# Strong force, where is the seperation?

In class I got given a diagram like this: (albeit without the Electrostatic force line)

However the teacher told us the nucleons are typically separated when the force is zero. So as the string force crosses the x axis. (as does our textbook)

However initially this did not make sense to me because electrostatic force will still be repelling (in the case of a proton-proton "bond"/interaction) and so surely the separation distance must be on the positive side to counteract the electrostatic force?

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The answer is in your graph. You need to ADD the red line you added to the black strong nuclear force line, which will only slightly alter the overall shape. In other words the total sum of the electric and nuclear force at the 'typical nuclear separation' line is say -10 (in some arbitrary units) from the nuclear force, plus +2 from the electric, for a total of -8 at the min point, which does not move the minimum energy point much to the left or right.

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 See my other comments... – FrankH Oct 18 '11 at 7:53

Simply draw another graph i.e. the resultant force by adding the blue and red lines; remember red is attractive and blue is repulsive, so effectively you need to subtract the modulus of both.

Its quite clear that at the distance marked as "typical separation" you would find your minima i.e. maximum attractive force or least potential energy.

Regards,

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 But the force is the negative derivative of the potential energy so the force should be zero when the nucleons are in equilibrium and this would correspond to the lowest potential energy where the slope would be 0. See my other comment also. – FrankH Oct 18 '11 at 7:52 ya...so what's the problem if $U$ is 0 at "typical separation"... – Vineet Menon Oct 18 '11 at 11:57 Force should be 0 at typical separation and U, potential energy, should be at a minimum with slope 0 at that point. That is why I think the OP graph is mislabeled. It is not Force, but potential energy on the Y axis. Then the dotted line at typical nuclear separation makes sense. – FrankH Oct 18 '11 at 15:08