When an object moves on a surface, an obstructing force is produced in the opposite direction. That force is called friction. It is not a property of one surface on its own. It only exists where two surfaces meet.
Have you ever slipped while walking on the road or inside the house? Try to remember the place or the conditions. What sort of place did you slip in? Was it dry or wet, smooth or rough, on something that rolls or on something that does not?
Generally, wet, smooth and rolling objects produce less obstruction between the surfaces that are in contact. The opposite is also true: dry, rough and non-rolling objects produce more obstruction between surfaces in contact.
No surface is actually smooth
Here is the one mechanical picture that explains every single fact in this topic, so it is worth getting properly. However smooth an object may look, it is never perfectly smooth. Take a hand lens and look closely at a piece of wood, or at the surface of your desk, or at a sheet of glass. On that surface you will see raised parts and sunken parts.
The raised part of a surface is called a projection and the sunken part is called a depression. When one object is dragged over the surface of another, the projections of one get caught in the depressions of the other. That catching is what produces an obstruction in the direction opposite to the object's motion, and that obstruction is friction.
Hold on to that picture, because it is doing more work than it looks. Every fact in the rest of this topic is the same picture seen from a different angle. A rough surface has larger projections, so more catching and more friction. Pressing down harder pushes the projections deeper into the depressions, so more catching again. Oil fills the depressions so nothing can drop into them. And rolling means the two surfaces never drag across each other at all. Four separate facts, one mechanism.
What friction depends on, and what it does not
The amount of friction depends on the nature of the surfaces in contact and on the pressing force on the surface. Friction is greater on a rough surface than on a smooth one, and if the pressing force is greater then the friction is greater.
Now read that sentence again and notice what is missing from it. The area of contact is not on the list. This is the surprise waiting inside Activity 6.7, and most students predict the opposite. Lay a brick on its big flat face and drag it, then stand it on its narrow edge and drag it again. The area touching the ground has changed enormously, and the spring balance reads very nearly the same both times.
The projections and depressions explain why. Turning the brick on its edge does reduce the number of places where the two surfaces meet, but the whole weight of the brick is now pressed through that smaller area, so each remaining contact is squeezed much harder and catches much more firmly. Fewer contacts, each gripping more tightly, and the two changes cancel out. Stack a second brick on top instead and nothing cancels: the pressing force has doubled, and the reading roughly doubles with it.
Friction is our friend and our enemy
Why is there more chance of slipping when the pattern on the sole of your shoe or slipper has worn away? Could we walk, or could vehicles run on the road, if there were no friction at all?
| Where friction helps | What it is doing there |
|---|---|
| Tyre patterns | We have all seen the many patterns made on vehicle tyres. The reason for making them is precisely to increase the friction between the road and the wheel. Without them the vehicle slips. |
| Brakes | When you ride a bicycle you have to brake. As the brake is applied, friction occurs between the rubber block and the wheel, and the bicycle stops. Could a moving vehicle be stopped at all if there were no friction? |
| Nails, nuts and bolts | When two pieces of wood are joined by hammering in a nail, or by using a nut and bolt, it is friction that holds the joint together and stops it working loose. |
Because friction makes so many tasks in our daily life easy, it can fairly be counted as our friend. But there are disadvantages too. Have you a memory of any occasion when friction caused you difficulty? The friction produced between surfaces in motion converts mechanical energy into heat energy and so reduces the efficiency of machines. In that condition, more force is needed to do the work, and the machines wear away quickly and break. On the basis of these effects, friction is taken as both an enemy and a friend.
Friction does not change its behaviour depending on who is watching. It always does exactly one thing: it opposes the sliding of one surface over another. Whether we call that friendly or hostile depends entirely on whether the sliding was something we wanted. The friction between a tyre and the road is the same force in both of its famous roles. It is what lets the car set off without spinning its wheels, and it is what wears the tread off the tyre over the next thirty thousand kilometres. Nobody switched anything on or off. We simply liked one result and disliked the other. That is worth knowing, because it means the question is never is friction good or bad, but do I want this particular sliding to happen here.
Most students file friction under things that stop motion, and that filing is only half right. Think about what walking actually is. Your foot pushes backwards against the ground, and the ground pushes you forwards. That forward push exists only because friction stops your foot from sliding backwards. On wet ice there is almost no friction, your foot slides, and no forward push is produced at all. It is not that walking becomes difficult. It becomes impossible, which is exactly what happens when somebody slips. The same is true of a car: the engine can spin the wheel all it likes, and it is only friction that turns that spinning into the car actually going somewhere. So friction starts motion as often as it stops it.
Four ways to reduce friction, and why they are all one idea
Look at a door hinge and at a bicycle chain and try to work out why oil and grease are put on them. Since friction reduces the efficiency of machines, various measures are adopted to reduce it. What measures have you seen being used in machines around you?
| Measure | How it works, in terms of projections and depressions |
|---|---|
| Polishing a rough surface | Smaller projections and shallower depressions, so there is less for the two surfaces to catch on. This is the finding of Activity 6.8, where a rough table gives a bigger reading than a wet one. |
| Oil and grease | Smooth substances like mobil oil and grease are used in machines. They fill the depressions and hold the two surfaces slightly apart, so the projections have nothing to drop into and the friction falls. |
| Wheels and ball bearings | In a ball bearing, balls are placed between two rings and one ring turns on the other. Less friction acts when an object rolls than when it rubs, because rolling surfaces never drag across each other at all. This is used in fans, electric motors and other rotating machines. |
| Streamlining | To reduce the friction between a fast-moving object and the air or water around it, the front of the object is made narrow. Making such a shape is called streamlining, and aeroplanes, ships, rockets and high speed trains are all shaped this way. |
Read that table downwards and notice that all four are the same instruction expressed four ways: stop the projections from catching in the depressions. Polishing makes them smaller, oil fills the gaps so they cannot meet, rolling means they never slide past each other, and streamlining reduces how much air has to be pushed aside and scraped along the surface. If you understand the mechanism you never have to memorise the list, because you can rebuild it.
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