# How to count magnetic repulsion

I have two equal flat round magnets. I know amount of force $F$ which attracts iron objects to one of them and geometric characteristics of magnets. I want to fix first of magnet and some additional mass in the air by second magnet. To do so, I am going to orient magnets so that second magnet repulse first and additional mass. But I need to establish dependence between distance between magnets and value of additional mass this construction can held. How to gain this dependence?

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Possible duplicate: physics.stackexchange.com/q/17309/2451 and links therein. – Qmechanic Jan 10 '13 at 21:48

To a good approximation, normal magnets can be treated as dipole magnets, in which case the force between them can be found in this wikipedia article. To avoid link-only answers, here it is: $$\mathbf{F} = \dfrac{3 \mu_0}{4 \pi r^5}\left[(\mathbf{m}_1\cdot\mathbf{r})\mathbf{m}_2 + (\mathbf{m}_2\cdot\mathbf{r})\mathbf{m}_1 + (\mathbf{m}_1\cdot\mathbf{m}_2)\mathbf{r} - \dfrac{5(\mathbf{m}_1\cdot\mathbf{r})(\mathbf{m}_2\cdot\mathbf{r})}{r^2}\mathbf{r}\right].$$
How can i find $m_1$, $m_2$ and $\mu_0$? I'm planning to use some strings to establish equilibrium. – freopen Dec 26 '12 at 12:48
$\mu_0$ is the magnetic permeability of free space. $\mathbf{m}_i$ are the magnetic dipole moment vectors of the two magnets; once you know their direction (e.g. using an iron-filing experiment) you only need the product $m_1m_2$ of their magnitudes and that you can find with a single force reading. – Emilio Pisanty Dec 28 '12 at 1:13