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 August 7, 2007

SCIENCE SERIES FRICTION

# FRICTION helps you to go, stop

 This is proof that friction produces heat.

Have you ever wondered what causes a match to ignite when you strike it against the side of a matchbox? Or what forces are at work that cause a speeding car to come to a safe and complete stop? In both examples, you have friction to thank for the pleasant results. Friction is the force that opposes the relative movements of two surfaces that are in contact. Friction converts kinetic energy into thermal (heat) energy.

Indeed, friction is at work all around us. It is so commonplace that often we take advantage of its properties without thinking about it. For example, friction helps you walk from place to place. As you walk your shoes cause friction by pushing against the floor in a backward motion, which moves your body forward. The floor actually pushes back against your every step. You know that there is sufficient friction there because your foot doesn't slip.

The equal but opposing pushing forces (foot against floor, floor against foot) is explained by Sir Isaac Newton's Third Law of Motion, which in simplified terms states: ``For every action there is an equal and opposite reaction.''

In the eyes of physics, whenever you move an object you are performing mechanical work, and mechanical work transfers energy.

 A microscopic view of two surfaces. In the upper diagram friction makes movement difficult, while in the lower diagram, a lubricant separates the surfaces so they can move more freely.

For example, if you exert force to push a box along a flat surface you're performing mechanical work by expending energy. Obviously, when you stop pushing, the box instantly stops moving. So where did all the energy you used in doing the work go?

To answer that, you have to consider what caused the box to stop. It was the combination of two things: (1) you stopped pushing, and (2) friction between the box and the flat surface. Friction is a force which tries to stop two surfaces sliding past each other and friction creates heat in the process. You can reproduce this effect by rubbing your hands against each other. So a lot of the energy you exerted against the box to move it forward was converted to heat.

Stop that car!

Friction between a car's tires and the road surface helps to roll the car forward, similar to your walking motion discussed above. When bringing the car to a stop, the friction between the brakes and the wheel, and the friction between tires and the road, cause the car to decelerate and to eventually stop. The better the condition of the tires and road, the more friction will be created, and the quicker the car stops. This too is Newton's Third Law.

A car going around a corner relies on a centrifugal (sideways) force, caused when the car tries to change direction. If the road is wet, or the car is going too fast, there may not be enough friction and the car may not be able to complete the turn safely.

To answer our matchbox query, it is friction that causes the phosphorus-filled head of the safety match to burst into flames. At room temperature though, it is not hot enough to start the reaction, but it is close. Simply rubbing the match over the rough strip on the side of the matchbox generates enough heat to start the reaction.

Friction on ice

Friction occurs because no two surfaces are perfectly smooth at the microscopic level. The diagrams to the left show how this can lead to friction. But separating the surfaces by inserting a layer of lubricant such as water or oil in between reduces the friction. This is why wet floors are slippery. There is a thin layer of water separating your feet from the floor so the floor and your feet don't grip each other well.

You also experience this when you try to pick up ice cubes using your fingertips. The ice is very smooth to begin with, but the surface has a very thin layer of water on it, which gets bigger as the cube starts to melt. The layer of water separates the roughness of your fingertips from any surface imperfections on the ice cube, which may cause the cube to slip through your grip.

Ice skaters guide themselves by the sharp blades of their skates that cut grooves in the ice. They control their movements and direction by pushing against the sides of the grooves.

Earth's brakes

An interesting effect due to friction is the slowing down of the Earth's rotation. But what could the Earth be rubbing against? The answer is not very obvious. It is the friction caused by the flow of water that produces high and low tides each day that produces a lot of heat. Heat must come from somewhere. It comes from the Earth's rotational kinetic energy, so the Earth's spin is slowing down very, very gradually. Come back in a few million years and you'll need a new watch!

If you learn only one thing from reading this article, it should be that you could not read Learning Post without friction, because every time you try to pick up the paper it would slip out of your hands, and turning the pages of Learning Post would be impossible.

Next time: Look ma, no rails! The secrets of the maglev train will be revealed.

 Knowledge you can use How do you make waterproof matches? Simply use clear fingernail polish to paint over the entire match head and stem. You can also use melted candle wax, but clear fingernail polish is a bit easier to use and works best because it's harder than candle was.

Corrin Funnell is a laser physicist with a specialty in laser spectroscopy. He has taught in the UK, Egypt, at Thailand's own Harrow International School, where he became head of the physics department. Currently, he is head of Physics at Island School, Hong Kong.

Read our other science series here.