physics question

Friction is created because two objects are not perfectly smooth, and in places, they actually weld together. When an object is stationary, welding between the surfaces takes place, and it takes a lot more force to break these bonds. Once an object is moving, the force of friction is the force needed to keep the imperfections between two surfaces sliding over each other. As you increase the normal force, you increase the pressure between two objects and thus you cause more welding to occur between two surfaces. This is where the proportionality between force friction and normal foce comes from. You are right in that these forces arise because of the electric force. When two objects slide over each other, parts of each are transfered, and thus you have to overcome molecular bonds and any bonds that form between the surfaces. It''s a macroscopic description that works well unless you are changing the interactions between the objects significantly by having an extremely large normal force. I think that for most forces experienced in everyday life, the F = kN formula works fine.

Brendan

AP-
There is no restriction on the range of mu, except that it has to be positive (or zero in a frictionless environment). There have been experiments done where they cut brass in a vaccumm, and when they put it back together the object welds to itself very strongly. If you measured mu, it is well over 100. The only way to determine mu is by experimentation. The easiest way of doing this is having an incline at an angle theta, material A on the incline, and material B wrapped around some mass. Change the inclination until the block slides down the incline at a constant speed after being given a small push (to overcome static friction). The tangent of the angle is the coefficient of friction. You can go through a derivation of this to show that it is true by finding mu in terms of theta such that the forces in the plane of the incline are zero.

Brendan

Hmm... new question... Where does the normal force come from? Does it also arise from electrostatic interactions? Or is it one of the mysterious strong or weak forces that I can''t seem to find any information on? It doesn''t seem like an inverse-square force could behave like the normal force, so I don''t know what to think.

The normal force isn''t a fundemental force. Its just a side effect of Newton''s second law (2nd? the one about forces coming in pairs). If two objects are in contact, they exert some force upon each other. A component of that force will be perpendicular to the contact surface, that is the normal force.

CM

I hope I haven''t missed someone''s comment, but I think the fact that the friction is less or equal to kN, is very important. The frictional force is only ever kN just on the point of the surfaces sliding, or if they''re moving. I know it''s obvious, but it can cause a lot of confusion amongst people learning about it.

A good, but not accurate model of friction is this one, it explains the formula(sort of).

Take two surfaces with interlocking smooth triangular teeth. To move against each other, the top surface must be pushed up. This pushing is against the normal force from the top applied to the bottom. So as the normal forces increase, the friction must increase to overcome this force. If you think about it the forces are proportional. In this case the coefficient of friction is determined by the height of the teeth.

I have taken first year college physics; I have no trouble understanding how to use the friction and normal forces, I just can''t figure out, theoretically, where they come from. Obviously, they are not fundamental forces. I have always thought that friction can be reduced to a microscopic normal force, like in the example of microscopically rough surfaces. I still wonder, though, what actually causes the normal force. Just saying that it is a side-effect of Newtons second law isn''t enough; it doesn''t exist because I say so. in Newtonian mechanics there must be an upward force on a body resting on a flat surface under gravity, and this force must come from one of the fundamental forces.
So really, the question I am asking is "Which fundamental force(s) is responsible for the normal force, and what is the mechanism that causes it?"

The fundamental force that causes the contact forces like the normal force is the electrostatic force.
The reason your arm won''t go through a table is because the molecular bonds in the table repulse the molecular bonds in your arm.

The question about where the normal force comes from is a really good one. The first explanation I heard for it was the electrostatic force, but the more i thought about that, the less I liked it. If you put two electrically neutral blocks one on top of the other, you''ve got as much electric attraction as repulsion going on, after all.
I heard a second explanation for the normal force in an astrophysics class. According to this professor, the normal force is an entirely quantum mechanical phenomenon; you can''t stick your hand into a table because the energy bands in the space that would be shared by your hand and the table are already full of electrons; sticking your hand in would mean there were too many electrons for the (discrete) number of energy levels available, which is forbidden by QM.
Well I was curious about the fundamental forces question, just as you were, so I asked, "All right, but which of the 4 fundamental forces does this correspond to?"
He answered: "None of them."
I was obviously very surprised by this answer; the word "fundamental" must not mean the same thing to him as it does to me :-)

By the way, when I was a freshman, my first physics professor gave friction as an example of an area of classical mechanics that was still poorly understood. It has something to do with these "microwelds" but I don''t think anyone properly understands exactly why they happen.

Thank you, Muse and tenten. tenten, that''s what I always assumed was the explanation, but do you know if there is experimental evidence to back it up? I guess there isn''t, if Muse''s professor thinks it must be explained in terms of quantum mechanical phenomena. Muse, I won''t say that that answer quenches my thirst for knowledge, but at least I know as much about it as I can right now. Yargh, I''m frustrated.