Protons all have charge \( +1.6 \times 10^{-19} \text{ C}\).

But they are extremely close in the nucleus. The repulsive force between them is huge.

There is no choice but to invent a new very strong short range force that is attractive within the nucleus.

This is the strong nuclear force.

If we look at the behaviour of the strong interaction it is

repulsive for distances shorter than 0.7 fm

equilibrium is about 0.7 fm and

the force is attractive between 0.7 and 2.5 fm

with a maximum attractive force at about 0.9 fm

and effectively zero beyond 2.5 fm

The resultant force on protons becomes electrostatic repulsion beyond about 2 fm.

Exach fundamental force has an exchange particle.

These particles are bosons and exist only during the interaction. They carry momentum and charge between particles.

In the strong nuclear force the exchange particles are pions. The particles are mesons quark anit-quark pairs.

In the diagram above a \( \pi^+\) exchange particle carries the positive charge from the proton to the neutron.