Why protons attract inside a nucleus?

What is the reason for proton- proton attraction inside the nucleus according to nuclear physics? (Sanjeev Asked)

Answer: Inside the nucleus where the nucleons are very close to each other, the force which holds them together is the nuclear force, which is the strongest force in nature. The electrostatic force between protons is negligibly small compared to the nuclear force. But the nuclear force is of a very short range, 10^-15 m.

The nuclear force is charge independent, i.e; the nuclear force between proton and proton, proton and neutron as well as neutron and neutron are almost the same.

Yukawa’s meson theory suggest that the nuclear force is an exchange force. The nucleons are bound because of constant exchange of the mesons.

The nuclear force is only felt among hadrons. At small separations between nucleons (less than ~ 0.7 fm between their centers) the force becomes repulsive, which keeps the nucleons at a certain average separation, even if they are of different types. At distances larger than 0.7 femtometer (fm) the force becomes attractive between spin-aligned nucleons, becoming maximal at a center-center distance of about 0.9 fm. Beyond this distance the force drops essentially exponentially, until beyond about 2.0 fm separation, the force drops to negligibly small values.

At short distances (less than 1.7 fm or so), the nuclear force is stronger than the Coulomb force between protons; it thus overcomes the repulsion of protons inside the nucleus.

However, the Coulomb force between protons has a much larger range due to its decay as the inverse square of charge separation, and Coulomb repulsion thus becomes the only significant force between protons when their separation exceeds about 2 to 2.5 fm.

(There are many terms introduced in the explanation. You can discuss them as comments for obtaining further details, if required)

Reference:

1. http://hyperphysics.phy-astr.gsu.edu/hbase/forces/funfor.html
2. http://en.wikipedia.org/wiki/Nuclear_force