Of the four fundamental forces, the gravitational force is…
A helium nucleus packs two protons and two neutrons into a space about a hundred-thousandth of the atom's width. Both protons are positive, and like charges repel. The nucleus should blast itself apart. It doesn't; helium nuclei last forever.
Why does any nucleus hold together? You now have every piece of the answer.
You've already seen that like charges repel, and the repulsion grows as charges get closer. Crammed into a nucleus, two protons repel each other hard.
Could gravity hold them in? No. The protons have mass, so gravity does attract them to each other. But gravity is by far the weakest force, and between two protons its attraction is hopelessly smaller than their electrical repulsion.
The real glue is the strong force. Every proton and neutron contains quarks, so the strong force acts on all of them. Neighboring protons and neutrons in a nucleus sit within nuclear range of one another, the strong force's home ground. And the strong force is the strongest of the four. At that distance, it beats the protons' electrical repulsion.
The neutrons matter too. A neutron carries no charge, so it adds no repulsion. But it contains quarks, so it adds strong-force glue. Neutrons are binding with no cost.
Back to the helium nucleus: two protons repel each other electrically, but the strong force among all four particles is stronger. The nucleus stays bound.
A helium nucleus holds two protons and two neutrons. The protons repel each other electrically. Why does the nucleus stay bound?
Every proton and neutron in the nucleus contains quarks, so the strong force acts on all four particles.
Neighboring protons and neutrons sit within nuclear range of one another, so the strong force binds them together.
The strong force is the strongest of the four forces, stronger at that distance than the protons' electrical repulsion.
The strong force between neighboring protons and neutrons overpowers the protons' electrical repulsion, so the nucleus stays bound.
In the same helium nucleus, what do the two neutrons contribute?
A neutron carries no electric charge, so it adds no electrical repulsion.
A neutron contains quarks, so the strong force binds it to its neighbors.
The neutrons add binding without adding repulsion: extra glue for the nucleus.
Which force holds the protons and neutrons of a nucleus together?
A carbon nucleus holds six protons, all repelling one another electrically. Why does it stay bound?
A student says gravity is what holds a nucleus together. Is the student correct?
A lithium nucleus holds three protons and four neutrons. The protons repel one another electrically. Why does the nucleus stay bound?
A student says: 'Gravity holds the nucleus together — the protons pull on each other's mass.' What is wrong with the claim?
Imagine the strong force stopped acting inside an oxygen nucleus. What would happen?