What is the source of the force that moves the tectonic plates along this plate boundary

Mantle convection

Explanation:

The source of the force that moves tectonic plates along plate boundaries is as a result of the mantle convection.

Heat energy produced in the mantle especially from radioactive decay is a very important source of internal heat that drives plate motion. Denser and cold materials close to the over-riding lithosphere above sinks deep into the mantle. Hotter and lighter materials rises to the surface. This sets up a convection cell by which the plates above are driven.

Yes it would be a physical change.