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vanillacoke

physics question

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Does anyone have any information on the contact friction force? I''ve always wondered where it comes from, and my physics book just says that F=k*N is a good approximation. I assume that it comes from the electrostatic force, but I''d like to read about it.

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Guest Anonymous Poster
Frictional force opposing the direction of motion when a body slides on a rough plane could be expressed as k*N, where k is some coefficient of friction in the range 0-1 (usually written as mu) and N is the normal contact force (usually R), the component of the weight and any other force on the body, which acts perpendicular to the plane on which the body is sliding. For a block of mass M sliding down a plane at an angle theta to the horizontal, the normal reaction force is M*g*cos(theta), and the frictional force is acting parallel to (but up) the plane, resisting the motion.

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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

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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

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Guest Anonymous Poster
"k is some coefficient of friction in the range 0-1"

Actually k can be larger than one.

Don''t believe it? try this:

Take an empty casette for a tape for an old casette recorder.
Hold it (on the sides) between your thumb and indexfinger so one corner points directly down and one directly upwards - after a few tries this is actually possible and it looks almost supernatural.
How is this possible? the friction between the casette and your finger is > 1

read it in a phisics book and tried it myself so I guess its true.

Ulf Livoff

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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.

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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

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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.

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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?"

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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.

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