Question 3
Alida runs her toy car on dry marble floor, wet marble floor, newspaper and towel spread on the floor. The force of friction acting on the car on different surfaces in increasing order will be :
- wet marble floor, dry marble floor, newspaper and towel
- newspaper, towel, dry marble floor, wet marble floor
- towel, newspaper, dry marble floor, wet marble floor
- wet marble floor, dry marble floor, towel, newspaper
wet marble floor, dry marble floor, newspaper and towel.
Reason — The force of friction depends on several factors, including the roughness of the surfaces and presence of moisture.
Movement on towel offers high friction because its surface is usually rough, providing significant resistance to motion.
Newspaper is smoother than a towel but still provides some resistance due to its texture.
Dry marble is smoother than newspaper, so it has less friction compared to the previous two surfaces.
Wet marble has the least friction among the listed surfaces because water acts as a lubricant, reducing the friction between the car's tires and the surface of the wet marble.
Hence, the order of increasing friction will be : wet marble floor < dry marble floor < newspaper < towel.
Chapter Overview: Friction
Building on force concepts, this chapter explores friction in detail: its causes (surface irregularities), types (static, sliding, rolling, fluid), factors affecting it, and its role as both helpful and harmful. Methods to increase and reduce friction are practical exam topics.
Board Exam Weightage: 4-6 marks | Difficulty: Moderate
Key Formulas & Concepts
| Concept | Details |
|---|---|
| Friction Order | Static friction > Sliding friction > Rolling friction |
| Cause | Interlocking of microscopic surface irregularities |
| Factors | Nature of surfaces (rough/smooth) and weight of object |
| Streamlining | Shape that reduces fluid friction (used in fish, birds, aircraft, cars) |
Must-Know Concepts
- Friction opposes relative motion between surfaces in contact
- Even smooth surfaces have microscopic irregularities that cause friction
- Friction does NOT depend on the area of contact
- Static friction prevents a stationary object from moving; sliding acts during motion; rolling acts on rolling objects
- Friction is a necessary evil: essential for walking, writing, braking; but causes wear, heat, energy waste
- Ball bearings convert sliding friction to rolling friction in machines
Types of Friction
| Type | When It Acts | Magnitude | Example |
|---|---|---|---|
| Static | Object at rest | Highest | Book on a tilted desk |
| Sliding | Object sliding | Less than static | Pushing a box along the floor |
| Rolling | Object rolling | Least (among solids) | Ball rolling, car wheels |
| Fluid (Drag) | Object in fluid | Depends on speed/shape | Swimming, parachute, airplane |
Common Mistakes to Avoid
- Thinking friction only has disadvantages (it is essential for many activities)
- Confusing static and sliding friction
- Believing that smooth surfaces have zero friction (friction can be reduced but never eliminated)
- Forgetting about fluid friction in liquids and gases
Scoring Tips
- Always discuss both advantages AND disadvantages when answering about friction
- Make a two-column chart: methods to increase vs reduce friction
- Remember the order: static > sliding > rolling
- Connect streamlining to real-world design: fish, birds, planes, racing cars
Frequently Asked Questions
Q: How many marks does this chapter carry in the exam?
A: Approximately 4-6 marks in the annual exam.
Q: What question types are commonly asked?
A: MCQs (1 mark), Very Short Answer (2 marks), Short Answer (3 marks), and Long Answer / Diagram (5 marks).
Q: Is this chapter important for competitive exams?
A: Yes, concepts from this chapter appear in NTSE, Olympiad, and other science competitions.