# Effects of fast moving object on air masses

• mumfordzisk
In summary, you are asking how an object moving faster than the speed of sound through air would affect a hypothetical mass. Friction and compression heat would be devastating, while relativistic mass increase is negligible. You can try to predict the effect by looking into what a space shuttle or a ballistic reentry spacecraft looks like on re-entry, but size is not a big factor.f

#### mumfordzisk

I'm writing a book and I have a question about the effects of a fast moving object on an air mass. Strictly speaking, I am wondering how to compute the effects of a human-sized (70 kilo, roughly,) object moving in excess of 30,000 miles an hour on a hypothetical mass. I'm recently out of college and I have a passing knowledge of physics: AP physics in high school, dad was a physics prof, so I understand general theory, I'm just vastly out of practice. If someone could point me in the right direction vis a vis formulas/what to keep in mind when trying to work on a problem like this, it would be greatly appreciated.

By the way, it's entirely possible based on my limited ability that I have missed a variable of some kind.
Thanks.

Welcome to PF, Mumfordzisk.
I'm not entirely sure what your question means. If you're referring to a mass moving in air at that speed, then compression and friction heat will be devastating. Relativistic mass increase, on the other hand, is negligible at such a small percentage of c.

Just to clarify, I'm working within that perfect physics problem world, and therefore am ignoring the effects of air friction and compression. I'm simply looking to understand how to quantify the effect such an object would have on a system.

Oh, okay. That's way out of my field of knowledge. Sorry, but you'll have to wait for a real scientist to respond. I'm strictly a 'hands-on' guy.

Just to clarify, I'm working within that perfect physics problem world, and therefore am ignoring the effects of air friction and compression. I'm simply looking to understand how to quantify the effect such an object would have on a system.

I'm confused- you have a question about "effects of a fast moving object on an air mass", but you want to neglect the "effects of air friction and compression". What exactly are you asking?

Hm... Let me try anew.
I'm attempting to find the 'pull,' so to speak, that an object moving through an air mass would generate on that mass, similar to the pushed air mass of a subway train. Is there a way to predict this, or have I bitten off a theoretical bite that is too big to chew? In this context I am not concerned with the detrimental effects of friction or compression. I understand that these affect the system, and did not mean to exclude them entirely-obviously in a frictionless system the air mass would not be 'pulled' by the object. Anything that points me in the right theoretical direction would be appreciated.

Ok, well that is drag (friction and compression effects), and yeah, it is a pretty complicated thing to calculate from scratch.

I somewhat expected that- can you offer me a place to start, something to read, some details I MUST keep in mind, something along those lines? I have no problem doing research/legwork, I simply have no idea where to start, beyond a basic idea that with sufficient velocity/mass an object would have SOME kind of movement effect on a gaseous mass.

The topic you are looking for is called "gas dynamics" and you can probably find a used textbook cheap or at the library. It covers the dynamics of high speed flow: rocket engines, supersonic aircraft, etc. Here's one: https://www.amazon.com/dp/0131206680/?tag=pfamazon01-20

You might also try googling for class notes that a professor posts online. I can't remember if we got into the dynamics of high speed flow around blunt bodies - sharp angles are actually relatively easy in high speed flow.

You may also just try looking into what the space shuttle or a ballistic reentry spacecraft (the US's early space program) looks like on re-entry. Size really isn't a big factor, the temperature of the air at the leading edge of the craft is more a function of geometry and what it does to the shock wave.

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Very cool. Thanks much, this is a good place to begin. I imagine I'll be back at some point when I hit a wall. Thanks for your help.

a ballistic reentry spacecraft (the US's early space program)
Gotta love that Dyna-Soar... :tongue2:

Hm... Let me try anew.
I'm attempting to find the 'pull,' so to speak, that an object moving through an air mass would generate on that mass, similar to the pushed air mass of a subway train. Is there a way to predict this, or have I bitten off a theoretical bite that is too big to chew? In this context I am not concerned with the detrimental effects of friction or compression. I understand that these affect the system, and did not mean to exclude them entirely-obviously in a frictionless system the air mass would not be 'pulled' by the object. Anything that points me in the right theoretical direction would be appreciated.

I think also, you will consider the shape of the object into observation

Welcome to PF, Mumfordzisk.
I'm not entirely sure what your question means. If you're referring to a mass moving in air at that speed, then compression and friction heat will be devastating. Relativistic mass increase, on the other hand, is negligible at such a small percentage of c.

yeah, like in the tunguska meteorite in Russia...i'm in russia i got o know bout bout, you could google this for more on it..

the tunguska meteorite in Russia...
Boy, that sucker put a couple of generations of lumberjacks out of business.