Energy-momentum of non-free classical particle

[atyy]

Let's discuss only classical fields and particles.

For fields, E²=p²+m² applies only if the field is free. In the presence of sources, we have to use the energy-momentum tensor.

For particles, does E²=p²+m² apply only when they are free, or does it work even if they are acted on by a force?

 

[inottoe]

The energy-momentum tensor has to be used for all forms of mass/energy that are in a force field.

 

[atyy]

Thanks!