David Hudson asked:
Why do things burn up on entering Earth's atmosphere?
Dave - The main reason why things heat up when they hit the Earth’s atmosphere is they've got huge amounts of kinetic energy - they're going incredibly fast.
When they bash into the Earth’s atmosphere, most of the heating is actually because the air they bash into hasn’t got time to get out of the way, so the air gets compressed; and when you compress air, it gets hotter. You may have noticed this if you've ever pumped up a bicycle tyre very, very quickly: the end of the pump gets hot.
So, the air in front of the inbound object - such as a meteor or even an asteroid - heats up, and that starts to erode the surface of the object and you get this tail of hot, burning material, which you see as a shooting star.
With very small things, because the friction is so much larger compared to their mass, they tend to lose their speed much more gently very high up in the atmosphere, so they slow down more gently and don’t get as hot. And once they slow down enough, they just drift down like dust through the atmosphere.
So, it is conceivable that something like bacteria clinging to a small dust grain could survive re-entry from space, whereas a big lump of rock would melt very quickly.
It's also to consider that the faster a gas is going the more kinetic energy it has and increase in kinetic energy means an increase in temperature Pmb, Sun, 20th Oct 2013
Regarding the two explanations given above, they are completely equivalent. One is based on a microscopic level and the other on a macroscopic level.
It doesn't matter what I understood.
"Of course it does. You confused ram pressure with fluid pressure in this particular case."
By the way. For those of you who have a problem with using the term "temperature" other than as the scalar (i.e. Cartesian scalar/invariant) that one uses in thermodynamics, i.e. as in reentry temperatures I strongly suggest that you not get bogged down with concerns about who used a word correctly or not and train your thoughts back to the real question, i.e. Why do objects burn up on entering the Earth's atmosphere?
Back in the depths of ancient history, what you said was this
Having read this thread with interest, I suspect that if anyone asks me this question I shall say "friction". Naive? Perhaps, but it may be what the person is looking for.
can u go very slow and not heat up dd, Sun, 21st Dec 2014
When rockets & the shuttle leave the earth...they take advantage of the way earth spins (+20,000mph in one direction) and leave with the earths spin (rather than against it) to not get burned GOING INTO space ....by taking advantage of this spin the rocket consenquently is now going +20,000 mph. When you slide this speed on the atmosphere molecues upon rentry... Plasma burn.... Now to drop with zero speed youd have to slow down back to 0....meaning the power it took you to go into space now needs to be used to slow down (where are we going to bring these tools as every rocket drops booster or stage rockets during launch)....so unless the object is very small...it's totally impractible (although satiliettes have achieved RETROGRATE (going a speed oposite of earths rotation) orbit).....but if you slow you speed to zero just as an apple falling so too would you ....like Felix you might go the speed of sound but you will slow to TERMINAL velocity and survive. There are organizations looking into making a sort of personal ecape suits for emergency escape to earth from space. jim jones, Fri, 11th Sep 2015
To Pmb: when he said putting a gas cylinder in a car, he literally meant having a cylinder of gas probably sitting in the passenger seat or something. If he starts driving his car, this cylinder isn't going to heat up. If the molecules in the cylinder started moving faster then the gas would heat up. But that wouldn't happen unless the cylinder was being compressed or heated with a flame or something. The cylinder would gain kinetic energy as it's keeping pace with the vehicle. The gas molecules might get slightly compressed upon acceleration due to inertia, but other than that there would be no *notable or significant* change in temperature. Simba, Sat, 9th Apr 2016