# Nuclear binding FORCE or ENERGY?

• B
I was reading that the reason the nucleus of the atom is so heavy is that there is so much energy being used to keep the nucleons together with the strong nuclear force, and energy=mass. I can understand that. But it is energy which equals mass, not force, right? The strong nuclear force is just exerting a force to keep the nucleons together, not energy. The force may translate potential into kinetic energy if it was actually doing work across a distance. But these nucleons aren't going anywhere. They are stationary. So why is there talk of energy?

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Henryk
Gold Member
Actually, force is a classical physics concept and it has no equivalent in quantum theory. In quantum mechanics, the quantity that can be measured are called 'observables' and are defined, They include position, momentum, angular momentum and, most importantly, energy. The importance of energy is that, no matter what the system is, the stationary states (i.e. allowed quantum states) are characterized by a definite energy value. Other quantities may or may not be well defined for quantum states but that depends on details.
You say that the nucleons aren't going anywhere, so why talk about energy. Well, the energy in question is binding energy. That is the difference between all the nucleons separate and put together to form a nucleus of an atom.
In a way, this is something like binding energy that keeps you on the surface of the Earth. You are not leaving the Earth, you are stuck to being on the surface. If you want to go into space, you have to reach the escape velocity (that is 11.2 km/s), therefore the energy binding you to Earth is (1/2)* your mass * (square of escape velocity).

Sophrosyne
I see. That makes sense. Thank you.

Actually, force is a classical physics concept and it has no equivalent in quantum theory.
So then when there is talk of strong or weak nuclear FORCE, is it safe to say that that term "force" there is not really being used in the classical physics sense? It seems to be more of a sense of "interaction". Correct?

Dale
jtbell
Mentor
So then when there is talk of strong or weak nuclear FORCE, is it safe to say that that term "force" there is not really being used in the classical physics sense? It seems to be more of a sense of "interaction". Correct?
Correct. "Strong force", "weak force", "electromagnetic force" in the context of elementary particle and nuclear physics should be understood as historical linguistic artifacts. When physicists started studying the strong and weak interactions nearly a hundred years ago, during the same period that quantum theory itself was being developed with great struggle, their background was of course in classical physics. So they first tried to explain things classically. When that failed, they continued to use classical terms in connection with the quantum concepts that replaced the classical ones.

dlgoff
Correct. "Strong force", "weak force", "electromagnetic force" in the context of elementary particle and nuclear physics should be understood as historical linguistic artifacts. When physicists started studying the strong and weak interactions nearly a hundred years ago, during the same period that quantum theory itself was being developed with great struggle, their background was of course in classical physics. So they first tried to explain things classically. When that failed, they continued to use classical terms in connection with the quantum concepts that replaced the classical ones.
OK. That’s making more sense now. Thanks.