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Isn't Air resistance very similiar to friction? So why is air resistance an exception that depends on contact area compared to other frictional forces?

Zheer
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    Does this answer your question? Does the contact area affect friction forces? – Charlie Mar 20 '20 at 17:29
  • @Charlie The link explains dry friction, but I don't think it explains air resistance. – Bob D Mar 20 '20 at 17:33
  • This might also be relevant: https://physics.stackexchange.com/q/154443/, I can't find anything that directly compares friction and air resistance though. – Charlie Mar 20 '20 at 17:40
  • For air resistance once must move a given volume of air out of the way per unit length moved. Details of the air-surface interaction have some impact, but you still have to move all the air. For friction see the answer above. So, no, there are major differences. – Jon Custer Mar 20 '20 at 17:48
  • Re, "Isn't air resistance [similar to sliding] friction?" In a word, No. In the case of sliding friction, you're talking about forces between two solid surfaces that are in close contact with each other. In a first-order approximation of "air resistance," you're talking about the force needed to push a massive fluid (air) out of your way, which is not any kind of "friction" at all. – Solomon Slow Mar 20 '20 at 18:16
  • Also, you're talking about two different kinds of "area." In the case of air resistance, you're talking about area in a plane perpendicular to the direction of motion--proportional to the amount of air that must be pushed out of the way for each unit of distance moved. In the case of sliding friction, you're talking about area in a plane that is parallel to the direction of motion--proportional to the number of molecules or atoms of the two surfaces that are able to interact with each other. – Solomon Slow Mar 20 '20 at 18:21
  • @Solomon Slow, so you mean that the fluid or air tries to stay in equilibrium state by pushing, but what about friction? i have thought that it is friction that causes that – Zheer Mar 20 '20 at 19:52
  • What other frictional forces are you thinking of apart from dry friction? There is viscous drag which also depends on contact area. And there is rolling resistance which varies with contact area but it not proportional to it.. – sammy gerbil Mar 20 '20 at 22:14
  • Another related question: Dependence of Friction on Area – BioPhysicist Mar 20 '20 at 22:54
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    @Zheer. Air has mass. You can't make air move without exerting force on it. And you can't move through it without making it move out of your way. – Solomon Slow Mar 21 '20 at 20:28

2 Answers2

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Because the fact that a portion of the area is deflecting air and thus feeling drag does not prevent (on average) other air around it to be deflected by neighbouring areas, thus increasing the total drag on the object.

For friction, however, since it is proportional to the normal weight, and since the total weight is held constant (I assume), the fact that some portion of area is feeling the weight means that it is holding part of the shared weight, removing part of the weight from their neighbours, thus decreasing the friction force per unit area.

In a word, in the first case more area displaces more air, while in the second more area sustains the same weight.

Guillermo BCN
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The link that @Charlie provided ( physics.stackexchange.com/q/154443 )already provides the details of the reasons for the independence of dry contact friction on surface area. The following will rather elaborate on the difference between the mechanisms of air resistance and dry friction.

Both air resistance (a.k.a. air drag) and dry contact friction are dissipative forces. That is, they dissipate the macroscopic kinetic energy of the moving object(s) involved and convert into other forms (heat, light, etc.). However the mechanism by which the energy is dissipated differs as well as the dependency upon surface area as you already know.

In the case air resistance, the moving object has to "push" or compress the air in front of it while moving it out of the way. All other things being equal, the larger the projection of the surface area of the object in the direction of motion, the more air that has to be pushed away and therefore the greater the air resistance. The work the object needs to do to push the air results in a loss of macroscopic kinetic energy of the object. The main result in an increase in the local temperature of the air (an increase in its internal microscopic kinetic energy) sometimes, though technically erroneously, referred to as heat.

In the case of dry contact kinetic friction, the relative motion between the surfaces raises the temperature of those surfaces (increases the internal microscopic kinetic energy of the materials). The elevated temperatures then result in heat transfer to within the materials and to the environment.

Hope this helps..

Bob D
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