Physic Lecture, Week 1, FRICTION!

Some of you may be interested in the material, but not know about the science behind it, or how to even begin solving such problems. This lecture is for you.

With every pop quiz there is a load of concepts integrated into the question. And a lot of material if you wish to dig deeper than the strict "how do I solve it" mentality engineers have :) So I hope to expound upon the material in the question, and hopefully explain how one solves such problems.

Things taken for granted in this particular problem are balancing of forces, an understanding of the "normal force", decomposition of vectors (well, sorta, more on that next week) and probably a few things I'm over looking due to familiarity with the subject.

So, on to the actual "lecture" and feel free to ask ?'s and discuss!

---------------------- Part 1, Types of Friction, and coefficients ------------------------------

This week's problem focuses primarily upon friction, and how it interacts with the motion of objects. To understand how to solve this problem, one must examine the various components of the frictional force that motion across a surface.

Nobody knows, precisely, what causes friction, though there are a couple of general ideas about it. Part of the problem is that there are actually two kinds of friction! There is "static" friction and "kinetic" friction. These vary in strength depending on the materials involved but always (as far as I know) the static friction is stronger than the kinetic friction.

Have you ever noticed that it's harder to start moving an object, than it is to keep pushing it once it's moving? You'll push and pull and tug on that old refrigerator until it finally budges, and then once it's moving you have a much easier time of it. Of course, at some point you stop pushing again, and once again, it's really hard to start sliding. This is due to the differences in the two types of friction.

Friction in general is believed to be a combination of the two surfaces catching and snagging against eachother, even on an atomic scale. The rougher the surfaces, the more they catch, the stronger the friction. Unfortunately there are exceptions to this rule. For instance if you take two highly polished and clean steel plates, and place them together, they can actually fuse together far stronger than if they were rough pieces of metal.

This is because there are two other mechanisms for friction. One is sorta like a suction cup action, where small cracks and fissures in the surface of a material, even a smooth material, shift and flex when the objects are brought into contact. This flexing can create a sort of suction seal that needs to be broken before the object moves.

Then there is of cold welding or bonding. This is the main cause of the two steel plates sticking together. If you bring the material close enough the molecules between the two objects will actually form weak bonds, melding the two materials together.

Of these three basic mechanisms for friction, two of them work best when the object is stationary, allowing them time to adhere (though all three work all the time). And as a stationary has 3 strong frictional mechanisms at work, the resulting force due to friction, for a stationary object, is much higher than if the object is moving, where only the "catch and snag" mechanism works efficiently.

The strength of these two types of friction are contained and communicated by their "coefficients." These are constants, reffered to with the greek letter mu (µ) and are usually determined empirically. The larger the coefficient, the stronger the frictional force. Typically this value is less than one, however if you use adhesives, the value can exceed 1.

To calculate the frictional force then, you need to know the strength of each type of friction, and correctly use the right type! If the object is sliding past the surface, you use kinetic friction. If the object is stationary on the surface, you use the "static" friction. There are cases where this can be hard to determine. Take, for instance, a walking man. The man is moving, so you would think to use the kinetic friction coefficient. But you'd be mistaken. The key is where the man touches the ground. Usually a walking mans feet do not slide across the surface. They stick firm, an the man then sorta falls past his feet. Since his feet are not moving across the surface themselves you have to use the static friction coefficient.