When I move at speed through the air, it's cold. When an object reenters the atmosphere, it burns up. Why?

Satisfaction
0
There are (at least) 3 different competing mechanisms going on here.
The first is recycling the air that is conducting with your skin. This is why you feel colder when there is wind or when you move. When the air is cooler than your skin and could be kept completely still what would happen is heat is conducted from your skin to the air, warming the air up and cooling you down. The air eventually equalizes temperature to you and you will stop feeling cold.
Generally air is moving a little so warm air leaves and cooler air comes in and there is a constant loss of heat to your surroundings. When it is windy (or you are moving) you have a very tough time retaining your blanket of warm air; it is constantly replaced with new, cold air. This is functionally identical to convection; in convection the hot air natural rises and cold air takes it place, here you are forcing the hot air to leave and replacing it with cooler air with a breeze. A reply has informed me that this has a specific name, forced convection.
If the air was warmer than your skin then this process would reverse and wind would make you feel warmer than sitting still. (you can experiment with this to some extent in the bath, run a hot bath and sit still in it then move your arm and the water should feel warmer).
Clothes do a lot of work in keeping you feeling toasty by trapping this blanket of warm air, keeping the cool air at bay.
When a spacecraft reenters the atmosphere it is going incredibly fast, much faster than the surrounding atmosphere's sound speed. Basically the air in front of the craft can't 'get out of the way' so it is compressed by the spacecraft. A gas being compressed heats up and this causes the dramatic heating of objects re-entering the atmosphere. This is entirely separate from friction.
Friction is when the air passing over the surface of an object rubs against that surface, converting some of the bulk kinetic energy of the flow into random motion (heat). Supersonic aircraft get heated up a lot by friction from the surrounding air.
Whether frictional heating or compressional heating is more important depends a lot on the relative speed, the properties (density, temperature) of the air, the coefficient of friction of the object, the cross sectional area of your object and the wetted (surface) area of the object.
There would never be a speed which would balance this for all objects as it would need a unique solution for each combination of variables.

Post a Comment

0Comments

Feel free to write

Post a Comment (0)