When you try to slide a heavy object across your floor and have difficulty, you talk of friction. Or, you might be getting ready to paint and have to sand something smooth. You speak of friction making the sandpaper and the surface you’re smoothing warm. But, do you really know what friction is?
Friction is the force that creates a resistance to two objects moving across one another. The underlying force that creates friction is the electromagnetic force. The electromagnetic force creates friction in two ways.
The first way the electromagnetic force creates friction is the attraction of the molecules of one object to the other where they are in contact. Molecules of different materials in contact with one another will be attracted to each other to differing degrees. When you move one object across another the electromagnetic bonds of attraction between the surfaces of each object have to be broken. This breaking of the bonds of attraction takes force to overcome and the resistance of the bonds to that force is part of what causes friction.
The second impact of the electromagnetic force, and in most cases the most significant, is the bonds that make up the surface of each object. Sandpaper run over a rough surface erodes part of that surface, breaking the electromagnetic bonds that hold that surface together. When those bonds are broken parts of the surface being sanded comes off. The tighter those bonds are the more difficult it is to break bits off.
If the electromagnetic bonds of the material are strong and the surface is smooth the sandpaper simply slides across the top of the material, like in the first cause of friction, and removes very little material, and causes little friction. If the bonds are weak and the surface is rough the sandpaper will remove a lot of material but have little friction. The highest amount of friction comes when the electromagnetic bonds are strong and the surface is rough. In that case the only way to move the sandpaper across the material is to actually break those bonds and remove some of the material, and this is much harder because those bonds create more friction.
Sandpaper, however, is just an obvious example of this type of friction. All surfaces, at a microscopic level, are rough. Sandpaper is one of the most obvious example but even what we might consider a very smooth surface has microscopic pits and valleys and peaks. Just like the different examples with sandpaper above, the smoother the surface is the less friction it will present. And, the weaker the bonds of the material on the surface the less friction you will have when two materials are moved across one another.
To reduce friction we can put something between the surfaces that will help to prevent bonds from forming or to help break apart the bonds that form. An example of this is when we coat two surfaces with oil. The oil fills in the tiny peaks and valleys of the rough surfaces and creates a film between the surfaces. That film allows the two surfaces to actually slide on the oil, rather than on the materials that makes up the objects. As you probably know, this greatly reduces the friction and the force required to move the objects.
You probably have noticed that if you try to move an object over another when they are stationary to one another it is usually fairly hard to get started. But, when you have them moving across one another less force is required. This difference is due to the two types of friction recognized by scientists, static and kinetic.
Static friction is the friction of two objects that are at rest relative to one another. Static friction is usually much greater than kinetic friction. To get something moving you have to overcome this friction and, once things are started, you are working with kinetic friction that uses less force. This is one reason it is easier to keep things moving once you’ve started them moving. (Another reason is inertia, which would be another topic.)
This discussion has dealt with friction associated with moving relatively flat surfaces over one another, but there are other types of friction. Rolling friction, or rolling resistance, has to do with objects rolling over a flat surface. Fluid friction has to do with the resistance of fluids flowing past a surface.
As you can probably tell from the description of what causes friction, the amount of friction is dependent on a number of factors. The types of each material and the force holding the materials together are probably the most important. Scientists and engineers can predict the amount of friction between two objects by looking up numbers, called the coefficient of friction, for the two materials and using a relatively simple equation.
Friction is very important. Friction keeps us connected to the world around us. Without the friction of the tires of our car on the pavement, the car wouldn’t move. Without the friction of our shoes on the ground, we couldn’t walk. We can see this in the winter when there’s ice on the roads or the sidewalk. The ice drastically reduces the amount of friction and our car tires spin without moving us forward and, if we’re not very careful, our shoes slide on the ice and we could have a nasty fall.
Friction even plays a part in making sure you don’t slide out of your chair.
So, next time you have to move a heavy object, don’t curse friction. Just try to figure out a way to over come it and make your job easier.