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Do You Ever Look Up in the Sky, and Wonder…

space_ritual.jpgSure, there are all sorts of facts about space in the new edition of The World Almanac, but we don’t say what would happen if a satellite fell to earth. We don’t contradict claims that the moon landing was really a hoax. And we also don’t explain your chances of being sucked up by a black hole. Go to Get a Straight Answer for facts on these and many other important space issues brought to you by retired NASA astrophysicist David P. Stern. One I found particularly amusing is after the jump.
239. Sticking a hand out a window...

I am not attached to any school or college and just hope that you can settle a work place argument. If you put your hand out of a car doing 70mph your hand will cool down due to the wind chill. But if you're traveling at thousands of mph then your hand (if you could stick it out of the window) would heat up due to friction. Is this idea correct? And does it mean that there exists a speed at which the temperature of your hand would neither increase or decrease? And more importantly, if this is true, what is that speed?

Reply

I am afraid I don't know the answer, but in any case, your question is trickier than it seems, so much that I am not sure it has a good answer. Let me explain. When you stick your hand out of the window, it will usually emit or receive heat. Some possible processes:

  • It is cooled or heated by the air passing by it, through conduction of heat.
  • It is heated by the resistance of the moving air (overcoming resistance absorbs energy).
  • It radiates heat or receives heat by radiation.

Let me start with conduction. What is the temperature of the air outside? Here in Maryland it is February right now, the air temperature is close to the freezing point and if you stick your hand--which receives blood at about 98 deg. F--out of a car window, it is likely to be cooled, Stick it out on a summer day in Baghdad, when the air is at 120 deg F., and I am not sure what will cool what.

And which side of the car will you be sitting there? All warm objects radiate heat, including your hand, and this cools them down--but meanwhile, they also receive radiation. If you sit on the sunny side (in Baghdad), your hand probably receives more than it gives out. If it is in deep shade, maybe not (though if hot walls are nearby, heated by sunlight, they radiate towards you, too).

And heating need not be evenly distributed. Consider a bullet. A pistol bullet flying below the speed of sound pushes air out of its way and encounters resistance, which heats it up. A rifle bullet is supersonic, and in addition also compresses air in its front, creating much greater heating. But the heating is not evenly distributed, the tip gets much hotter than the rest (in either case). It is not just "what speed", but what part of the object, too.

A practical problem: an airliner at 35,000 feet. The outside air there is very cold, say 40 below zero, and the airplane skin is quite cold--in spite of the friction of air rushing by, at close to the speed of sound. But I read somewhere that in airplanes going at 3 times the speed of sound, their skin can get warm enough to be weakened (at least at the front edge). Sticking out a hand from such a plane would not be a good idea--besides, the force would be so high, even at jetliner speed, your hand would be ripped off before it could get much heating.

Anyway, those are some thoughts. May the Force be with you.

Get a Straight Answer (David P. Stern)

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