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How do I know that this Force is repulsive? If I solve for the force: $$F=-\frac{\delta U(r)}{\delta r}=-\frac{\delta}{\delta r}\frac{a}{r^2}=-a\frac{\delta}{\delta r}r^{-2}=-a(-2)[r^{-3}]=\frac{2a}{r^3}$$

The force looks attractive because as the two particles get closer, the force becomes stronger (similar to gravity):$$F \propto \frac{1}{r^3}$$

But the correct answer is repulsive. Can somebody tell me how I can determine an attractive from repulsive force?

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One way to decide this is to look at the potential instead of the force, because we know that systems tend towards states of lowest energy.

Given $U(r)=\frac{a}{r^2}$ we see that for fixed $a \gt 0$, the only way to decrease $U$ is to increase $r$ (that's why the sign of $a$ is important here!), so a state where the two masses are further apart has less energy and is therefore preferred, leading to the interpretation of a repulsive potential.

If you insist on interpreting the force, just think of one of the masses as fixed and as $r$ pointing in the direction of the other mass. Since you correctly get that $$ F \propto \frac{1}{r^3} $$ you see that there is no relative sign, therefore the force is pointing towards the same direction as $r$, which also means that the other mass will move away from the first one. So, also here we get that the force is repulsive.

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  • $\begingroup$ I completely neglected that U wants to be as low as possible. Thanks for answering so quickly $\endgroup$ – Jess L Mar 19 '17 at 1:57
  • $\begingroup$ @JessL Yeah, like I said, I find it easier just to check how I can make $U$ as small as possible, since the interpretation of $F$ always needs some sense of direction. $\endgroup$ – Wojciech Morawiec Mar 19 '17 at 2:04
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By looking at the sign of $U(r)$. If infinite is taken as the reference point, the force is attractive if $U(r)<0$ and repulsive if $U(r)>0$

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Actually the correct conclusion depends on the magnitude and direction of the force. Your expression $F=2a/r^3$ should really be $$ \vec F=\frac{2a}{r^3}\hat r $$ showing the force points away from any one mass, and hence is repulsive.

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