Friction Class 8 Notes: CBSE Science Chapter 9

Friction: The force that opposes the relative motion between two surfaces of objects when they come in contact. Frictional force always acts in a direction opposite to the direction of applied force.

What are the causes of Friction?

Frictional force comes into play between two surfaces due to irregularities on the surfaces of the objects coming in contact, causing interlocking of micro-level irregularities between the two surfaces and ploughing of harder surfaces into smoother surfaces.

Force of Friction

• Opposes Motion: Friction is a force that opposes the motion of one surface moving over another.

• Depends on Surface: The roughness or smoothness of surfaces affects the amount of friction between them.

• Acts in Opposite Direction: Friction always acts in the opposite direction of the movement, slowing objects down.

• Generates Heat: When two surfaces rub against each other, friction can produce heat.

• Varies with Weight: The heavier an object, the more friction it experiences due to the increased contact force between surfaces.

Factors Affecting Friction:

a. The roughness of the surface is present.

b. The extent by which the two surfaces can be pressed together.

c. Nature of the surfaces coming in contact.

Friction is a necessary evil. why? 

Friction has advantages so considered a friend but due also has some disadvantages,  then it is a foe. Depending on the circumstance, friction could be of help or could be a problem. Therefore, it is a necessary evil.

How can we Increase Friction?

a. We can increase friction by pressing the surfaces together very strongly. For example, when we apply brakes while riding a bicycle or car, the brake pads are pressed against a moving part of the wheel which leads to increased friction force.

b. Friction can also be increased by increasing the roughness of the surfaces which are coming in contact. For example, the threading of shoes and tires is done to increase friction which causes its movement on the road without slipping.

How can we reduce friction?

1. Frictional force acting between the sliding surfaces of two objects can be reduced by making the surfaces in contact smooth by polishing them.

2. Sliding friction between the moving parts of vehicles and machinery can be reduced by applying oil, grease, graphite or any other lubricant. The rolling frictional force is usually smaller than the sliding frictional force. Hence, sliding friction is generally replaced by rolling friction by using rollers, like ball bearings are used between the hub and the axles in the moving parts of machines and vehicles.

3. Friction can be reduced by providing wheels, hence used in suitcases, and school bags of kids.

Wheels Reduce Friction:

a. Static Friction: When a body is at rest, the force of friction acting on it is known as static friction. This force is always equal and opposite to the applied force on the object. The force which is acting, when the body is just at the point of sliding on the surface is known as limiting friction.

b. Sliding friction: The frictional force which opposes the sliding motion between two surfaces that are in contact. Sliding friction is generally smaller than static friction.

c. Rolling Friction: The frictional force which is present between the two surfaces when one body rolls over the other body. The rolling frictional force is usually smaller than the sliding frictional force.

Fluid Friction

• Air, water, and other fluids exert frictional force (drag) on objects moving through them.

• The frictional force depends on the object's speed, shape, and the nature of the fluid.

• Objects moving through fluids lose energy due to friction, so efforts are made to reduce this friction.

• Special shapes, inspired by nature (birds and fish), are designed to minimise energy loss due to drag.

• Vehicles, including aeroplanes, are designed with shapes that reduce fluid friction for better efficiency.

• The body of aeroplanes, boats, fishes, and birds can move through fluids because of their streamlined shape, to reduce the friction due to fluid and avoid any energy loss.

Effects of Friction:

a. Friction generates heat.

b. Friction is responsible for causing wear and tear in tyres. 

c. Frictional force always opposes motion.

Advantages of Friction:

a. It is due to friction between pen and paper which allows us to write on paper.

b. Frictional force between our feet and the ground allows our movements like standing, walking, and running.

c. Frictional force between the surface of the road and the tyres of our vehicles allows the vehicles to move on the road without slipping.

Disadvantages of Friction:

a. Due to friction, moving objects tend to stop or slow down as their motion is opposed.

b. Due to friction heat is generated leading to wastage of energy in machines.

c. Friction is responsible for the wear and tear of moving parts of machinery, soles of shoes, and tyres on the road after some time.

5 Important Topics of Science Class 8 Chapter 9 you shouldn’t Miss!

Importance of Class 8 Science Chapter 9 Notes

• They explain the concept of friction in simple terms, helping students understand how friction works in daily activities like walking and using machines.

• The notes highlight different types of friction such as static, sliding, and rolling, making it easier for students to differentiate between them.

• They provide clear explanations of how factors like surface type and force affect friction, helping students grasp the science behind them.

• These notes assist in exam preparation by organising key points and examples, making revision more efficient.

• They help students relate friction to real-life applications, deepening their understanding of the concept.

Tips for Learning the Class 8 Science Chapter 9

• Understand the concept of friction by relating it to everyday activities like walking, writing, and using brakes.

• Memorise the types of friction: static, sliding, and rolling friction, and know examples of each.

• Focus on factors affecting friction, such as the type of surface and the force applied, and how they change friction.

• Learn about ways to reduce and increase friction, using examples like lubricants and rough surfaces.

• Practice diagrams showing the effects of friction, such as forces acting on an object.

• Solve questions at the end of the chapter to apply the concepts you have learned.

• Summarise key points in your own words for better retention and quick revision before exams.

Conclusion 

The Revision Notes for Class 8 Science Chapter 9, "Friction," provide a clear and simple explanation of the concept of friction and its role in our daily lives. The Notes cover key topics such as types of friction, factors affecting friction, and their practical applications. Friction Class 8 Notes help students understand how friction works in various situations and its importance in real-life applications. They are a valuable resource for exam preparation, ensuring students grasp the core concepts and perform well.

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