2012-02-04 - Friction

Frictions is basically the most interesting force in the universe, it affects just about everything and is constantly switching between being helpful and detrimental. The textbook definition of friction is a force that resists movement but we can take that one step further. The physical definition of friction comes out of the fact that apart from some small manufactured surfaces nothing is perfectly smooth; every surface has microscopic bumps and indents. As the imperfections of two such surfaces collide they cause resistance to the movement. You can think of it like a truck running into a sand pile or a ball rolling into a pot-hole. In order to overcome that obstacle more force is required and so friction is the force required to overcome the microscopic obstacles caused by moving two imperfect surfaces over each other. That’s sounds much more interesting doesn’t it?

Now at this point you would not be remiss in thinking that friction is a bad thing. After all it’s a force that resists movement and in most cases movement is good. Friction is the force that makes it difficult to push that large box across the floor. You require oil in your car to prevent friction from wearing out the moving parts of your engine. Friction is the force that causes scrapes when you slip and slide along a gravel road. You have to replace the tires on your bike every so often because friction has worn them smooth. The strange thing though is that even though friction resists movement it is also required for several specific and important forms of movement.

It would be impossible to drive without friction. Alright maybe not impossible, but it certainly wouldn’t work anything like it does now. As you press down on the gas pedal your car’s engine burns gasoline to create energy which is eventually used to turn the wheels of your car. As the wheel turns there is friction between it and the road at the point of contact which resists movement. If you have ever watched a car move you might have noticed that the actual direction of the movement of the tire as it contacts the road is in fact in the opposite direction to that of the car. This means that the force of friction is actually in the same direction as the car’s movement and so friction pushes the car forward. If you try driving with smooth tires on a smooth surface the tires will most likely just spin because there isn’t enough friction to move the car, this is where the idea of traction comes from. It should be pointed out that you also can’t have too much friction or else it will prevent the wheels from turning at all.

Walking also require friction, without it your foot would just slide out from under you. If you pick up an object by simply holding it between your fingers or hands (i.e. most glasses) friction is the force that is counteracting gravity and preventing it from falling. If there was no friction movement would be possible only by pushing off of something and the only way to stop would be to somehow create and equal and opposite force say by running into something. Without friction it would be impossible to start a fire by rubbing two sticks together and we probably would have died out thousands of years ago.

That is why friction is the most interesting force in the world. On paper it seems like a bad thing but in reality it is required for so many things that we do.