# Is nuclear force always attractive?

I read in my high school physics's textbook that nuclear force holds the nucleus. Is the nuclear force another name for the strong force? If it holds the nucleus it should be attractive. Am I right?

• AFAIK, the strong force is often called "(strong) nuclear force", so yes. I also took the liberty to remove your second question from this post to make it more focused. You can always ask a new one. May 12, 2021 at 8:03

The nuclear force (or nucleon–nucleon interaction, residual strong force, or, historically, strong nuclear force) is a force that acts between the protons and neutrons of atoms. Neutrons and protons, both nucleons, are affected by the nuclear force almost identically. Since protons have charge +1 e, they experience an electric force that tends to push them apart, but at short range the attractive nuclear force is strong enough to overcome the electromagnetic force. The nuclear force binds nucleons into atomic nuclei.

Note "residual strong force. If you study further into physics you will become familiar with the residual electromagnetic forces holding atoms and molecules into solids and fluids. The quantum mechanical locations of the electrons around the atom form "orbitals" that allow for the positive charge to shine through allowing the attraction of neutral atoms and molecules to form bonded solids. This will prepare you to visualize in analogy that the strong QCD force which binds quarks into the protons and neutrons, can spill over the color neutral protons and neutrons, allowing for attraction enough to bind into the nuclei of the periodic table.

The nuclear force cannot be directly calculated using the QCD mathematics because of the many particle complexity. But there exist models and field theories( with pion exchanges between nucleons, for example) that fit observations and data well.

• Thanks anna. why sometimes nuclear forces are repulsive? I thought it's always attractive. May 12, 2021 at 13:22
• If you read the various links, the repulsive comes from the electromagnetic component , the positive charge of the protons is repulsive and that some distance it is stronger than the residual QCD force. It depends on the nuclear model used to solve for specific cases. May 12, 2021 at 13:34
• Thanks for your help. May 12, 2021 at 13:44

"Nuclear force" is a vague term and it doesn't really have a well-defined technical meaning. That said, to the extent that it's talked about as

nuclear force holds the nucleus

then the text is talking about the strong nuclear force. This does indeed hold the nucleus together, and there are many settings in which it can be considered attractive.

(In reality, the strong nuclear force is a complicated object, and we can only talk about how it "holds protons and neutrons together" in an approximate sense because when the strong force is relevant, the particle content of the protons and neutrons themselves is not well-defined. For a starter look into how that looks like, see this thread.)

However, the strong force cannot be only attractive: if it did, since it's much stronger than the electromagnetic repulsion, the nucleus would collapse. Instead, it is attractive if the nucleons are reasonably close to each other, but it becomes repulsive if they get too close. For more details, see How does the nature of nuclear force change between attractive or repulsive based on distance? and its linked threads.