Lecture on Friction - Class 8 Science

Introduction to Friction

• Friction is the force opposing motion.
• Examples:
  • A ball rolling down a slope eventually stops due to friction.
  • A cycle coasts to a stop if pedaling ceases.

Understanding Friction

• Friction enables walking by providing grip between feet and ground.
• Friction decreases on smooth surfaces (e.g., banana peel) leading to slipping.
• Friction is reduced on wet floors, causing slipperiness.
• Direction of friction is always opposite to the direction of motion.

Measuring Friction

• Spring balance measures force by the extent a spring stretches.
• Frictional force between surfaces can alter spring balance readings as surfaces smoothen (e.g., wrapping a brick in polythene).

Nature of Friction

• Friction arises from surface irregularities interlocking.
• More surface roughness increases friction due to better interlocking.

Static vs Sliding Friction

• Static friction: Opposes initial movement of an object, higher than sliding friction.
• Sliding friction: Opposes motion of a moving object, less than static friction.

Everyday Examples

• Grooves in shoes increase friction, aiding in movement.
• Smooth shoes reduce friction, increasing slip risk.
• Cabaddi players: Increase hand friction by rubbing them in the soil.
• Treaded tires: Increase friction to prevent slipping.

Methods to Decrease Friction

• Sprinkling powder on carrom boards: Reduces friction by smoothening surface.
• Lubricating machines: Grease reduces friction, minimizing wear and heat release.

Rolling Friction

• Rolling objects experience less friction than sliding ones.
• Ball bearings: Used in machinery to reduce friction.

Fluid Friction

• Air and water resist motion; this resistance is fluid friction or drag.
• Factors affecting fluid friction:
  • Speed of the object
  • Shape of the object (streamlined shapes minimize friction)
  • Nature of the fluid

Friction as a Necessary Evil

• Friction enables walking, writing, and various other daily activities.
• Disadvantages include wear and tear of shoes, tires, and machinery parts due to friction.

Conclusion

• Friction is both beneficial and problematic, necessitating strategies to either increase or decrease it based on specific needs.