I am replying to this old post because I have the EXACT same problem. The only solution I have found is to unregister and reregister the physics entity which I'd rather not due as I tend to get crashes. If someone from Conitec is reading this I'd like to request that the physics engine have a new function to zero out the forces applied to a physics entity once that entity has been disabled. This way I could apply new forces once the entity was enabled again. I'm going to post this in several appropriate places in hopes of having this issue resolved.