NCERT Science Notes - Class 8
Chapter 9 - Friction

Welcome to AJs Chalo Seekhen. This webpage is dedicated to Class 8 | Science | Chapter 9 - Friction. The chapter explores the force that opposes motion between two surfaces in contact. It explains the different types of friction: static, sliding, and rolling, and how each type affects movement. The chapter discusses the factors influencing friction, such as the nature of surfaces and the force pressing them together. It also highlights the advantages and disadvantages of friction in daily life, from enabling us to walk without slipping to causing wear and tear on machinery. Understanding friction is essential for grasping its role in various real-world applications.

NCERT Science Notes - Class 8 Chapter 9 - Friction notes ajs, cbse notes class 10 ajslearning, cbse notes ajs, ajs notes class 10, ajslearning, ajs chalo seekhen

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NCERT Science Notes - Class 8
Chapter 9 - Friction

9.0 - Introduction

Movement of Objects:

You have learned in Class VII that objects can move at different speeds.
The distance an object moves in a unit of time indicates how fast it is moving.

Changes in Motion:

A moving object, like a rolling ball, can slow down, change direction, or both.
These changes in speed or direction happen in everyday activities.

Applying Force:

To move an object like a football, you kick or push it.
To make a ball move faster, you apply a greater force.
A goalkeeper stops a ball by applying force, and a hockey player changes its direction with a flick of the stick.
Fielders in cricket stop the ball by applying force as well.

Concept of Force:

Force is applied when an object is kicked, pushed, thrown, or flicked.
This chapter explores what force is and what it does to the bodies on which it is applied.

9.1 - Force of Friction

Understanding Friction through Activity 9.1

In this activity, you'll observe how friction works when you push a book on a table. Here's what happens:

Push the book gently in one direction. After moving a short distance, the book stops.
Now, push the book in the opposite direction. Does the book stop again? Yes, it does.

This shows that a force is acting on the book to oppose its motion, preventing it from continuing to move. This force is called friction.

Key Observations:

When you push the book to the left, friction acts to the right.
When you push the book to the right, friction acts to the left.

In both cases, friction always opposes the direction of motion. The friction in this case is happening between the surface of the book and the surface of the table. Now, does friction remain the same on all surfaces? Does it change if the surface is smooth or rough? These are questions we will explore further to understand how surface texture affects friction.

9.2 - Factors affecting Friction

Understanding Factors Affecting Friction through Activity 9.2

In this activity, you'll explore how different surfaces affect the force of friction. Here's what to do:

Tie a string around a brick and pull it using a spring balance (as shown in Fig. 9.3).
Note the reading on the spring balance when the brick just starts to move. This reading gives you an idea of the force of friction between the brick and the floor.

Now, repeat the activity with different materials wrapped around the brick:

Wrap a jute bag around the brick and pull it again. Check if the force required to move the brick changes.
Wrap a piece of polythene around the brick and pull it. Once again, note the change in the spring balance reading.

Key Observations:

Jute bag wrapping increases friction, meaning you will likely need more force to move the brick.
Polythene wrapping decreases friction, so the brick will move more easily with less force.

Explanation: The roughness or smoothness of the surface in contact affects the amount of friction. Rough surfaces, like jute, create more friction and require more force to move the object. Smooth surfaces, like polythene, reduce friction, making it easier to move the object. This demonstrates how friction depends on the type of surface.

What is a Spring Balance?

A spring balance is a tool used to measure the force acting on an object. Here's how it works:

It contains a coiled spring that stretches when a force is applied.

As the spring stretches, a pointer moves along a graduated scale.

The reading on the scale shows the magnitude of the force being applied.

Spring balances are useful for measuring forces like the pull of gravity or friction. The more the spring stretches, the greater the force being applied to it.

Activity 9.3 : How Surface Affects Friction

Objective: To understand how the surface over which an object moves affects the distance it travels due to friction.

Materials Needed:

A wooden board or flat surface (to act as an inclined plane)
Books or bricks (to support the inclined plane)
A pencil cell
A smooth cloth
A thin layer of sand
Sandpaper (optional)

Steps:

Set up the inclined plane using a wooden board, supported by books or bricks.
Mark a point A on the inclined plane where the pencil cell will start moving.
Release the pencil cell from point A and let it move down the plane.
Measure and note the distance it travels on the table before coming to rest.
Spread a smooth cloth on the table and repeat the activity. Measure and compare the distance covered.
Spread a thin layer of sand on the table and repeat the process.
Optional: Wrap the pencil cell in sandpaper and repeat the activity.

Observations:

The distance traveled by the pencil cell changes depending on the surface.
The smoother the surface, the farther the pencil cell moves.
Rougher surfaces (like the sand or sandpaper) create more friction, reducing the distance covered.
Smooth surfaces (like the smooth table or cloth) allow the cell to travel farther.

Conclusion:

The nature of the surface and its smoothness greatly affect the distance an object travels.
The smoother the surface of both the inclined plane and the object, the less friction there is, allowing the object to travel farther.

What is Friction?

Friction is the force that opposes the motion of one object moving over another. It is caused by the irregularities or bumps on the surfaces in contact (Fig. 9.5). Even smooth surfaces have tiny irregularities, which interlock when two surfaces touch.

Rough surfaces: They have more irregularities, so the force of friction is higher.
Smooth surfaces: Though smoother, they still have minute bumps that cause some friction, but much less than rough surfaces.

Effect of Pressure on Friction: When two surfaces are pressed together more firmly, the interlocking becomes stronger, increasing the friction. For example, dragging a mat is harder when someone is sitting on it because the weight presses it down, creating more friction.

Static Friction vs. Sliding Friction:

Static Friction: This is the friction you must overcome to move an object from rest. It is generally higher because the irregularities have had time to interlock firmly.
Sliding Friction: Once the object is in motion, the contact points don’t lock as firmly, making sliding friction lower than static friction. This is why it’s easier to keep an object moving than to start moving it.

Example: If you try to push a box, it’s harder to get it started (overcoming static friction) but easier to keep it sliding (dealing with sliding friction). This happens because the irregularities on the surfaces don’t have time to interlock fully once the object is in motion.

Friction: A Necessary Evil

Friction is essential in our daily lives, but it also has some negative consequences.

Friction is Necessary:

Holding objects: It's easier to hold an earthen pot (kulhar) than a glass tumbler because the rough surface of the pot provides more friction. If the tumbler is greasy, it becomes harder to hold due to reduced friction.
Walking: We can walk on a regular surface because friction between our feet and the ground prevents slipping. On wet or muddy surfaces, the reduced friction makes walking difficult.
Writing: Whether using a pen, pencil, or chalk, friction allows us to write. The chalk's rough surface rubs off particles, which stick to the blackboard due to friction.
Starting or stopping motion: Vehicles move or stop due to friction between their tyres and the road. Without friction, they couldn’t be started, stopped, or even change direction.
Building and fixing: Friction helps us fix a nail to the wall or tie a knot. It is crucial for construction.

Friction as an Evil:

Wear and tear: Friction causes wear and tear of materials like screws, ball bearings, or even shoe soles. The steps of foot over-bridges at railway stations wear down over time due to friction (Fig. 9.7).
Heat production: When you rub your palms together, they feel warm due to friction (Fig. 9.9). Striking a matchstick produces fire, and even a mixer jar becomes hot after prolonged use due to friction (Fig. 9.10).
Energy loss: In machines, the heat generated by friction leads to energy loss, which reduces efficiency.

While friction is unavoidable and necessary in many activities, we also need ways to reduce its negative effects.

9.4 - Increasing and Reducing Friction

Friction can be both increased and reduced depending on the situation:

Increasing Friction:

Shoe soles: The soles of shoes are grooved [Fig. 9.11 (a)] to provide better grip and prevent slipping. Similarly, treaded tyres on vehicles like cars, trucks, and bulldozers help maintain a firm grip on the road.
Brake systems: In bicycles and automobiles, brake pads increase friction with the wheels when pressed, stopping the vehicle.
Sports and gymnastics: Kabaddi players rub their hands with soil for better grip, and gymnasts use coarse substances on their hands to increase friction for secure holds during performances.

Reducing Friction:

Carrom boards: Sprinkling fine powder on a carrom board reduces friction, allowing the pieces to slide smoothly [Fig. 9.12].
Lubricants: Oil, grease, or graphite is applied to machines to reduce friction. A thin layer of these substances prevents direct rubbing between moving parts by reducing the interlocking of surface irregularities [Fig. 9.13].
Oil in hinges: A few drops of oil on door hinges allow the door to move smoothly by reducing friction.
Air cushion: In some machines, where oil cannot be used, an air cushion is employed to reduce friction.

Boojho wonders if friction can be completely eliminated by polishing surfaces or using large amounts of lubricants. Paheli points out that friction can never be entirely eliminated because no surface is perfectly smooth, and some irregularities will always exist.

9.5 - Wheels Reduce Friction

Wheels significantly reduce friction, making it easier to move objects. You've likely noticed that luggage fitted with rollers is much easier to pull, even for a child [Fig. 9.14]. Let's explore why this happens.

Activity 9.4:

Place a few cylindrical pencils parallel to each other on a table.
Then place a thick book on top of these pencils [Fig. 9.15].
Push the book, and you'll observe that the pencils roll as the book moves.
You'll notice that it's much easier to move the book this way compared to when you slide it without the rollers.

This activity shows that rolling friction is much less than sliding friction. The resistance encountered when one body rolls over another is called rolling friction. Because rolling friction is smaller, it's easier to roll objects than to slide them.

Practical Applications:

Wheels are among the greatest inventions because they take advantage of the reduced friction that comes with rolling.
In machines, sliding is often replaced with rolling to reduce friction. For example, ball bearings are commonly used in the hubs and axles of ceiling fans, bicycles, and other machinery to reduce friction and make the motion smoother [Fig. 9.16].

By using rollers or wheels, resistance to movement is minimized, making tasks much more efficient and easier.

9.6 - Fluid Friction

Fluid friction refers to the resistance or drag exerted by fluids (liquids and gases) on objects moving through them. Though air is light and water may seem smooth, both exert significant frictional forces on objects in motion within them. These forces result in energy loss.

Key Factors Influencing Fluid Friction (Drag):

Speed of the object: The faster the object moves through the fluid, the greater the frictional force acting on it.
Shape of the object: The design and shape of an object affect how much resistance it encounters while moving through the fluid.
Nature of the fluid: The viscosity (thickness) of the fluid also influences the amount of friction.

Minimizing Fluid Friction: To reduce the energy loss caused by drag, objects that need to move efficiently through fluids are designed with special shapes. Scientists take inspiration from nature, where the bodies of birds and fish have evolved streamlined forms to reduce drag and save energy when moving through air and water, respectively. For example, the aeroplane [Fig. 9.17] has a shape similar to a bird's body. This streamlined shape helps it cut through the air with reduced drag, conserving energy and improving efficiency. Similarly, vehicles like cars, trains, and ships are also designed to minimize fluid friction by incorporating streamlined designs.

NCERT Science Notes - Class 8 | Science | Chapter 9 - Friction

NCERT Science Notes - Class 8 | Science | Chapter 8 - Force and Pressure

NOTES

9.0 - Introduction