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Is friction independent from area of contact?

by shaan_aragorn
Tags: contact, friction, independent
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Jan11-06, 09:25 AM
P: 43
Now, my textbook says that force of friction is independant from the area of contact. But, when the tyres of my bicycle get deflated i have to exert extra force than otherwise. Why?
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Jan11-06, 09:36 AM
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My guess would be that the rubber deforms more when deflated, and therefore more work is done while deforming the tyre. I'm not sure, like I say thats only my opinion.
Jan11-06, 09:45 AM
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The extra force required to cycle with flat tires could be explained independently of area of contact. For one thing, you are doing extra work in pushing a deformation in the rubber around the wheel, with each revolution. It's also not inconceivable that the surface properties of the rubber are different when it is under tension (from the air pressure) from when it isn't (no air pressure).

Jan11-06, 10:52 AM
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Is friction independent from area of contact?

You've overlooked a couple things. If the coefficient of friction is held constant, increasing surface area does not increase the force of friction. That's technically true, but in practice:

There are exceptions: Friction and area of contact

The coefficient of friction depends upon surface roughness, molecular adhesion, and deformation effects. A fully inflated tire has greater resistance to deformation than a softer tire. In other words, the coefficient of friction is different for a flat tire than a fully inflated tire.

The hyperphysics explanation about snow bothers me a little. Decreasing your tire pressure also improves your traction in sand, as well.
Jan11-06, 11:14 AM
P: 83
If you enlarge the area, the weight ofe object is spread over a larger area. Thus, however the area gets bigger, the pressure per square foot/meter/whatever decreases. The total pressure, i.e. the weight remains constant. However, tires need deforming, as said before.
Jan11-06, 11:43 AM
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Just anticipating the next question likely to follow the original one ...

If friction is independent of the area of contact, why do dragsters have wide tires?

This page has two lousy responses, but the third is very good (and very interesting).

Wide tires

In general, tires are a poor example of the principle you're asking about. However, if you understand the idea of static friction, you do realize the reason speeds in the quarter mile have gotten so much better over time has more to do with improvements in tire technology than the ability to build more powerful engines.

(This subject almost makes me homesick for my hometown. Aaah, the smell of burning rubber when they fired up the factory furnaces on Monday mornings. )

Trivia: The study of friction and lubrication is called 'tribology'.

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