# Average relative speed

1. Aug 25, 2010

### Oojee

1. The problem statement, all variables and given/known data

For a gas in which all molecules travel with the same speed v, show that average relative speed = 4/3v (rather than sqrt(2) v which is the result obtained when we consider the actual distribution of molecular speeds.)

2. Relevant equations

3. The attempt at a solution

http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/menfre.html#c5

2. Aug 26, 2010

### hikaru1221

How do you approach this problem? Show us your work so far.
By the way, I doubt the correctness of the method provided in the link.

3. Aug 26, 2010

### Oojee

I used the method given in the link & for the case of same speed for each particle this method gives zero average relative speed. I cannot think of any other approach to the problem that is why I posted the question here.

4. Aug 26, 2010

### hikaru1221

IMHO, the method in the link is a fallacy, or at least, it's a source of misunderstanding. The wrong step is that from this equation:
$$v_{rel}=\sqrt{\vec{v_1}^2-2\vec{v_1}\vec{v_2}+\vec{v_2}^2$$
we CANNOT go straight to this equation:
$$<v_{rel}>=\sqrt{<\vec{v_1}^2>-<2\vec{v_1}\vec{v_2}>+<\vec{v_2}^2>$$
simply because these two are different:
$$<\sqrt{\vec{v_1}^2-2\vec{v_1}\vec{v_2}+\vec{v_2}^2}>\neq \sqrt{<\vec{v_1}^2>-<2\vec{v_1}\vec{v_2}>+<\vec{v_2}^2>$$
That this method yields the correct result, I think, is just a mathematical coincidence.

We have the relative speed between 2 particles: $$v_{rel}=|\vec{v_1}-\vec{v_2}| = 2v|cos\phi |$$ where $$\phi$$ is one-half of the angle occupied by 2 vectors $$\vec{v_1}$$ and $$-\vec{v_2}$$. Now some questions:
1. Because of the randomness of the gas, in the viewpoint of each molecule, the average speed of all other molecules relative to it (let V denote this average speed) is the same for every molecule; that is, the average speed in the viewpoint of each molecule doesn't depend on which molecule considered. Do you agree?
2. From that, what can you deduce about the relation between the average relative speed of the whole system and V? Do we only need to compute V in order to calculate the needed average relative speed?
3. Calculating V: Pick an arbitrary molecule with velocity $$\vec{u}$$. You can use $$\vec{u}$$ as a fixed axis and calculate $$<|cos\phi |>$$. Notice the uniform distribution of velocities in every direction, which leads to spherical symmetry.

P.S.: The < > sign means average.