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I am talking here about dry friction between solid objects, for example a ruler and a table, not anything lubricated or fluid.

I noticed that with a ruler and a table for example, if you drag the ruler like it was a knife, it is much easier to do it than if still holding the ruler in the same position you drag it sideways (like if you are scrapping something).

Also I noticed that when I am washing dishes, if I leave two metal objects (ie: to flat metal areas) in contact, they don't move much, but the same objects, if I try to find deformations that make the area of contact between them smaller, then they can be easily pushed around from rest, or spun.

This also applies to tyre sizes (ie: for dramatic effect, dragster cars with HUGE rear tyres and tiny front tyres).

Also I did some experiments with a paper, ie: holding it down with a finger make it much easer to slide than if I make sure more of its area is in contact, but doing that also is a downforce on the paper, so I guess I can sum that on the normal force.

My best guess is that it all has to do with the normal force, but I am not sure at all... Can someone quench my curiosity here?

Qmechanic
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speeder
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2 Answers2

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Deformities in the surface cause more friction. As it's rightly said friction is a necessary evil, friction between the surface helps in forward movement as well. Like when we walk on a rough surface we get more forward push.A slippery and smooth surface is really difficult to walk on. A very nice article with Free Body Diagram is here, which you can check pls: How friction helps in walking

Anupam Majumdar
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To state it simply, friction is the resistance to motion of an object within a system, in this case a ruler on a desk.

As you suggest in your question the normal force to the surface is important to friction, the equation is:

Coefficient of friction = force required to maintain constant velocity / normal force

however turning the ruler on its side does not change the force on the desk, as the weight of the ruler is unchanged.

Unfortunately this coefficient is not calculated from a formula, it is empirically measured for each system for which it is needed to be known.

All this means is that your two systems, ruler on side, and ruler on flat have different coefficients of friction.

I realise this may have not have fully satified your curiosity, but I think it is all I can accurately say.

Hope this was in some way helpful.

James
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