I have a question about work and internal forces. My textbook seems to contradict what I'm finding online:

my textbook claims:
Notice about the work by internal forces:
the work by the internal forces is in general not equal to zero although the sum of the internal forces is always equal to 0. Since the work also depends on the relative displacement of the two pointmasses in the material system, the work provided by these 2 internal forces will generally not be equal to .

This gets refuted by multiple sources on the internet who claim that the work done by internal forces is always 0

Question

I have a question about work and internal forces. My textbook seems to contradict what I'm finding online:

my textbook claims:
>Notice about the work by internal forces:
>the work by the internal forces is in general not equal to zero although the sum of the internal forces is always equal to 0. Since the work also depends on the relative displacement of the two pointmasses in the material system, the work provided by these 2 internal forces will generally not be equal to .

This gets refuted by multiple sources on the internet who claim that the work done by internal forces is always 0

anonymous · Answer

Maybe, that's what you call as internal energy.. Our bodies do a lot of internal work for example, don't they ?