NCERT Explained: Class VIII Science, Chapter 12 - Friction: The Force That Opposes Motion

Introduction to the Topic

Have you ever wondered why a rolling ball gradually slows down and eventually stops, even if no one touches it? Or why it is so difficult to walk on a wet marble floor or a patch of ice? The answer to these questions lies in a fundamental force of nature called Friction. In the NCERT Class VIII Science curriculum, Chapter 12 focuses on this invisible force that plays a crucial role in our daily lives.

Friction is the force that opposes the relative motion between two surfaces in contact. While it might seem like a nuisance when it wears out your favorite pair of shoes or the tires of a bicycle, life would be impossible without it. Without friction, you wouldn't be able to write with a pen, walk without slipping, or even stop a moving vehicle. This chapter explores the causes of friction, the factors that affect it, its various types, and how we can manipulate it to suit our needs.

Key Concepts Explained

1. What Causes Friction?

At a glance, many surfaces look perfectly smooth. However, if you were to look at them through a powerful microscope, you would see that every surface has tiny bumps and ridges called irregularities. When two surfaces come into contact, these irregularities interlock with each other. This interlocking creates a resistance to movement, which we call friction.

To move one object over another, we must apply enough force to overcome this interlocking. This is why rough surfaces, which have more significant irregularities, offer more friction than smooth surfaces. For example, pushing a heavy box across a concrete floor (rough) is much harder than pushing it across a polished wooden floor (smooth).

2. Factors Affecting Friction

There are two primary factors that determine the strength of the frictional force:

• Nature of the Surfaces: Friction depends on how rough or smooth the surfaces are. A smooth surface has fewer irregularities, leading to less interlocking and thus less friction. A rough surface has more irregularities, leading to greater friction.
• The Force Pressing the Surfaces Together: Friction increases if the surfaces are pressed harder against each other. For instance, it is much easier to push an empty crate across the floor than a crate full of heavy books. The weight of the books increases the downward force, causing the irregularities of the crate and the floor to interlock more firmly.

3. Types of Friction

Friction is not a one-size-fits-all force. It manifests in different ways depending on the state of motion:

• Static Friction: This is the friction that acts on an object when it is at rest. It is the force you must overcome to start moving an object. For example, when you try to push a heavy sofa, the initial resistance you feel before the sofa starts sliding is static friction.
• Sliding Friction: Once an object starts moving, the irregularities on its surface do not get enough time to interlock deeply with the irregularities of the floor. Therefore, sliding friction is slightly less than static friction. This is why it is easier to keep an object moving than it is to start it from rest.
• Rolling Friction: When one body rolls over the surface of another, the resistance to its motion is called rolling friction. Rolling significantly reduces friction compared to sliding. This is why suitcases are fitted with rollers and why the invention of the wheel was a turning point in human history.

4. Friction: A Necessary Evil

Friction is often described as a 'necessary evil' because it has both advantages and disadvantages.

• It allows us to walk without slipping by providing grip between our feet and the ground.
• It enables us to write on paper or a chalkboard. The friction between the pen tip and paper rubs off the ink/graphite.
• It allows vehicles to stop when brakes are applied.
• It enables us to light a matchstick by rubbing it against a rough surface, generating heat through friction.

• It causes wear and tear. Soles of shoes, tires of vehicles, and machine parts wear out over time due to constant friction.
• It produces heat, which can damage sensitive machine parts or lead to energy loss.
• It opposes motion, meaning more energy (fuel or physical effort) is required to move objects.

5. Increasing and Reducing Friction

Depending on the situation, we may want to either increase or decrease friction:

• To Increase Friction: We make surfaces rougher. Examples include the treaded tires of trucks and earthmovers, the spiked shoes (cleats) worn by athletes for better grip, and the coarse surface of brake pads in bicycles and cars.
• To Reduce Friction: We use lubricants like oil, grease, or graphite. These substances form a thin layer between the surfaces, preventing them from interlocking directly. Another method is using ball bearings in fans or motor axles to convert sliding friction into rolling friction.

6. Fluid Friction (Drag)

Friction is not limited to solid surfaces. Liquids and gases (collectively called fluids) also exert a frictional force on objects moving through them. This fluid friction is often called drag. The amount of drag depends on the speed of the object, its shape, and the nature of the fluid.

To minimize drag, nature and engineers use streamlining. Birds and fishes have evolved streamlined bodies to move easily through air and water. Similarly, airplanes, rockets, and high-speed cars are designed with specific shapes to cut through the fluid with minimal resistance.

Summary & Key Takeaways

• Friction is a force that opposes the relative motion between two surfaces.
• It is caused by the interlocking of irregularities on the surfaces in contact.
• Friction depends on the nature of materials and the force with which the surfaces are pressed together.
• Static friction is the strongest, followed by sliding friction, while rolling friction is the weakest.
• Friction is essential for daily tasks like walking and writing, but it also causes wear and tear and energy loss.
• Lubrication and ball bearings are used to reduce friction, while treading and spiking are used to increase it.
• Fluid friction (drag) is reduced by giving objects a streamlined shape.