📖 Chapter Overview
Friction is a force that opposes the relative motion (or tendency of motion) between two surfaces in contact. It arises due to the interlocking of irregularities and, in some cases, due to adhesion between surfaces. Friction can be helpful (walking, writing, gripping) and harmful (wear and tear, heat, energy loss).
Static friction
Sliding (kinetic) friction
Rolling friction
Fluid friction (drag)
Factors affecting friction
Remember: For the same pair of surfaces and normal force, typically: static friction > sliding friction > rolling friction.
🔑 Key Points
- Friction opposes motion or the attempt to move.
- Origin: microscopic irregularities and interlocking of surfaces; also adhesion.
- Types: static (objects at rest), sliding/kinetic (surfaces slide), rolling (wheels/rollers), fluid friction (drag) in air/water.
- Factors: nature of surfaces (roughness), normal reaction (how hard surfaces press), presence of lubricants, speed/shape in fluids.
- Useful effects: walking, holding objects, writing with pencil, nails/screws grip.
- Harmful effects: wear of parts, heating, wastage of energy, noise.
- Methods to reduce friction: lubrication, polishing, ball bearings, air‑cushion, streamlined shapes.
- Methods to increase friction: treads on tyres, soles with grooves, chalk on hands, roughening surfaces.
📐 Concepts & Relations (Idea Level)
| Concept | Relation / Notes | Implication |
|---|---|---|
| Limiting static friction | Maximum friction just before motion starts | Requires larger force to start motion |
| Sliding (kinetic) friction | Acts during motion; usually less than limiting static | Easier to keep moving than to start |
| Rolling friction | Much smaller than sliding | Use wheels/ball bearings to reduce friction |
| Fluid friction (drag) | Depends on speed, shape, and fluid nature | Streamlining reduces drag |
Approx idea: Frictional force for dry surfaces is roughly proportional to how hard the surfaces are pressed together (normal reaction). Coefficient of friction (μ) is a ratio (no unit) used in higher classes.
🧪 Everyday Examples
| Situation | Type | Key Idea |
|---|---|---|
| Walking without slipping | Static | Foot and ground grip due to friction |
| Writing with a pencil | Sliding | Friction leaves graphite on paper |
| Ball bearings in fans | Rolling | Rolling reduces friction vs sliding |
| Car design with smooth curves | Fluid drag | Streamlining reduces air resistance |
| Chalk on hands for gymnastics | Increased friction | Dry surface gives better grip |
📊 Comparison Table
| Type | When it acts | Magnitude (typical) | Example |
|---|---|---|---|
| Static friction | Before motion starts | Highest (limiting value) | Box just about to move |
| Sliding friction | During sliding motion | Less than static | Block sliding on table |
| Rolling friction | During rolling | Least among three | Cart with wheels |
📚 30 Important Questions & Answers
Q1. What is friction?
A contact force that opposes relative motion between two surfaces in contact.
Q2. Name the main types of friction.
Static, sliding (kinetic), rolling, and fluid friction (drag).
Q3. Why does friction arise?
Due to interlocking of microscopic irregularities and adhesion between surfaces.
Q4. Which is larger: static or sliding friction?
Static (limiting) friction is generally larger than sliding friction.
Q5. Which type of friction is the least?
Rolling friction is usually the least.
Q6. Give two advantages of friction.
It enables walking and allows gripping/holding objects.
Q7. Give two disadvantages of friction.
Causes wear and tear; produces heat and wastes energy.
Q8. State two ways to reduce friction.
Use lubricants; use ball bearings or rollers.
Q9. State two ways to increase friction.
Make surfaces rough; use grooved/treaded tyres or chalk on hands.
Q10. What is fluid friction?
The frictional force (drag) experienced by objects moving through fluids (liquids or gases).
Q11. How does speed affect fluid friction?
Drag increases as speed increases.
Q12. What is streamlining?
Designing shapes with smooth curves to reduce fluid resistance.
Q13. Why are ball bearings used?
They replace sliding with rolling friction, which is much smaller.
Q14. Why do bicycle brakes have rough pads?
To increase friction and stop the wheel effectively.
Q15. Why does a matchstick light when rubbed on the box?
Friction produces heat that ignites the chemicals on the match head.
Q16. What happens to friction if surfaces are polished?
Polishing reduces irregularities and thus reduces friction.
Q17. Give one example where friction is undesirable.
In machine parts where it causes wear and energy loss.
Q18. Give one example where friction is desirable.
While walking or running, we need friction to push against the ground.
Q19. What is limiting friction?
The maximum static friction just before the object starts moving.
Q20. Does friction depend on the area of contact (rough idea)?
For dry rigid surfaces, friction mainly depends on how hard surfaces are pressed (normal force) and nature of surfaces; area has a minor role.
Q21. Why do tyres have treads?
Treads increase friction, improving grip and preventing skidding.
Q22. Why is it difficult to move a heavy box initially but easier to keep it moving?
Overcoming higher static friction is harder; once moving, sliding friction is lower.
Q23. What is drag in air and water called?
Air resistance in air; water resistance in water—both are fluid friction.
Q24. Name a device that uses air cushion to reduce friction.
Hovercraft (idea level).
Q25. How does chalk help gymnasts and rock climbers?
Chalk absorbs moisture, increases roughness, and improves grip (more friction).
Q26. Why are oiling and greasing done in machines?
Lubricants form a thin film between surfaces, reducing direct contact and friction.
Q27. Which is more: rolling friction or sliding friction? Why use wheels?
Sliding friction is more; wheels convert sliding into rolling, reducing friction.
Q28. State one everyday example of fluid friction.
A cyclist feels air pushing against them while riding fast.
Q29. Why do ships and airplanes have streamlined shapes?
To reduce drag and move efficiently through water/air.
Q30. A crate needs a 120 N pull to just start moving on a floor. While moving, only 90 N is needed to keep it moving at steady speed. What does this show?
Limiting static friction (120 N) is greater than sliding friction (90 N).