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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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: $$\mathbf{F} = \dfrac{3 \mu_0}{4 \pi r^5}\left[(\mathbf{m}_1\cdot\mathbf{r})\mathbf{m}_2 + (\mathbf{m}_2\cdot\mathbf{r})\mathbf{m}_1 + (\mathbf{m}_1\cdot\mathbf{m}_2)\mathbf{r} - \dfrac{5(\mathbf{m}_1\cdot\mathbf{r})(\mathbf{m}_2\cdot\mathbf{r})}{r^2}\mathbf{r}\right].$$
However, be warned that while equilibrium levitation is possible using magnets, stable equilibria are impossible due to Earnshaw's theorem. Magnetic levitation is indeed possible but you need fancier schemes than just two magnets. The wikipedia article on it is a good starting point.
Emilio Pisanty
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