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When an object moves through air, the air closest to the object’s surface is dragged along with it, pulling or rubbing at the air that it passes. This rubbing exerts a force on the object opposite to the direction of motion—friction drag.

The thin layer of air closest to the surface of a moving object is called the boundary layer. This is where friction drag occurs.

Reference

What is the difference between this drag and the drag that appears when an object is in a free fall? if it is the same, how a molecule of a fluid that is sticked to the object can produce friction and thus heat?

Thank you

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That is the same force, both in free fall through the air/atmosphere and, say, in wind tunnel.

Heat is transferred to object in free fall/wind tunnel via transfer of momentum from air. Momentum is being transferred due to collision of stationary particles (surface of object and molecules stuck to surface) with surrounding particles.

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  • $\begingroup$ Great, Thank you and even in a free fall there will be a gradient of velocity like an horizontal plane right? $\endgroup$ – Tonylb1 Jun 12 '15 at 23:11
  • $\begingroup$ just rotate the system 90 degrees and nothing will change. Except that gravity now acts same way as propulsion force moves a jet $\endgroup$ – aaaaaa Jun 12 '15 at 23:12
  • $\begingroup$ both are good haha but I cannot vote up I need 15 reputation $\endgroup$ – Tonylb1 Jun 12 '15 at 23:15
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There is no difference between free fall and say an aircraft wing moving horizontally, as regards the air molecules closest to the surface.

Friction between the surface and the air will still cause the air molecules closest to the surface to be essentially stationary.

Take the Blackbird SR-71 spy plane , it stretches a significant amount because of heat caused by air friction, and also a meteorite heats up for exactly the same reason, even though it is in free fall.

When an object moves through air, the air closest to the object’s surface is dragged along with it, pulling or rubbing at the air that it passes. This rubbing exerts a force on the object opposite to the direction of motion—friction drag

In both cases there is a gradient of air velocity normal to the surface, we try to control the laminar flow for the plane to minimise drag, but the meteorite is out of our control, obviously enough.

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  • $\begingroup$ Great Thanks, so when a molecule of a fluid stick to the surface of the free fall object, all the kinetic energy before it sticks will be converted to heat right? $\endgroup$ – Tonylb1 Jun 12 '15 at 23:09
  • $\begingroup$ Well, energy must be conserved. So take the case where the air is still and the plane/meteorite moves through it. As the air molecule hits the surface, it transfers its k.e. into heat, $\endgroup$ – user81619 Jun 12 '15 at 23:13
  • $\begingroup$ Thank youu both answers are good but I have to choose haha $\endgroup$ – Tonylb1 Jun 12 '15 at 23:16

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