Friction Class 8 Notes Science Chapter 9

Ever wondered why a ball eventually stops rolling on the ground? The answer lies in the concept of friction. Whether it's a car's brakes, a bicycle slowing down, or the slipperiness of a banana peel, the interaction between objects and surfaces determines their motion. A Man Falls Down When He Steps on a Banana Peel 

Force of Friction

• When a force is applied, frictional force always acts in the opposite direction, resisting the object's motion.
• In activities like a book on a table, friction between surfaces creates a force that slows down the book.

Friction isn't the same for all surfaces, it changes.

• Smooth surfaces have less friction, making sliding easier.
• Rough surfaces generate more friction, making it harder for things to move.
• The force of friction depends on how smooth or rough the surfaces are in contact. 

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Factors Affecting Friction

• Nature of Surfaces: The type of surfaces in contact is crucial. Smoother surfaces generally have less friction compared to rough surfaces.
• Weight of the Object: Friction is influenced by the weight or mass of an object. Heavier objects often encounter greater friction than lighter ones.
• State of Surface: The condition of a surface, whether dry, wet, or oily, can impact friction. Dry surfaces usually exhibit more friction than wet or oily ones.
• Roughness of Surface: The roughness of surfaces in contact directly affects friction. Irregular or rough surfaces generate more friction.
• Applied Force: The force applied to an object influences the frictional force. As the applied force increases, so does the friction opposing the motion.
  
  Question for Short Notes - Friction

  Try yourself:

  What determines the force of friction between two surfaces?

  • A.

    The speed at which the objects are moving.
  • B.

    The weight or mass of the objects.
  • C.

    The color of the surfaces in contact.
  • D.

    The temperature of the surrounding environment.

  Explanation

  - The force of friction between two surfaces is determined by the weight or mass of the objects.
  - Heavier objects often encounter greater friction than lighter ones.
  - This is because the weight of an object increases the normal force between the surfaces, resulting in a stronger frictional force.
  - Therefore, the weight or mass of the objects affects the force of friction.

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  Friction : A Necessary Evil 
  

  Consider holding a kulhar (earthen pot) versus a glass tumbler. Friction plays a role in making the tumbler easier to hold.
• Surface Condition: Greasiness or a film of cooking oil on a glass tumbler affects holdability, highlighting the influence of friction.
• Importance of Friction: Imagine attempting to hold a glass without any friction; it would be challenging or impossible.
• Mobility: Moving on a wet muddy track or wet marble floor is difficult due to reduced friction. Friction is crucial for walking.
• Writing: Writing with a pen or pencil relies on friction. Chalk on a blackboard sticks due to friction between the chalk and the board.
• Motion Control: Friction is necessary for starting, stopping, and changing the direction of motion for automobiles on the road.
• Construction: Building construction involves the use of friction, from fixing nails in walls to tying knots.
• Drawback of Friction: Friction wears out materials like screws, ball bearings, and shoe soles, leading to wear and tear.

 Shoe Soles Lead to Wear and Tear Due to Friction 

• Heat Generation: Friction can produce heat, as felt when rubbing palms together or striking a matchstick against a rough surface.

 Striking a Matchstick

• Energy Wastage: Machines operating with friction generate heat, resulting in energy wastage. Minimizing friction is essential.

Increasing and Reducing Friction

When Increasing friction is necessary?

• Shoe Design: The grooves on shoe soles enhance grip on the floor, promoting safe movement.

Grooves on Shoe

• Tyres and Brakes: Treaded tyres on vehicles increase ground grip, and brake pads intentionally increase friction for effective braking.

Treaded tyres

• Sports Techniques: Kabaddi players use soil on hands for a better grip, and gymnasts apply substances to enhance friction for improved performance.
  
  Kabaddi Player using Soil 

When Increasing friction is necessary?

• Carrom Board and Hinges: Sprinkling fine powder on a carrom board and applying oil on door hinges reduce friction, enabling smoother movements.

Fine Powder on Carrom

• Mechanical Lubrication: Using grease, oil, or graphite between moving parts in machines forms a thin layer, preventing direct rubbing and ensuring smooth movement.

Mechanical Lubrication

• Lubricants: Substances like oil, grease, and graphite that reduce friction are called lubricants.
• Air Cushion: In some cases, an air cushion is used between moving machine parts to minimize friction.
  Air Cushion Used in Ship

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Wheels Reduce Friction 

Rolling Luggage attachés and luggage pieces with rollers make it easy for even a child to pull them, reducing the resistance to motion.

Rolling Luggage

• Rolling Friction: When one body rolls over another, the resistance to motion is termed rolling friction.
• Advantages of Rolling: Rolling is easier than sliding a body over another, making luggage with rollers convenient to pull.
• Wheel Invention: The wheel, considered one of mankind's greatest inventions, is efficient due to reduced rolling friction.

Wheel Invention

• Comparison of Frictions: Rolling friction is smaller than sliding friction, making rolling preferable.
• Ball Bearings: To further reduce friction, machines use ball bearings. Examples include ceiling fans and bicycles, where ball bearings are used between hubs and axles.
  Ball Bearing

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Fluid Friction

• Fluids: In science, gases and liquids are referred to as fluids. Frictional force exerted by fluids on objects in motion through them is called drag.
• Factors Affecting Drag: The frictional force (drag) in a fluid depends on the object's speed with respect to the fluid, as well as the shape of the object and the nature of the fluid.
• Energy Loss: Objects moving through fluids must overcome friction, resulting in energy loss. Efforts are made to minimize friction by giving objects special shapes.
• Nature-Inspired Designs: Scientists take hints from nature, observing birds and fishes, which have evolved shapes to minimize energy loss in overcoming fluid friction.
• Vehicle Designs: All vehicles, including airplanes, are designed with shapes that reduce fluid friction, drawing inspiration from natural forms to enhance efficiency.

Airplane Design