Class 8 Science Notes Chapter 12 (Friction) – Science Book

Dear Reader, let's get straight into the important points for Chapter 12: Friction. Remember, understanding the 'why' behind things is just as crucial as remembering the 'what' for your exams.

Chapter 12: Friction - Detailed Notes for Exam Preparation (Based on NCERT Class 8 Science)

1. What is Friction?

• Definition: Friction is a force that opposes motion (or attempted motion) between surfaces in contact.
• Direction: It always acts in the direction opposite to the direction of motion or the direction in which motion is trying to occur.
• Origin: It arises when two surfaces slide (or try to slide) over each other.

2. Cause of Friction:

• Microscopic View: Friction is primarily caused by the interlocking of irregularities (tiny hills and valleys) present on the surfaces in contact.
• Even Smooth Surfaces: Even surfaces that appear very smooth have minute irregularities visible only under a microscope.
• Interlocking: When one surface moves or tries to move over another, these irregularities get caught or 'lock' into each other, resisting the movement. The stronger the interlocking, the greater the friction.

3. Factors Affecting Friction:

• Nature of the Surfaces in Contact:
  • Rougher surfaces have larger irregularities, leading to stronger interlocking and thus, greater friction. (Example: Moving a box on sandpaper vs. a polished floor).
  • Smoother 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 deeper the irregularities interlock.
  • Therefore, friction increases if the surfaces are pressed harder. (Example: It's harder to push a heavy box than a light box of the same material, because the heavy box presses down harder on the floor).
• Important Note: Friction is largely independent of the area of contact, as long as the force pressing the surfaces together remains the same. (This might seem counter-intuitive, but it's due to how pressure distributes).

4. Types of Friction:

This is a very important section for exams!

• (a) Static Friction:

  • Definition: 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.
  • Nature: It is a self-adjusting force. It increases as the applied force increases, up to a certain maximum limit.
  • Example: Pushing a heavy table that doesn't move initially. The force you apply is balanced by static friction.
  • Limiting Friction: The maximum value of static friction that can be overcome just before an object starts moving.

• (b) Sliding Friction (Kinetic Friction):

  • Definition: The friction that comes into play when one surface slides over another.
  • Magnitude: Sliding friction is slightly less than the maximum static friction (limiting friction). This is why it's often easier to keep an object moving than to start it moving from rest.
  • Example: The friction acting on the table once it starts sliding across the floor.

• (c) Rolling Friction:

  • Definition: The friction that comes into play when one body rolls over the surface of another body (like wheels, ball bearings).
  • Magnitude: Rolling friction is much smaller than both static and sliding friction. This is because rolling replaces the interlocking of large surface areas with much smaller contact points/areas, reducing resistance significantly.
  • Example: Pushing a suitcase with wheels compared to dragging the same suitcase without wheels.

• Comparison (Crucial!):
  Static Friction (up to its limit) > Sliding Friction > Rolling Friction

5. Friction: A Necessary Evil

Friction has both advantages and disadvantages.

• Advantages (Why it's Necessary):

  • Allows us to walk (friction between shoes/feet and ground).
  • Enables writing (friction between pen/pencil and paper).
  • Allows vehicles to move and stop (friction between tires and road; friction in brakes).
  • Helps in holding objects.
  • Used to light a matchstick.
  • Transmission of power using belts in machines.

• Disadvantages (Why it's an Evil):

  • Opposes motion, requiring more energy to move objects.
  • Causes wear and tear of moving parts (like soles of shoes, tires, machine parts).
  • Produces heat (e.g., rubbing palms, machine parts getting hot), which often represents wasted energy and can damage components.
  • Reduces the efficiency of machines.

6. Methods of Increasing Friction:

Sometimes, we need more friction.

• Making surfaces rougher:
  • Treads on tires (cars, bicycles) provide better grip.
  • Soles of shoes are grooved for better grip.
  • Brake pads in vehicles are made of materials that create high friction.
• Applying substances: 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 and reduce wear.

• Polishing Surfaces: Makes surfaces smoother, reducing irregularities.
• Lubrication:
  • Applying substances called lubricants (like 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 in some cases (e.g., air hockey table).
• Using Wheels or Ball Bearings:
  • These devices convert sliding friction into much smaller rolling friction.
  • Used in vehicles, luggage, fans, bicycles, and many machines. Ball bearings consist of small metal balls placed between moving parts.

8. Fluid Friction (Drag):

• Definition: Fluids (liquids and gases) also exert a frictional force on objects moving through them. This is often called drag.
• Factors Affecting Fluid Friction:
  • Speed: Fluid friction increases significantly with the speed of the object moving through the fluid.
  • Shape of the Object: Objects with streamlined shapes experience less fluid friction. This shape is pointed in the front and tapers towards the back (like a teardrop). Examples: bodies of fish, birds, airplanes, racing cars. Nature often provides streamlined shapes for efficient movement in fluids.
  • Nature of the Fluid: Denser or more viscous fluids generally exert more friction. (Example: Moving your hand through water vs. air).

Key Takeaways for Exams:

• Know the definitions of friction and its types precisely.
• Understand the cause of friction (interlocking).
• Remember the factors affecting friction (nature of surface, force pressing them).
• Memorize the comparison: Static > Sliding > Rolling.
• Be able to list advantages and disadvantages (necessary evil concept).
• Know the common methods for increasing and decreasing friction with examples (lubrication, wheels, streamlining).
• Understand fluid friction (drag) and the factors affecting it, especially the importance of streamlined shapes.

Study these points well. Focus on understanding the concepts so you can apply them to different scenarios presented in questions. Good luck!