With physics I calculate it per frame with a fixed value, not in time.
However, I have time slices that do th trick for me. if I got 600 FPS, it renders only at that rate. The udates of my game are in 60 FPS. When I render with 30 FPS, I just double the frames so the game update would still run at 60 FPS (doing 2 updates and 1 rendering).
In this case you have frame based motion thats always the same. (Time Sliced something it was called,)