what is air friction?

Answer 1

Air Friction

Air friction, or air drag, is an example of fluid friction. Unlike the standard model of surface friction, such friction forces are velocity dependent. The velocity dependence may be very complicated, and only special cases can be treated analytically. At very low speeds for small particles, air resistance is approximately proportional to velocity and can be expressed in the form
f drag = -bv where the negative sign implies that it is always directly opposite the velocity. For higher velocites and larger objects the frictional drag is approximately proportional to the square of the velocity:
f drag = -1/2(c*pAv^2)

where ρ is the air density, A the cross-sectional area, and C is a numerical drag coefficient.

Answer 2

When something moves through the air it is moving through a medium. Just like there is friction when you slide your hand across a table, there is friction when you move your hand through the air, only much less.

At slow speeds the friction is minor, but at high speed the friction can cause serious drag, as well as heat.

An example would be the SR-71 spyplane. At top speed there is so much friction from the air molecule sliding over the cockpit that you would burn yourself if you reached up to touch the glass.

Answer 3

It is the force acting in the direction opposite of motion that is caused by the moving object having to, literally, push the air out of it's way. Stick your hand out the window of a moving car and you'll feel it.

Rbanzai is correct about the SR-71. At night, at top speed, one can actually see the leading edges of the wings glowing dimly due to the heat generated by air friction.

Doug