To gain from multithreading, you need to be a) doing enough work, and b) need to update the result infrequently.
For a physics system, a is probably satisfied, but unless your fps is very low, b is not. You need to synchronism every frame, and the cost of that is more than the benefit of parallelization.
Do you really need to 'synchronism every frame'?
I think 99.9% of the time things will look fine if you don't synchronize every frame.