CBSE Class 8 Science, Friction

Friction is a force that opposes motion between any surfaces that are touching. Friction can work for or against us. For example, putting sand on an icy sidewalk increases friction so you are less likely to slip. On the other hand, too much friction between moving parts in a car engine can cause the parts to wear out. Other examples of friction are illustrated in the two Figures below

Friction between the graphite in a pencil and a sheet of paper leaves a mark on the paper.

Friction between a bicycle brake pad and the rim of a wheel causes the wheel to stop turning.

Did you ever rub your hands together to warm them up, Why does this make your hands warmer? The answer is friction.

Friction occurs because no surface is perfectly smooth. Even surfaces that look smooth to the unaided eye make look rough or bumpy when viewed under a microscope. Look at the metal surfaces in the Figure below. The aluminum foil is so smooth that it's shiny. However, when highly magnified, the surface of metal appears to be very bumpy. All those mountains and valleys catch and grab the mountains and valleys of any other surface that contacts the metal. This creates friction.

Rougher surfaces have more friction between them than smoother surfaces. That's why we put sand on icy sidewalks and roads. Increasing the area of surfaces that are touching also increases the friction between them. That's why you can't slide as far across ice with shoes as you can on the thin blades of skates. The greater surface area of the soles of the shoes cause more friction and slow you down.

Heavier objects also have more friction. Can you explain why?

Heavier objects press together with greater force, and this causes greater friction between them.

You know that friction produces heat. That's why rubbing your hands together makes them warmer. But do you know why? Friction causes the molecules on rubbing surfaces to move faster, so they have more energy. This gives them a higher temperature, and they feel warmer. Heat from friction can be useful. It not only warms your hands. It also lets you light a match as shown in the Figure below. On the other hand, heat from friction between moving parts inside a car engine can be a big problem. It can cause the car to overheat.

How is friction reduced between the moving parts inside a car engine?

To reduce friction, oil is added to the engine. The oil coats the surfaces of the moving parts and makes them slippery. They slide over each other more easily, so there is less friction.

Friction is always parallel to the surface in contact.

In some cases, friction is desirable while in other cases, friction reduces the effectiveness of machines. Without friction, you would not be able to walk, the wheels on a vehicle would have no grip on a road surfacing and the vehicle would not be able to move forward.

On the other hand, friction is undesirable in most of the cases. Friction causes mechanical parts to seize and wear out.

Activity 1: Take two pieces of paper. Wad one of the pieces of paper into a ball. Leave the other piece of paper like normal.

Hold the pieces of paper above your head. Drop the pieces at the same time. The wadded piece will fall to the ground immediately. The flat piece of paper will flutter down more slowly because it has more surface area to create friction or drag on the air as it drops.

To begin, fill the bottle with rice. Try sticking the chopstick down into the bottle. When you grab the chopstick and pull up, what happens? The stick should just come right out.

How do you make the rice stick to the chopstick so you can just pick it up? You've got to add more friction. Here's how.

Put the lid on the bottle and tap the bottom of the bottle on a hard surface. Do this a few times to compact the rice and get rid of the air pockets. Now you should have room to add a bit more rice to the bottle.

Once you've added a bit more rice, stick the chopstick back into the bottle. This time, when you pull up on the chopstick, you should be able to pick up the whole bottle.

Fig: Holding up a bottle with just a chopstick

Concept: Friction applied by rice on the chopstick.

1.   Define friction, and explain why it occurs.

2.   Identify three factors that affect friction.

3.   Why does friction warm your hands when you rub them together?

4.   Outside wooden steps may get slippery when they are wet. How could you make them less slippery?

Friction is the force that opposes motion between any surfaces that are in contact. There are four types of friction: static, sliding, rolling, and fluid friction (Air/Viscous friction). Static, sliding, and rolling friction occur between solid surfaces. Fluid friction occurs in liquids and gases.

Static friction acts on objects when they are resting on a surface. For example, if you are hiking in the woods, there is static friction between your shoes and the trail each time you put down your foot (see Figure below). Without this static friction, your feet would slip out from under you, making it difficult to walk. In fact, that's exactly what happens if you try to walk on ice. That's because ice is very slippery and offers very little friction.

Can you think of other examples of static friction?

One example is the friction that helps the girls climb the rock wall in the picture above. Static friction keeps their hands and feet from slipping.

Sliding friction is friction that acts on objects when they are sliding over a surface. Sliding friction is weaker than static friction. That's why it's easier to slide a piece of furniture over the floor after you start it moving than it is to get it moving in the first place. Sliding friction can be useful. For example, you use sliding friction when you write with a pencil. The pencil "lead" slides easily over the paper, but there's just enough friction between the pencil and paper to leave a mark.

How does sliding friction help you ride a bike?

There is sliding friction between the brake pads and bike rims each time you use your bike's brakes. This friction slows the rolling wheels so you can stop.

Rolling friction is friction that acts on objects when they are rolling over a surface. Rolling friction is much weaker than sliding friction or static friction. This explains why most forms of ground transportation use wheels, including bicycles, cars, 4-wheelers, roller skates, scooters, and skateboards. Ball bearings are another use of rolling friction. You can see what they look like in the Figure below. They let parts of a wheel or other machine roll rather than slide over on another.

The ball bearings in this wheel reduce friction between the inner and outer cylinders when they turn.

It is found that:

Rolling friction < sliding friction < static friction.

Fluid friction is friction that acts on objects that are moving through a fluid. A fluid is a substance that can flow and take the shape of its container. Fluids include liquids and gases. If you've ever tried to push your open hand through the water in a tub or pool, then you've experienced fluid friction. You can feel the resistance of the water against your hand. Look at the skydiver in the Figure below. He's falling toward Earth with a parachute. Resistance of the air against the parachute slows his descent. The faster or larger a moving object is, the greater is the fluid friction resisting its motion. That's why there is greater air resistance against the parachute than the skydiver's body.

Activity 1:

Take a few pencils which are cylindrical in shape. Place them parallel to each other on a table. Place a thick book over it . Now push the book. You observe the pencils rolling as the book moves. Do you feel it easier to move the book in this way than to slide it? Do you think that resistance to the motion of the book has been reduced? Have you seen heavy machinery being moved by placing logs under it?

Concept: Rolling friction

1.   List four types of friction.

2.   You can move heavy boxes by sliding them over the ground. Or you can put them on a dolly, like the one in the Figure below, and then roll them over the ground. Explain which way makes it easier to move the boxes.

3.   What is a fluid? Give an original example of fluid friction.

•   Friction between pen and paper enables us to write on the paper.

•   Friction between our feet and the ground allows our movements like standing, walking and running.

•   Friction between the surface of the road and tyres of our vehicles allow the vehicles to move without slipping.

•   Friction causes moving objects to stop or slow down

•   Friction produces heat causing wastage of energy in machines.

•   Friction causes wear and tear of moving parts of machinery, soles of shoes, etc.

In real life, there are circumstances where we have to increase the friction and minimize the friction. Friction can be increased by increasing the roughness of the surfaces in contact. For example, treading of shoes and tires is done to increase friction. When friction is undesirable we have to reduce the friction. Friction can be minimized by using lubricants like oil and grease and by using ball bearing between machine parts. A substance that is introduced between two surfaces in contact, to reduce friction, is called a lubricant. Fluid friction can be minimized by giving suitable shapes to the objects moving in the fluids.

There are instances in daily life where friction is a necessity. For example, without friction, we cannot hold objects in our hands; we cannot walk and cannot light a match stick. Sometimes friction is not desirable, for example friction between machinery parts, which causes wear and tear. As friction is advantageous to us it is considered as a friend but due to its disadvantages it is a foe. Depending on the circumstance, friction can be a help or a hindrance. Thus it is a necessary evil.