Complicated question concerning air speeds/friction. can you help?

Question

If you're standing still outside anywhere, there's a constant temperature. let's just use 70 degrees F.

When you're in a car going approx. 55MPH and you stick your arm out the window. The air is a cool temperature, cooler than if you weren't moving. Probably around 50 degrees give or take.

Now, when a shuttle comes back from the moon and enters the earth's atmosphere. It bursts into flames from the friction of the atmosphere/oxygen. that must mean that it's very hot. that's going really fast. i'm not sure of the speed.

my question is: at what speed/velocity does the temperature change from being cool, to getting warm? that is.. at what speed does the friction from the air start to warm things up instead of cool them down?

Answer 1

Its not quite like that.

When you stick your arm out the window, the air is the same temperature as if you are still. The difference is that you are forcing air over your arm by sticking it out the window so there is more convective heat transfer. When standing still only natural convection is acting on you.

As for heat due to friction - that is always acting - there is heat energy being generated due to friction when you stick your arm out the window of a car but it is a negligibly small amount of energy and so has no affect on your arm's surface temperature. The faster you go, the larger this friction energy will become, but it does not contribute significantly to surface temperature till speeds greater than the speed of sound.

Answer 2

both ways air is resisting your motion, obviously the re-entry case is quite high in magnitude. upon re-entry into atmosphere speed is very high > so temp due to friction reaches red hot conditions > we make the outer case strong and choose this property of friction for deceleration rather than employing own tricks.

in car, human beings sweat continually to maintain a fixed body temp (~37 C), that sweat envelops the entire body surface. when air circulates over the body > it causes forced "evaporation of sweat" due mainly to FORCED convection (other agents being temp, humidity etc) >> since water changes from liquid to vapour state huge amount of latent heat MUST be supplied for it >>no source around to facilitate it >> body itself provides for it >> huge loss from body >> person feels thermal comfort

Question > will an animal also feel like that in a moving car?