Friction
Focused last-minute revision notes with clear concepts, exam traps, key comparisons and compact practice.
Chapter Snapshot
- Friction opposes relative motion or the tendency of surfaces to move.
- It arises mainly from microscopic irregularities and interactions between surfaces.
- Friction depends on the nature of surfaces and how strongly they are pressed together.
- Static friction prevents motion from starting; sliding friction acts during sliding; rolling friction acts during rolling.
- Rolling friction is usually less than sliding friction.
- Friction is useful for walking, writing, gripping and braking.
- Friction also causes heat, wear and energy loss.
- Lubricants, polishing and ball bearings reduce friction.
- Treads, spikes and rough surfaces increase friction when grip is needed.
- Fluids exert drag; streamlined shapes reduce drag.
What You Will Learn
- Explain why friction occurs
- Identify useful and harmful effects
- Differentiate static, sliding and rolling friction
- Explain ways to increase or reduce friction
- Explain fluid friction and streamlining
Last-Minute Chapter Map
Friction can be managed by changing surface contact or the way an object moves through a fluid.
1. What Is Friction?
Friction is a force that opposes relative motion or the tendency of one surface to move over another. Even surfaces that look smooth have microscopic irregularities that interact.
2. Factors Affecting Friction
Friction depends on the nature or roughness of the surfaces and how strongly they are pressed together. Rougher surfaces generally produce greater friction than smoother ones under similar conditions.
3. Static, Sliding and Rolling Friction
Static friction acts before sliding begins and adjusts up to a limit. Sliding friction acts when surfaces slide. Rolling friction acts when an object rolls and is generally smaller than sliding friction, which is why wheels are useful.
4. Friction Is Useful
Without friction we could not walk, grip objects, write with a pencil, tie knots or stop vehicles with brakes. Tyre treads and shoe soles are designed to provide sufficient friction.
5. Friction Can Be Harmful
Friction causes wear of machine parts and tyres, produces unwanted heat and wastes mechanical energy. Engineers reduce it where smooth motion is required.
6. Reducing Friction
Lubricants such as oil or grease form a layer between surfaces. Polishing reduces surface irregularities. Ball bearings replace sliding with rolling, which usually reduces friction.
7. Fluid Friction
Air and water also oppose motion; this resistance is called drag. Streamlined shapes reduce drag by allowing fluid to flow more smoothly around the object. Birds, fish, aircraft and fast vehicles show streamlined forms.
Types of Friction
| Type | When It Acts | Typical Relative Size |
|---|---|---|
| Static | Before sliding begins | Can be highest up to limiting value |
| Sliding | During sliding | Usually less than maximum static friction |
| Rolling | During rolling | Usually much less than sliding |
Exam Focus
Must Revise Before the Exam
- Cause of friction
- Factors affecting friction
- Static/sliding/rolling friction
- Advantages and disadvantages
- Lubricants
- Ball bearings
- Increasing grip
- Fluid friction and streamlining
Common Mistakes
Key Terms
Key Terms
Quick Revision
Quick Revision
- Friction opposes motion or attempted motion.
- Rougher surfaces usually give more friction.
- Static friction acts before sliding; sliding during sliding; rolling during rolling.
- Rolling friction is usually least.
- Increase friction for grip; reduce it in machines.
- Lubricants and ball bearings reduce friction.
- Air/water drag is fluid friction.
- Streamlining reduces drag.
Practice Questions
Which friction is usually smallest?
A. static
B. sliding
C. rolling
D. all always equal
View Solution
Correct Answer: C. rolling
Rolling friction is generally less than sliding friction.
Which device reduces friction by replacing sliding with rolling?
A. brake pad
B. ball bearing
C. sandpaper
D. spike
View Solution
Correct Answer: B. ball bearing
Ball bearings reduce friction by using rolling motion.
Tyre treads are designed mainly to:
A. reduce all friction
B. increase grip
C. remove wheels
D. cool the road
View Solution
Correct Answer: B. increase grip
Treads increase friction and improve grip.
Resistance offered by air or water is called:
A. drag
B. pressure only
C. magnetism
D. buoyancy only
View Solution
Correct Answer: A. drag
Fluid friction is commonly called drag.
Why is it difficult to walk on a smooth wet floor?
View Solution
Water reduces effective friction between the foot and floor, so there is less grip and a greater chance of slipping.
Why are wheels used to move heavy objects?
View Solution
Rolling friction is usually less than sliding friction, so wheels reduce the force needed to move the object.
Why are racing bicycles and aircraft designed with smooth streamlined shapes?
View Solution
Streamlined shapes reduce drag from air, allowing more efficient and faster motion.
Chapter Summary
Chapter Summary
- Friction opposes motion or attempted motion.
- Rougher surfaces usually give more friction.
- Static friction acts before sliding; sliding during sliding; rolling during rolling.
- Rolling friction is usually least.
- Increase friction for grip; reduce it in machines.
- Lubricants and ball bearings reduce friction.
- Air/water drag is fluid friction.
- Streamlining reduces drag.