9. Friction

You might have seen a driver of a car or a truck slowing down the vehicle at a traffic signal. You, too, slow down your bicycle whenever needed by applying brakes. Have you ever thought why a vehicle slows down when brakes are applied? Not only vehicles, any object, moving over the surface of another object slows down when no external force is applied on it. Finally, it stops. Have you not seen a moving ball on the ground stopping after some time? Why do we slip when we step on a banana peel? Why is it difficult to walk on a smooth and wet floor?

Friction is the force that opposes motion between two surfaces in contact. It is what causes a moving vehicle to slow down when brakes are applied. Similarly, when a ball rolls and eventually stops, it's due to friction between the ball and the ground. The concept can also be observed in everyday life, such as slipping on a banana peel or struggling to walk on a wet floor.

Imagine sliding down a hill on a smooth sled versus a rough one. The smoother the surface, the less friction there is, and you go faster. However, if that hill were muddy (increasing friction), you would slow down considerably.

You saw that if you apply the force along the left, friction acts along the right. If you apply the force along the right, the friction acts along the left direction. In both cases, the force opposes the motion of the book. The force of friction always opposes the applied force. In the above activity, the force of friction acts between the surface of the book and the surface of the table.

Friction always acts in the opposite direction to the applied force. For example, if you push a book to the right, friction pushes back to the left, which slows the book down and eventually stops it. This opposition is crucial in understanding how various objects move in contact with different surfaces.

Think about playing football. When you kick the ball forward, the grass provides friction that can slow it down or redirect its path, showing how friction interacts with motion in sports.

Is the friction the same for all surfaces? Does it depend on the smoothness of the surfaces? Let us find out. Activity 9.2: Tie a string around a brick. Pull the brick by a spring balance. You need to apply some force. Note down the reading on the spring balance when the brick just begins to move. It gives you a measure of the force of friction between the surface of the brick and the floor. Repeat this activity by wrapping a piece of jute bag around the brick. Now wrap a piece of polythene around the brick and repeat the activity.

Friction can vary significantly depending on the surfaces in contact. For example, a rough surface like jute will create more friction compared to a smooth surface like polythene. This is because friction arises from the irregularities of the surfaces; with more irregularities, there are more points of contact that cause friction.

Consider trying to slide different types of containers over a kitchen counter—plastic vs. wood. The plastic will glide more easily due to lower friction because it has smoother surfaces compared to the wood, which has more texture and imperfections.

Recall now some of your experiences. Is it easier to hold a kulhar (earthen pot) or a glass tumbler? Suppose the outer surface of the tumbler is greasy; would it become easier or more difficult to hold it? Just think: would it be possible to hold the glass at all if there is no friction?

Friction is essential in many daily tasks. For instance, gripping a glass tumbler is easier because of the friction between your hand and the glass. Without friction, you wouldn't be able to hold or manipulate objects securely, leading to accidents and difficulty in everyday activities.

Think about climbing a mountain. The friction between your shoes and the rock is what keeps you from slipping and allows you to maintain your grip as you ascend. Without sufficient friction, climbing would be nearly impossible.

As you have seen in the previous section, friction is desirable in some situations. Have you ever thought why the sole of your shoe is grooved? It is done to provide the shoes better grip on the floor. Similarly, the treaded tyres of cars provide better grip with the ground. In some situations, however, friction is undesirable and we would want to minimise it.

Grooved shoe soles and treaded tyres are designed to increase friction, which helps prevent slipping and improves safety. On the other hand, in machines or moving parts, reducing friction is often desirable to enhance efficiency. Lubricants like oil or grease are applied to reduce friction, making movements smoother.

When cooking, imagine how oil in a pan prevents food from sticking and allows it to move easily. This is similar to how lubricants in machines minimize friction and enhance performance.

You know that air is very light and thin. Yet it exerts frictional force on objects moving through it. Similarly, water and other liquids exert force of friction when objects move through them. The frictional force exerted by fluids is also called drag.

Fluids such as air and water exert frictional forces on objects passing through them, referred to as drag. This friction depends on the object’s speed, shape, and the properties of the fluid. Reducing this drag is crucial for improving the efficiency of various vehicles, from airplanes to boats.

Consider a swimmer moving through water. They experience drag, which can slow them down. To swim faster, they need to streamline their bodies just as vehicles are designed with rounded edges to reduce air resistance while moving.

--