Chapter Overview & Weightage
Friction is one of the most practically relevant chapters in CBSE Class 8 Science. It explains why objects slow down, why we can walk without slipping, and why machines need lubrication. Understanding friction deeply prepares students for Physics topics in Class 9 and 11.
In CBSE Class 8 Science annual exams, Friction typically carries 8–12 marks. Expect questions on types of friction (2 marks), advantages and disadvantages (2–3 marks), methods of reducing friction (2 marks), and application-based questions (3–4 marks). This chapter is factual and conceptual — no calculations required at this level.
Key Concepts You Must Know
1. What is Friction? Friction is the force that opposes relative motion between two surfaces in contact. It acts parallel to the surfaces, opposing the direction of motion or tendency of motion.
Friction arises because all surfaces, however smooth they appear, have microscopic irregularities (bumps and hollows). These interlock with each other and resist motion.
2. Types of Friction
- Static friction: Acts when an object is at rest. Prevents the object from moving when a force is applied. Can vary from zero up to a maximum value.
- Sliding (kinetic) friction: Acts when an object is sliding on a surface. Always less than maximum static friction.
- Rolling friction: Acts when an object rolls on a surface. Much less than sliding friction — this is why wheels revolutionised transportation.
- Fluid friction: Friction in liquids and gases. Called drag when an object moves through air.
3. Factors affecting friction:
- Nature of surfaces (rough vs smooth)
- Force pressing the surfaces together (normal force)
- Does NOT depend on: area of contact (for most surfaces), speed (approximately)
Important Formulas
At Class 8 level, no formula is needed. But for completeness:
where = coefficient of friction, = normal force
This is introduced formally in Class 11 Physics.
Solved Examples
Example 1 — Advantage or Disadvantage?
Is friction between tyres and road an advantage or disadvantage? Explain.
Answer: Advantage. Friction between tyre and road allows the car to start, stop, and turn. Without this friction, tyres would spin in place (no grip) and the car could not be controlled. We actually need this friction for safe driving.
However, tyre friction also wears out tyres — this is a disadvantage of the same friction.
Key idea: The same friction can be both advantageous and disadvantageous depending on context.
Example 2 — Rolling vs Sliding
Why is it easier to pull a heavy box with rollers under it than to drag it across the floor?
Answer: When the box is dragged, sliding friction acts — this is large. When rollers are used, the box rolls — rolling friction acts — which is much smaller. Rolling friction is less than sliding friction because the contact area at any instant is a line (or point) rather than a surface.
This is why wheels, ball bearings, and rollers are used in machines.
Example 3 — Lubrication
How does a lubricant reduce friction?
Answer: A lubricant (oil, grease, graphite) fills the microscopic gaps between the two surfaces. Instead of the rough surfaces rubbing directly against each other, the surfaces now slide over the smooth lubricant layer. This reduces interlocking of surface irregularities and greatly reduces friction.
Advantages of Friction
- Walking is possible — friction between feet and ground provides grip
- Writing with a pen or pencil — friction holds ink on paper
- Braking a vehicle — friction between brake pads and wheels/disc slows the car
- Striking a match — friction ignites the match
- Climbing a tree — friction between hands/feet and bark provides grip
Disadvantages of Friction
- Wears out machine parts (bearings, gears)
- Generates heat (wastes energy — engines get hot)
- Reduces efficiency of machines
- Wears out soles of shoes and tyres
Methods of Reducing Friction
- Polishing surfaces — reduces microscopic irregularities
- Lubrication — oil, grease, graphite fills gaps between surfaces
- Ball bearings — convert sliding friction to rolling friction
- Streamlining — teardrop shape reduces fluid friction (used in cars, aircraft, submarines)
- Air cushions / hovercrafts — surface is replaced by a layer of air (nearly frictionless)
Difficulty Distribution
| Question Type | Marks | Difficulty |
|---|---|---|
| Define friction and types | 1–2 | Easy |
| Advantages and disadvantages | 2–3 | Easy |
| Methods of reducing friction | 2 | Easy |
| Application/scenario questions | 3 | Medium |
| Why rolling < sliding friction | 2 | Medium |
Friction is primarily a memorisation and understanding chapter. Almost all marks are accessible with thorough reading and a few practice questions.
Expert Strategy
Read each question carefully and identify whether it’s asking about static, sliding, or rolling friction, or fluid friction. Use the correct term — examiners award marks for precise vocabulary.
For “is friction an advantage or disadvantage” type questions, always mention BOTH — friction is often both helpful and harmful in the same situation. Nuanced answers score better.
Remember the general trend: Static > Sliding > Rolling friction. Wheels beat sliding; lubrication further reduces both. Streamlining reduces air/fluid friction — explain the physics, don’t just list the method.
Common Traps
Trap 1 — Friction always opposes motion. Friction opposes relative motion between surfaces. When you push a book across a table, friction on the book acts backwards. But friction on your foot from the floor acts forward (it’s what propels you forward when you walk). Direction of friction depends on which body you’re examining and which direction it would move without friction.
Trap 2 — More area = more friction. This is a common misconception. For most solid surfaces, friction does NOT depend on area of contact (a wide tyre and a narrow tyre have similar friction forces for the same weight). This surprises students because it seems counterintuitive. The reason is that fewer contact points per area with a larger surface area compensates for the larger area.
Trap 3 — Friction only slows things down. Friction can also start motion. When you push off the ground while walking or running, friction from the ground on your foot is what actually propels you forward. A world without friction would make it impossible to walk, drive, or even pick up objects.