students Will Achieve
Knowledge
- Define friction and explain its causes
- Identify different types of friction (static, sliding, rolling, fluid)
- Describe factors affecting friction
- Explain practical applications of friction
Understanding
- Analyze why friction is both necessary and problematic
- Explain why rolling friction is less than sliding friction
- Understand how lubricants reduce friction
- Evaluate methods to increase/decrease friction
Skills
- Conduct experiments to measure different types of friction
- Design solutions to friction-related problems
- Analyze real-world applications of friction
- Create models demonstrating friction principles
Prior Knowledge Assessment
Starter Questions:
- Why do vehicles slow down when brakes are applied?
- Why is it difficult to walk on a smooth, wet floor?
- What happens when you rub your palms together vigorously?
Understanding Friction
What is Friction?
Friction is the force that opposes the relative motion between two surfaces in contact. It acts parallel to the surfaces and opposite to the direction of motion or intended motion.
Causes of Friction
Surface Irregularities
Even smooth surfaces have microscopic bumps and ridges that interlock
Molecular Adhesion
Atoms and molecules on surfaces attract each other when in close contact
Normal Force
The force pressing the surfaces together affects friction magnitude
Factors Affecting Friction
| Factor | Effect on Friction | Example |
|---|---|---|
| Surface Roughness | Rougher surfaces increase friction | Sandpaper vs glass |
| Normal Force | More weight increases friction | Pushing heavy vs light box |
| Surface Area | Does not affect friction (for most cases) | Wide vs narrow tires on same car |
Experiment: Measuring Friction
Objective: Demonstrate how surface type affects friction
- Attach a spring balance to a wooden block
- Pull the block across different surfaces (table, carpet, sandpaper)
- Record the force needed to start moving (static friction)
- Record the force needed to keep moving (sliding friction)
- Compare results for different surfaces
Different Types of Frictional Forces
Four Main Types
Static Friction
Acts on stationary objects to prevent motion
- Strongest type of friction
- Example: Keeping objects from sliding on slopes
Sliding Friction
Acts when objects slide against each other
- Weaker than static friction
- Example: Brakes on bicycle wheels
Rolling Friction
Acts when objects roll over a surface
- Much weaker than sliding friction
- Example: Car tires on road
Fluid Friction
Acts when objects move through fluids
- Depends on speed and object shape
- Example: Air resistance on moving cars
Comparing Friction Types
| Type | Relative Strength | Practical Example | Methods to Reduce |
|---|---|---|---|
| Static | Strongest | Book on tilted table | Reduce surface roughness |
| Sliding | Medium | Dragging furniture | Use lubricants |
| Rolling | Weakest | Ball bearings | Use wheels/rollers |
Activity: Comparing Friction Types
Objective: Demonstrate differences between static, sliding, and rolling friction
- Measure force needed to start moving a block (static friction)
- Measure force needed to keep it moving (sliding friction)
- Place round pencils under the block and measure force (rolling friction)
- Compare the three measurements
Friction in Daily Life
Increasing Friction
Treaded Soles
- Shoes have patterns to increase grip
- Prevents slipping on smooth surfaces
- Different patterns for different sports
Brake Pads
- Rough material pressed against wheels
- Converts kinetic energy to heat
- Essential for vehicle safety
Road Surfaces
- Asphalt contains rough aggregates
- Special surfaces for wet conditions
- Grooves in airport runways
Reducing Friction
Lubricants
- Oil, grease between moving parts
- Forms protective layer
- Prevents direct surface contact
Ball Bearings
- Replace sliding with rolling friction
- Used in bicycles, fans, machinery
- Greatly reduces energy loss
Streamlining
- Smooth shapes reduce fluid friction
- Used in vehicles, aircraft, ships
- Inspired by nature (birds, fish)
Project: Friction Around Us
Objective: Identify and document friction applications in daily life
- Find 5 examples where friction is increased intentionally
- Find 5 examples where friction is reduced intentionally
- Photograph or sketch each example
- Explain the purpose of friction modification in each case
- Present findings to class
Evaluation Strategies
Formative Assessments
- Friction types matching quiz
- Experiment report evaluation
- Classroom participation in activities
- Concept maps of friction applications
Summative Assessments
- Chapter test (MCQs + short answer)
- Friction project presentation
- Case study analysis of real-world friction problems
- Design challenge: Improve a product by modifying friction
Extended Learning: Visit a Bicycle Repair Shop
Objective: Observe practical applications of friction concepts
- Interview mechanic about friction in bicycles
- Identify parts where friction is increased (brakes)
- Identify parts where friction is reduced (chain, gears)
- Document lubrication points and methods
- Write a report on findings
Frequently Asked Questions
Why is rolling friction less than sliding friction?
Rolling friction is less because:
- Only small areas of the rolling object contact the surface at any moment
- The contact points keep changing, preventing deep interlocking
- Less surface deformation occurs compared to sliding
- Energy loss through heat generation is minimized
Can friction be completely eliminated?
No, because:
- All surfaces have microscopic irregularities that cause friction
- Even in space, molecular forces create friction
- Superconductors can reduce friction greatly but not completely
- Some friction is always needed for control and motion
