The rules of friction
In the seventeenth century lived a scientist called Robert Hooke. He made many investigations on many different scientific topics from studying the stars with a telescope to studying tiny insects with a microscope. He also studied metal springs. Hooke set up a spring so that it hung down. He measured the length of the spring, put a weight on the end and measured the length of the spring again. He repeated this experiment with different weights, and made a discovery. He found that when weights were added to a spring it stretched in a special way. When a small weight was put on the spring it stretched a small amount. When double the weight was put on the spring, it stretched double the original amount. When three times the weight was added the spring stretched three times as much and so on. This meant that the spring could be used for measuring forces because it stretched in a regular way when it was pulled and returned to its original length when the pulling force was removed.

The instrument used to measure the pull of forces is called the forcemeter. It has a spring inside it with a pointer attached. As the spring is stretched the pointer moves along a scale. The units used in the scale are called newtons. This unit is named after another scientist. He was called Isaac Newton and he lived at the same time as Robert Hooke. Newton made many discoveries about forces so it was fitting for the unit that measured a force to be named after him.

When someone uses a forcemeter to measure the force of friction the following things happen. Imagine a forcemeter attached to a wooden block by a piece of string. The block is resting on the table and the forcemeter is next to it. Under the block the surfaces of the table and block meet. If you could examine the surfaces with a powerful microscope you would see that they are covered with ridges and grooves and that the ridges of one surface fit into the grooves of the other. When a person picks up the forcemeter and gently pulls on the side of the block, the ridges in the grooves are pulled to the sides of the grooves. They hold their position there and the spring stretches. The pointer moves along the scale and shows the frictional force due to the interlocking ridges and grooves.

When the person increases their pull on the block the ridges may hold firm and the spring will stretch more but eventually when a certain pulling force is reached, the ridges slip out of the grooves and the block begins to slide. When this happens the spring shortens and the pointer shows that a smaller force is needed to pull the block along the surface of the table. The strongest frictional force that held the block in place was the one that made the spring stretch the most.

When you try experiments on measuring friction remember the work of Hooke and Newton three hundred years ago and think about how the grooves and ridges help the block hold its place when it is pulled from the side.

Do scientists today work on many different science topics like Robert Hooke?
Most scientists don't. They work in just one science area. It may be astronomy or the study of insects but not both. There are a huge number of scientific topics to study, far more than in Robert Hooke's day.

How do scientists find out about the topics they like?
They start out just like you, studying a wide range of science topics and this continues all the way through school. Later at college they pick a few science topics in more detail and find which one they like the best. After college they begin studying that topic in great detail. If they are lucky they may make new discoveries just as Hooke and Newton did.

Why does a spring go back to its original length when a pulling force is removed?
When the spring is pulled, a force is set up inside the material from which the spring is made. This force is called a strain force. As the spring is stretched the strain force increases and matches the pulling force. The two forces balance. When the pulling force is removed, the strain force has nothing to keep it in balance so it pulls back on the spring. This makes the spring go back to its original length.

Do springs always return to their original length after they have been pulled?
No. When the spring is pulled to a certain length and the pulling force is removed, the spring does not get shorter again. This means that it can no longer be used for measuring forces.

Why can't an over-stretched spring be repaired?
A stretched spring goes back to its original length owing to the pull of its strain force. The inside of the spring is made from tiny metal particles, which can only be seen by the most powerful microscopes. The strain force is made by the way the particles are arranged. When a spring is over-stretched the arrangement of the particles is changed. This change destroys the strain force and the spring remains in its over-stretched length.

Do forcemeters all have the same size of spring?
No. Forcemeters are designed to measure a range of forces such as 0-10 newtons or 0-100 newtons. The forcemeter which is designed to measure small forces has a smaller spring than a forcemeter designed to measure large forces.

What happens to the frictional force if the area of the surfaces in contact is made smaller?
If the surfaces are dry, the frictional force remains the same even if the area is made smaller. Think about a film you have seen where someone slides down a roof. As they slide they may change their position. One moment they are lying flat against the roof with a huge area in contact. The next moment they are trying to sit up with a much smaller area in contact. Neither position nor surface area in contact makes any difference, the person keeps sliding without slowing down. If surface area did make a difference a person sliding down a long slide could begin stretched out on their back and move fast, then sit up and move more slowly, get onto their knees and move more slowly still, then stand up and stop. Alternatively they could move more slowly when stretched out and faster when standing on one foot. Neither of these situations happens - once a person is moving down a slide they just keep on going no matter how they change their area in contact with the slide.

What happens to the frictional force if the weight pushing on the surfaces is increased?
The weight pushes the ridges further into the grooves and makes a stronger frictional force to hold the surfaces together.

How do the brake blocks on a bicycle make a strong frictional force?
The brake blocks are attached to levers and wires. The largest levers are the ones on your handlebars. When you squeeze on them you create a force which passes to the brake blocks. Levers can make the force stronger, so the force which pushes the brake blocks onto the rim of the wheel is stronger than the one you squeezed on the handlebar. The force on the brake blocks is like an extra weight pushing on them so they press hard to the wheel rim and make a strong frictional force to stop the wheel moving.