# Friction....

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This is my question: Does the contact area between two bodies affects the amount of friction force applied back when you push it?!? An example is a box with dimensions 10x10x20 .If we push it when the any 10x10 face be in contact with the ground, a friction force F1 will be applied back.And if we push it when the any 10x20 face(a greater one) be in contact with the ground, a force F2 will be applied back too.... F1<F2?!? F1==F2?!? Thanks [Edited by - xissburg on October 8, 2005 5:04:44 PM]

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This is my explanation:
At first glance F1<F2 looks plausible, as F2 has a larger area of contact and therefore should generate more friction. But, with that larger area of contact you also get less preasure between the two surfaces (preasure = force / area) and that gives less friction.
Friction is independant of the contact area.. It is all about the force putting the box to the ground (e.g. gravity).

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Theoretically it is independent of surface area. However, in the real world of bodies that aren't perfectly rigid, surface area does have minor effects because of the deformation of surfaces. I'm almost inclined to say F1>F2 since it pushes in more.

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Quote:
 Original post by xissburgThis is my question: Does the contact area between two bodies affects the amount of friction force applied back when you push it?!?Thanks

Yes: real world objects have a mechanical gearing effect between surfaces. If this were not true, adding wider tires to cars would have no effect. In your physics engine you could add a "gearing factor" to control the behavior.

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well....I have nothing to say about you've said but, I've read in a car physics article that 'larger the tyre larger the maximum friction force'.It means that 'larger the contact area larger the maximum friction force'...

what do we do now?!? :)

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Coulombs friction model only works for objects which are completely rigid (idealized objects, effectively).

Depending on what you are doing, it may be good enough.

Tire 'friction' is an amazingly complex thing that is the subject of massive research for the tire companies themselves. The understanding of that is constantly evolving. New models are constantly being created.

In the tires case, yes, more surface area usually does mean more 'grip'. The simplest way to explain it, I think, is that
1) the rubber deforms to fit the texture of the ground. More rubber doing this grabs better.
2) It reduces the pressure on the tires footprint. You have the same load acting over a larger area. This can mean less tread squirm, cooler temperatures, etc.

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Thanks for your answers men. Consider my question resolved since the thing is too complex and I don't have any interest on exploring it :) .

Thanks

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Quote:
 Original post by xissburgThanks for your answers men. Consider my question resolved since the thing is too complex and I don't have any interest on exploring it :) .Thanks

If you add a few tweakable factors, you can get the desired behavior for a wide variety of objects required for your game, without the need for more complex models (thus keeping it simple).

Another interesting behavior for tires is "tire load sensitivity", where the effective coefficient of friction drops as downforce/load is applied! See M&M's RCVD for more info.

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