Quote:
Originally Posted by Bongle
This isn't _entirely_ true. In an ideal physics world, yes, but there are cases where surface area is proportional (or inversely proportional) to grip.
-Racing cars have very large, wide tires (even though this increases unsprung weight) because it means that if the tire hits an imperfection in the track, it doesn't lose grip. Since the track is always imperfect, this has the effect of effectively very slightly raising the car's coefficient of friction (although the ideal rubber-on-ashphalt coefficient remains the same)
-Skates have very little surface area because it takes a lot of pressure at the skate-ice interface to create the microscopic layer of ice that the skate glides on.
Likewise, I'm not sure the rubber-on-carpet case is a cut-and-dry Ff = uFn case. It might be like Velcro: if you have two big sheets together, it is much harder to pull one off sideways than if you have two small sheets together. Despite the normal force being the same, it takes much more force to move the two sheets relative to each other when there is more surface area in contact.
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Sort of true. However, your Velcro analogy isn't exactly a wise one. Velcro is designed to lock together. It's designed not to move in any direction. The more locks you have on a door, the harder it is to open, right? The same for Velcro. Rubber on carpet isn't designed that way, and neither is rubber on HDPE. And, with velcro, if it's on some smooth surface, it won't stick. There's a piece of HDPE in every field box because of this fact.
Skates are designed to create semi-frictionless motion (or to use friction to create that!). They need to melt the ice and have it immediately re-freeze. Ever wonder why ice is considered frictionless?
As for the racing car analogy, that's more the right track, but you're still off. I'll link to a thread shortly that will help. (There's also a quote from the spotlights--"A drag racer can have all the output torque in the world, but it does no good on ice."--Paul Copioli) I also remember reading that the wide tires are actually to dissipate heat.
http://www.chiefdelphi.com/forums/sh...surfa ce+area
This thread is the most recent in a series asking if you get more traction (aka friction) for a wider wheel. If you want to argue that with Copioli and Baker, go right ahead. Oh, and Baker and the TechnoKats did tests on tank treads vs. wheels. See linked thread.
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