Question

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☺ ☺ Give one example to show that friction due to air is much smaller than friction due to liquid?. ☺ ☺

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Answer 1

Wave your hands in the air quickly. Easy, right? Now do the same thing when your hands are submerged underwater. Difficult, right?
Other examples are present, bit I couldn't think of any. Maybe others can give better examples.

Answer 2

Wave your hands in the air quickly. Easy, right? Now do the same thing when your hands are submerged underwater. Difficult, right?

Other examples are present, bit I couldn't think of any. Maybe others can give better examples.

Why does friction not depend upon the area of contact?

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Haridev Vaikundamoorthy, Turing test negative

Answered Jul 16, 2016

That friction does not depend upon area of contact is a misnomer. Friction does not depend upon the apparent area of contactbut depends very much on the real area of contact. This can be explained easily -

When pressure acts upon two surfaces in contact (maybe due to weight) and the surfaces have relative motion, the lateral friction force tends to shear the crests on the respective material surfaces. This is because of atomic roughness of the surfaces. When you touch a surface, you are only touching the crests of the surface.

A macroscopic explanation is possible. The frictional shearing results in yielding of the material at the contact. Yielding is the phenomenon when a certain amount of load is applied after the elastic limit of the material, the material starts deforming continuously (flowing) under the applied stress, called plasticity. The friction force divided by the real area of contact equals roughly the yield stress, which is a material property and is a constant for a given material:

FfAreal=σy=constantFfAreal=σy=constant

or, Ff∝ArealFf∝Areal

Now, the real area of contact is directly proportional the normal force:

Areal∝NAreal∝N

From the two equations, we get that Ff∝NFf∝N

Notice that the apparent area of contact appeared nowhere in the equations. Friction force depends only upon the real area of contact, not the apparent area of contact. Since it is very difficult for us to estimate the real area of contact, we often use the last equation in engineering and physics. It serves good most of the times.