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# Is "friction" completely defined??

We say that f=k.N Where N is the normal reaction and k is a constant. How come? If the root cause of friction is "ATTRACTION AT RANDOM" how do all the factors cancel out to give such a simple relation? Does k have to be a constant for a material, like refractive index? Seems quite implausible because to me, the cause of friction at the atomic level is not very well defined. Your thoughts?

Note by Arshdeep Duggal
3 years, 7 months ago

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Yes, friction is completely defined. Friction is a term to cover all of the wide variety of effects that make it difficult for one surface to slide past another.

Well, it should be obvious why frictional force is almost proportional to the normal force. If you have a heavier object pressing against a surface, you'll need more effort (force) to get it moving. In other words, more normal force implies more frictional force.

You'll notice that I used the word 'almost' in the beginning of the last paragraph. This is because the co-efficient of friction $$\mu$$ is almost never constant. No surface is absolutely uniformly smooth (or rough). But generally $$\mu$$ doesn't change so much that you notice something dramatic. And no, the coefficient of friction is not a constant for a material. It is a constant for a surface (you can have two surfaces made from the same material but they can have different $$\mu$$'s. Normally the rougher surface would have the higher $$\mu$$).

You can experimentally see if the frictional force is proportional to the normal force. If you were to carry out that experiment and graph the frictional force against the normal force, you'd get a straight line. It may not be the best straight line in the world, but it is a straight line nevertheless.

The cause of friction is anything that makes it difficult for two surface to slide past one another, for example :chemical bonds (that's why glue causes friction), electric effects (like the Van der Waals force) and physical barriers (surfaces with 'peaks' and 'valleys'; these are the things that make surfaces rough).

Hope this helps! · 3 years, 7 months ago