Felix Baumgartner and the speed of sound

  • #1
Doesn't Felix have an advantage when breaking the sound barrier since the air in which he is moving is less dense, therefore the speed of sound itself is considerably slower?

(Felix Baumgartner is the man who recently jumped from 128,000 feet in an attempt to become the first man to break the sound barrier without the aid of a vehicle)
 
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  • #2
phinds
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You may be factually correct (probably are) but it is irrelevant because (as I understand it) they did not actually consider whether he was breaking the sound barrier in the medium he was IN, they just looked to see was he going faster than the speed that is the sound barrier in normal air.

I see there is a whole 'nother thread on this:

https://www.physicsforums.com/showthread.php?t=643395&highlight=Felix+Baumgartner
 
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  • #3
I saw this thread. The speed of sound was more of an after thought, and it wasn't discussed to the extent to how it varies with the medium in which it travels, so I decided to start a new thread. Thanks for the reply. I agree that the sound barrier by which he is measured is the sound barrier in normal atmospheric pressure. I just thought it was worth mentioning.
 
  • #4
phinds
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One of the statements in the other thread said the poster thought that they HAD adjusted for the medium he was in but I have no idea if that's true.
 
  • #5
My mistake then. I must have overlooked that bit.
 
  • #6
boneh3ad
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I am pretty sure they did take into account the medium he was in. The sound speed decreases with altitude because, for a thermally and calorically perfect gas, it varies with temperature. It was all a matter of hitting the local speed of sound before the air got dense enough that the drag became too high. From everything I have seen, he made it to roughly Mach 1.24 in the air he was in at the time.
 
  • #7
cjl
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This is a common misconception - the speed of sound is independent of density for an ideal gas. It's true that the speed of sound is slower at high altitude, but this is an effect due to temperature - the speed of sound (assuming the gas composition is constant, and it behaves as an ideal gas, both of which are very good assumptions for the atmosphere at that altitude) is only dependent on temperature. Specifically, it is proportional to the square root of temperature. Because it is colder at altitude, the speed of sound is slower.

That having been said, the low density did work to his advantage - not by lowering the speed of sound, but by decreasing the drag force he felt at a given velocity. Drag force is proportional to density, so the vastly less dense air at altitude decreased the drag force enough to allow his terminal velocity to be supersonic, at least at first.
 

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