# Kinetic theory equation

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#1
If i wanted to find the average speed of a molecule in a cube in 1 direction.
Would i need to square all the individual velocities in that particular direction divided by the total no. of molecules and this would give me the root mean squared speed in that particular direction?
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7 years ago
#2
http://en.wikipedia.org/wiki/Root_mean_square

RMS works like a normal average except you square everything at the start and square root it back at the end- this deals with negatives basically.

For a cube it depends whether the directions are at random. If so then because there are three different "directions" (one for each axis, all others can be resolved into these ones) then you need to divide by three. If they are all going in a straight line then it doesn't matter and you just use the RMS formula as it is.

Hope that helped- a clear explanation can be found here:
http://physicsnet.co.uk/a-level-phys...ry/derivation/
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#3
(Original post by lerjj)
http://en.wikipedia.org/wiki/Root_mean_square

RMS works like a normal average except you square everything at the start and square root it back at the end- this deals with negatives basically.

For a cube it depends whether the directions are at random. If so then because there are three different "directions" (one for each axis, all others can be resolved into these ones) then you need to divide by three. If they are all going in a straight line then it doesn't matter and you just use the RMS formula as it is.

Hope that helped- a clear explanation can be found here:
http://physicsnet.co.uk/a-level-phys...ry/derivation/
Thanks, but what doesnt matter and use what as it is?
0
7 years ago
#4
If they are all travelling randomly then you need to divide by three.

If not i.e. they are going at random velocities backwards and forwards, but in a straight line, then you don't. With both you still need to use RMS
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