Class 8 Science Notes Chapter 12 (Chapter 12) – Examplar Problems Book

Alright class, let's focus on Chapter 12: Friction, from your Science Exemplar book. This is an important topic, not just for your class exams, but also because concepts related to forces often appear in various government exams. Pay close attention to the details.

Chapter 12: Friction - Detailed Notes for Exam Preparation

1. What is Friction?

• Definition: Friction is a force that opposes the relative motion (or tendency of relative motion) between two surfaces in contact.
• Direction: It always acts in the direction opposite to the direction of motion or intended motion.
• Nature: It is a contact force, meaning it arises only when objects are physically touching.

2. Cause of Friction:

• Friction arises due to the interlocking of irregularities (microscopic hills and valleys) present on the surfaces in contact.
• Even surfaces that appear smooth have minute irregularities.
• When one surface moves or tries to move over another, these irregularities get interlocked, resisting the motion. The stronger the interlocking, the greater the friction.

3. Factors Affecting Friction:

• Nature of the Surfaces in Contact:
  • Rough surfaces have larger irregularities, leading to stronger interlocking and thus, greater friction.
  • Smooth surfaces have smaller irregularities, leading to weaker interlocking and less friction.
• Force Pressing the Surfaces Together (Normal Force):
  • The harder the surfaces are pressed together, the stronger the interlocking between irregularities.
  • Therefore, friction increases as the force pressing the surfaces together increases. (Example: It's harder to push a heavy box than a light one).
• Independence from Area of Contact (Generally): For solid surfaces, friction is largely independent of the area of contact, provided the normal force remains the same. (This is a simplification often used at this level).

4. Types of Friction:

• Static Friction:

  • The friction that comes into play when surfaces are at rest relative to each other, opposing the tendency of motion.
  • It prevents objects from starting to move.
  • It is a self-adjusting force; it increases as the applied force increases, up to a certain maximum limit.
  • Limiting Friction: The maximum value of static friction that must be overcome to start the motion.

• Sliding Friction (Kinetic Friction):

  • The friction that comes into play when one surface slides over another.
  • It acts opposite to the direction of sliding motion.
  • Sliding friction is generally less than static friction. This is why it's often easier to keep an object moving than to start it moving from rest.

• Rolling Friction:

  • The friction that comes into play when one object (like a wheel, cylinder, or sphere) rolls over the surface of another.
  • Rolling involves deformation of surfaces at the point of contact.
  • Rolling friction is much smaller than both static and sliding friction. This is the principle behind using wheels and ball bearings.

• Order of Magnitude: Static Friction (limiting) > Sliding Friction > Rolling Friction

5. Friction: A Necessary Evil

• Friction is often considered a 'necessary evil' because it has both advantages and disadvantages.
• Advantages (Why friction is necessary):
  • Allows us to walk and run (friction between shoes and ground).
  • Enables vehicles to move and stop (friction between tires and road, friction in brakes).
  • Allows us to write (friction between pen/pencil and paper).
  • Helps in holding objects.
  • Lighting a matchstick.
  • Transmission of power using belts.
• Disadvantages (Why friction is undesirable):
  • Wear and Tear: Causes rubbing surfaces to wear out (e.g., soles of shoes, tires, machine parts).
  • Energy Loss: Opposes motion, so extra energy is needed to overcome it. Much of this energy is wasted as heat.
  • Heat Generation: Can cause overheating in machinery, potentially leading to damage.
  • Reduces Efficiency: Lowers the efficiency of machines.

6. Methods of Increasing Friction:

• Sometimes, we need to increase friction for better grip or control.
• Making Surfaces Rougher:
  • Treads on tires provide better grip with the road.
  • Grooves on the soles of shoes prevent slipping.
  • Using brake pads with rough surfaces in vehicles.
  • Gymnasts apply coarse powder on their hands for a better grip.

7. Methods of Reducing Friction:

• Often, we need to reduce friction to improve efficiency, reduce wear and tear, and save energy.
• Polishing Surfaces: Makes surfaces smoother, reducing interlocking.
• Lubrication:
  • Applying substances called lubricants (e.g., oil, grease, graphite powder) between moving surfaces.
  • Lubricants form a thin layer, preventing direct contact and interlocking of irregularities. Air can also act as a lubricant (e.g., hovercraft).
• Using Wheels and Ball Bearings:
  • Wheels convert sliding friction into much smaller rolling friction (e.g., luggage with rollers).
  • Ball bearings are small metal spheres placed between moving parts of machines (like axles and hubs) to allow rolling instead of sliding, significantly reducing friction.
• Streamlining:
  • Giving objects special shapes (rounded in front, tapering at the back) to reduce fluid friction (drag). Discussed below.

8. Fluid Friction (Drag):

• Fluids (liquids and gases) also exert a frictional force on objects moving through them. This is called drag or fluid friction.
• Factors Affecting Fluid Friction:
  • Speed: Drag increases significantly with the speed of the object relative to the fluid.
  • Shape: The shape of the object greatly influences drag. Streamlined shapes experience less drag.
  • Nature of the Fluid: Denser or more viscous fluids exert greater drag.
• Streamlining: Objects that move through fluids frequently (like cars, airplanes, ships, submarines, birds, fish) have streamlined bodies to minimize drag and conserve energy.

Multiple Choice Questions (MCQs)

1. Friction is a force that:
   a) Aids motion between surfaces.
   b) Opposes relative motion between surfaces in contact.
   c) Acts perpendicular to the surfaces in contact.
   d) Only exists for rough surfaces.

2. Which of the following statements is INCORRECT?
   a) Static friction is self-adjusting.
   b) Sliding friction is less than static friction.
   c) Rolling friction is greater than sliding friction.
   d) Friction opposes the tendency of motion.

3. Four children were asked to arrange forces due to rolling, static and sliding frictions in decreasing order. Their arrangements are given below. Choose the correct arrangement:
   a) Rolling, Static, Sliding
   b) Rolling, Sliding, Static
   c) Static, Sliding, Rolling
   d) Sliding, Static, Rolling

4. The primary cause of friction between two surfaces is:
   a) The weight of the objects.
   b) The area of contact between surfaces.
   c) The interlocking of irregularities on the surfaces.
   d) The magnetic properties of the materials.

5. Applying oil or grease between moving parts of a machine helps to:
   a) Increase friction
   b) Convert static friction to sliding friction
   c) Reduce friction by forming a thin layer between surfaces
   d) Increase the wear and tear of parts

6. Which of the following is an example where friction is intentionally increased?
   a) Polishing a surface
   b) Using ball bearings in a bicycle hub
   c) Oiling the hinges of a door
   d) Making treads on vehicle tires

7. Fluid friction depends on:
   a) The speed of the object
   b) The shape of the object
   c) The nature of the fluid
   d) All of the above

8. Why are boats and aeroplanes given a streamlined shape?
   a) To increase friction
   b) To reduce fluid friction (drag)
   c) To make them look good
   d) To increase their weight capacity

9. A boy is pushing a heavy box which is not moving. The type of friction acting between the box and the floor is:
   a) Sliding friction
   b) Rolling friction
   c) Static friction
   d) Fluid friction

10. Which of the following is NOT an advantage of friction?
    a) Allows us to walk
    b) Causes wear and tear of machine parts
    c) Helps in braking vehicles
    d) Enables writing on paper

Answer Key for MCQs:

1. b
2. c
3. c
4. c
5. c
6. d
7. d
8. b
9. c
10. b

Study these notes carefully. Understand the definitions, the differences between the types of friction, and how we can manipulate friction to our advantage. Remember the concept of friction as a necessary evil and the specific examples. Good luck with your preparation!