laws of friction
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Posted by: ravindra.patil ®

01/06/2006, 07:00:40

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dear all,,,

Can any one plz tell me why the laws of static friction are independent of contact area & at the same time rolling friction is dependent upon the contact area ?????








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Re: laws of friction
Re: laws of friction -- ravindra.patil Post Reply Top of thread Forum
Posted by: ChrisMEngr ®

01/08/2006, 13:10:48

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This is a good question.

Sliding is different from rolling. Sliding friction is different from rolling friction. Yes they are both “friction” but there are different things going on in each of them.

Take two identical basketballs, one full of air and the other slightly deflated. Roll them along the ground with the same initial velocities. Which will roll furthest? The one that is more full will. What is the difference between the two balls? They both have the same weight. The only difference is that one has more contact surface area than the other. I found a site on the web that helps to illustrate what is happening.

http://webphysics.davidson.edu/faculty/dmb/PY430/Friction/rolling.html

It isn't necessarily the surface area but what comes along with the increased contact surface area. The resistant force to rolling is given a larger moment arm when the ball is flattest.

Contact area is not a factor in sliding friction. If you have a 10 lb load distributed over 1 in^2 then you will have 10 lbs on the sliding surface. One would think that if you have more surface area rubbing then it would be harder to slide. Lets say you have a contact area of 5 in^2. Yes it is larger than the 1 in ^2 section and there is more rubbing surface area but the 10 lb load is distributed over 5 in^2. So you actually have 2 lbs on each square inch instead of 10 on the 1 in^2 piece. So in the first case, you have only one square inch section rubbing. In the second you have 5. The first has a higher load and only one section. The second has a smaller load but more sections. In the end, they are equal.

Rolling friction on the other hand has to do more with surface area. Your rolling friction comes into play when your rolling object and the surface it is rolling on are not rigid. Due to the fact that they are not rigid, there will be some deformation. Depending on the rolling part and the surface it is rolling on, there can be a "plowing" effect on the rolling surface. The wheel will "sink" into the rolling surface and the wheel itself will deform and “smash” against the surface. The resistance to rolling due to surface deformation depends on the surface area and the geometry of the portion of the wheel is sinking into the rolling surface. Forces are resisting the wheel at a point that is not directly beneath the rotating axis. This creates a moment arm resisting motion. Let me know if that helped.

Chris








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Re: laws of friction Smile
Re: Re: laws of friction -- ChrisMEngr Post Reply Top of thread Forum
Posted by: ravindra.patil ®

01/08/2006, 22:35:35

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Thnx !!!!







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