CBSE Notes Class 8 Science Chapter 9 Friction

Friction is a key force that resists the relative motion between two surfaces in contact. It significantly impacts our daily activities, impacting everything from walking and driving to the operation of machinery. To fully grasp the concept of friction, it’s important to explore its various types—static, kinetic (or sliding), rolling, and fluid friction—each of which has unique properties and uses.

1.0Friction

• It is a force that resists the relative motion between two surfaces in contact. It consistently acts in the direction opposite to the applied force.
• It is always parallel to the surfaces in contact

2.0Causes of Friction

• Friction arises between two surfaces because of surface irregularities, the interlocking of microscopic features, and the way harder surfaces can dig into smoother ones.

3.0Factors Affecting Friction

• Surface roughness  
• The degree of contact between the two surfaces  
• Characteristics of the surfaces

Friction-A Necessary Evil: Friction can be seen as a friend because it offers advantages, but it also has its drawbacks, making it a foe at times. Depending on the situation, friction can either resist or obstruct us, which is why it's often viewed as a necessary evil.

Friction is useful in many cases:

1. Friction is required to hold vessels like glass tumblers.
2. Friction is useful for brakes and tyres.
3. You Could not tie a knot without friction.

4.0Increasing and Reducing Friction

• Increasing Friction: By applying greater pressure between the surfaces, such as when brakes are engaged on a cycle or car, the brake pads press against a moving part of the wheel, resulting in an increase in frictional force.
• Friction can be enhanced by increasing the roughness of the contacting surfaces. For instance, the treads on shoes and tires are designed to boost friction.
• Reducing Friction: Friction between the gliding surfaces of two objects can be minimized by smoothing the contact surfaces through polishing.
• Substances that decrease friction are known as lubricants.
• Sliding friction between the moving components of vehicles and machinery can be decreased by applying lubricants such as oil, grease, graphite, or other similar substances.
• Rolling friction is lower than sliding friction. Therefore, sliding friction is often substituted with rolling friction by using rollers, such as ball bearings, between the hub and axles in the moving parts of machines and vehicles.
• Friction is minimized by incorporating wheels, as seen in items like suitcases and children's school bags.
• Aeroplanes, boats, fish, and birds have specially shaped bodies known as streamlined shapes to minimize fluid friction and prevent energy loss during movement.

5.0Types of Friction

1. Static Friction: When an object is stationary, the frictional force is known as static friction and is always equal and opposite to the implemented force. The frictional force that occurs when the object is on the brink of sliding is referred to as limiting friction.
2. Sliding friction: The frictional force that resists the actual sliding motion between two contacting surfaces is known as sliding friction, which is generally less than static friction.
3. Rolling Friction: The frictional force that occurs between two surfaces when one body rolls over the other is called rolling friction, and it is typically less than sliding friction.

6.0Effects of Friction 

• Friction produces heat.
• Friction causes wear and tear.
• Friction opposes motion.

7.0Advantage and Disadvantage of Friction

Advantages of Friction

• Friction between a pen and paper enables us to write. Friction between our feet and the ground facilitates movements such as standing, walking, and running. Additionally, friction between the road surface and vehicle tires allows for movement without slipping.

Disadvantages of Friction

• Friction slows down or stops moving objects. It generates heat, leading to energy loss in machines. Additionally, friction contributes to the wear and tear of moving parts in machinery and the soles of shoes.

8.0Fluid Friction

• Fluids are the substance which can flow on application of force or pressure on them. Gases and Liquids are fluids.
• The force of friction exerted by the fluids on the objects moving through them is called fluid friction.
• The frictional force produced by fluids is known as drag.

Factors that affect the Fluid Friction:

• Speed of the object in the fluid
• Shape of the object moving in the fluid
• Nature of the Fluid.