Solved by KFrank in post #2 
              
              
                Hi Physics! 

The two formulations are equivalent.  Let’s call the “velocity” in the 
first, pytorch, formulation vPytorch_{t} and in your second proposed 
version vPhysics_{t}.  The two formulations only differ in a redefinition 
of v, namely vPhysics_{t} = -vPytorch_{t}, that drops out of the 
fin…