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QUESTION: In chapter- kinetic theory of gases,we take only x component is reversed,what happen when all three components is changed ,then how we find pressure.Also after taking x component reversed we take the molecule hits the parallel opposite wall to the wall that  in which molecules first hits,why we not take it hits the other wall.

I read in 11 class
sir, this is not a homework problem;its my doubt

ANSWER: I don't know why you doubt it...the coordinate system is indeed arbitrary.  If you want to do it in all 3 dimensions with a badly-aligned coordinate system then it's still not mathematically or physically wrong...just too difficult to calculate.

The coordinate system you draw in a problem is usually chosen to simplify the problem.  So you choose one direction to be perpendicular to the wall for collision, and the other two coordinate directions to be parallel and perpendicular to the motion of the molecule.  Problem solved.  It's just a matter of convenience.  Mathematical simplicity.

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QUESTION: sir thanks;just for surity i ask that molecule will not hits the another wall as its only x component is reversed not y and z component.Sir if i am wrong please tell me and what happen when it strikes another wall will it change the directioon of x component or not

That totally depends on the angle of the second wall relative to the first it a right angle, are the surfaces parallel?  If they're parallel, then the geometry is trivial and the basic answer is yes.  You'll have to figure that out for yourself, but at that point you're in a high vacuum condition...kinetic theory of gases doesn't apply so much as ballistic trajectory.  Also, kinetic theory for such situations is incomplete as it does not account for energy transfer to/from the wall but deals in averages and makes simplifying assumptions about coordinate systems to treat "average" collisions and not specific series of collisions like you're talking about.  It's just a framework that you can use to calculate the properties of a gaseous system, not something fundamental and absolute.


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Dr. Stephen O. Nelson


I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.


I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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