Friction notes by Abhinav Sir

CBSE Class 8 Science: Friction Notes & Questions

Comprehensive Notes for Chapter 9 (NCERT) / Chapter 12 (S. Chand)

By Abhinav Sir

1. Introduction to Friction

Friction is the force that opposes the relative motion or tendency of motion between two surfaces in contact. It acts opposite to the direction of motion or applied force and is measured in Newtons (N). Friction arises due to the interlocking of microscopic irregularities on surfaces and is essential for activities like walking, driving, and writing.

Examples:

• Friction between vehicle tyres and the road allows movement without slipping.
• Friction between a pen and paper enables writing.
• Rubbing hands together generates heat due to friction.

2. Causes of Friction

Friction is caused by:

• Surface Irregularities: Even seemingly smooth surfaces have microscopic ridges and grooves that interlock, creating friction.
• Molecular Adhesion: Attraction between molecules of different materials at contact points.
• Ploughing Effect: Harder surfaces may plough into softer ones, increasing friction.
• Deformations: Surfaces deform under pressure, adding to frictional resistance.

3. Factors Affecting Friction

The force of friction on:

• Nature of Surfaces: Rough surfaces (e.g., sandpaper) produce more friction than smooth surfaces (e.g., glass).
• Mass/Weight: Heavier objects press surfaces together with greater force, increasing friction.
• Normal Force: The perpendicular force pressing surfaces together increases friction.
• Surface Area: Friction is generally independent of contact area for solid surfaces, but actual contact points matter.
• Lubrication: Wet or lubricated surfaces (e.g., soapy water) reduce friction by minimizing interlocking.

4. Types of Friction

There are four main types of friction:

4.1 Static Friction

The force that prevents an object at rest from moving. It is the strongest type of friction and acts until the applied force exceeds the limiting friction.

Examples:

• A cup on a table.
• A car parked on a hill.
• Scenery hanging on a wall.

Note: Limiting friction is the maximum frictional force just before motion begins.

4.2 Sliding Friction

The force that opposes an object sliding over another surface. It is less than static friction due to less interlocking time.

Example: A log of wood sliding on a flat surface.

4.3 Rolling Friction

The force that opposes an object rolling over another surface. It is the weakest type of friction, as rolling reduces contact area.

Examples:

• Ball bearings in machinery.
• Skateboards or bicycle wheels.

4.4 Fluid Friction (Drag)

The frictional force exerted by fluids (liquids or gases) on objects moving through them, also called drag or air resistance.

Example: An aeroplane experiencing air resistance at high speed.

Factors Affecting Fluid Friction:

• Speed of the object relative to the fluid.
• Size and shape of the object.
• Nature of the fluid (e.g., water is more viscous than air).

5. Friction: A Necessary Evil

Friction is termed a "necessary evil" because it is essential for many activities but also causes inefficiencies.

5.1 Advantages of Friction

• Enables walking by providing grip between feet/shoes and the ground with grooved soles.
• Allows vehicles to move without slipping due to tyre-road friction.
• Facilitates writing by enabling grip between pen/pencil and paper.
• Generates heat (e.g., rubbing hands, lighting matchsticks).
• Holds nails in walls and supports ladders against walls.
• Assists in braking vehicles and parachuting safely.

5.2 Disadvantages of Friction

• Produces heat, damaging machines and wasting energy.
• Causes wear and tear of machine parts, tyres, and shoe soles.
• Resists motion, requiring more energy to move objects.
• Can cause forest fires due to friction between tree branches.
• Increases fuel consumption in vehicles.

6. Ways to Reduce Friction

To minimize friction and improve efficiency:

• Lubricants: Apply oil, grease, or polish to reduce surface interlocking (lubrication).
• Wheels and Rollers: Convert sliding friction to rolling friction (e.g., using a dolly for heavy boxes).
• Ball Bearings: Reduce friction in machinery by enabling rolling motion.
• Streamlined Shapes: Minimize fluid friction for objects moving through air or water (e.g., aeroplanes, fish).
• Polishing Surfaces: Smooth surfaces reduce irregularities and friction.
• Powder on Surfaces: Sprinkling powder (e.g., on a carrom board or dholak) reduces friction.

7. Ways to Increase Friction

To enhance grip or control:

• Roughen Surfaces: Increase surface roughness (e.g., sand on icy roads).
• Increase Mass/Weight: Heavier objects press surfaces together, increasing friction.
• Treads on Tyres: Patterns on vehicle tyres expel water and enhance grip.
• Grooved Soles: Shoe soles with grooves provide better traction.
• Brake Shoes: Used in vehicles to increase friction for stopping.

8. Spring Balance

A spring balance, invented by Christian Huygens, is a device used to measure the force or weight of an object. It consists of a coiled spring that stretches when a force is applied, with a pointer on a graduated scale indicating the force in Newtons. It is commonly used in markets to measure weight.

9. Practice Questions and Answers

9.1 S. Chand Questions

1. Name the force which always opposes motion.

Answer: Friction.

2. Out of sliding friction, static friction, and rolling friction: (a) Which one is the smallest? (b) Which one is the largest?

Answer: (a) Rolling friction is the smallest. (b) Static friction is the largest.

3. Which type of friction comes into play when a book kept on cylindrical pencils is moved by pushing?

Answer: Rolling friction.

4. Why is it more difficult to walk properly on a well-polished floor?

Answer: A well-polished floor is smooth, offering less friction, which reduces grip and makes walking difficult.

5. Why is it difficult to walk on a wet marble floor?

Answer: A wet marble floor has reduced friction due to a thin film of water acting as a lubricant, making it slippery.

6. When we try to push a very heavy box kept on the ground, it does not move at all. Which force is preventing this box from moving forward? Where does this force act?

Answer: Static friction prevents the box from moving. It acts at the contact surface between the box and the ground, in the direction opposite to the applied force.

7. Suppose your writing table is tilted a little. A book kept on the table starts sliding down. Show the direction of force of friction acting on the book.

Answer: The frictional force acts upward, opposite to the downward sliding motion of the book.

Diagram: Draw a tilted table with a book sliding downward. Indicate the frictional force (f) as an arrow pointing upward along the table surface, opposing the direction of motion.

8. Which will cause more friction: a rough surface or a smooth surface? Why?

Answer: A rough surface causes more friction because it has more irregularities that interlock with the other surface, increasing resistance compared to a smooth surface.

9. What is the cause of friction? Explain with the help of a labelled diagram.

Answer: Friction is caused by the interlocking of microscopic irregularities on surfaces in contact, molecular adhesion, ploughing effects, and deformations. When two surfaces (e.g., a book and a table) are in contact, their irregularities interlock, resisting motion.

Diagram: Show two surfaces with magnified irregular ridges and grooves interlocking. Label the applied force (F) moving the book and the frictional force (f) opposing it in the opposite direction.

10. Give examples to show that friction depends on the nature of two surfaces in contact.

Answer:

• A ball rolls farther on a smooth tiled floor (less friction) than on a rough carpet (more friction) due to fewer interlocking irregularities.
• Pushing a box on a concrete floor (rough, high friction) requires more force than on a polished wooden floor (smooth, low friction).

11. The friction between the brake pads and discs will be the maximum in the car which travels the distance of: (A) 5 m, (B) 5.5 m, (C) 4.8 m, (D) 5.2 m.

Answer: (C) 4.8 m. The car that stops in the shortest distance (4.8 m) experiences the maximum friction between brake pads and discs, as higher friction reduces stopping distance.

12. The weight of an object can be measured by a: (A) beam balance, (B) analytical balance, (C) spring balance, (D) physical balance.

Answer: (C) spring balance. A spring balance measures the weight (force) of an object in Newtons.

9.2 NCERT Questions

13. Fill in the blanks:

a. Friction opposes the ________ between the surfaces in contact with each other.
b. Friction depends on the ________ of surfaces.
c. Friction produces ________.
d. Sprinkling of powder on the carrom board ________ friction.
e. Sliding friction is ________ than static friction.

Answers:

• a. Relative motion
• b. Nature (smoothness or roughness)
• c. Heat
• d. Reduces
• e. Less

14. Four children were asked to arrange forces due to rolling, static, and sliding frictions in decreasing order. Choose the correct arrangement:

a) Rolling, static, sliding
b) Rolling, sliding, static
c) Static, sliding, rolling
d) Sliding, static, rolling

Answer: (c) Static, sliding, rolling. Static friction is the strongest, followed by sliding, and rolling is the weakest.

15. Alida runs her toy car on a dry marble floor, wet marble floor, newspaper, and towel spread on the floor. The force of friction acting on the car in increasing order will be:

a) Wet marble floor, dry marble floor, newspaper, towel
b) Newspaper, towel, dry marble floor, wet marble floor
c) Towel, newspaper, dry marble floor, wet marble floor
d) Wet marble floor, dry marble floor, towel, newspaper

Answer: (a) Wet marble floor, dry marble floor, newspaper, towel. Friction increases with surface roughness; wet surfaces reduce friction due to lubrication.

16. Explain why objects moving in fluids must have special shapes.

Answer: Objects moving through fluids (e.g., air, water) experience fluid friction (drag). Streamlined shapes, with pointed fronts and tapered rears, minimize resistance by reducing turbulence and allowing smooth fluid flow. Examples include aeroplanes and fish.

9.3 Additional Questions

17. Why is sliding friction less than static friction?

Answer: Static friction is greater because, when an object is at rest, surface irregularities have more time to interlock deeply, requiring a larger force to overcome. In sliding friction, surfaces are in motion, and interlocking is less due to limited contact time, requiring less force.

18. Give examples to show that friction is both a friend and a foe.

Answer:

• Friend:
• Friction enables walking by providing grip.
• Friction allows vehicles to stop when brakes are applied.
• Friction helps write by gripping the pen to paper.
• Foe:
• Friction causes wear and tear in machine parts and tyres.
• Friction produces heat, leading to energy loss in engines.
• Friction resists motion, requiring more force to move objects.