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I have a .json file exported by R.U.B.E, and I ran it on my device(iPhone), but I got different result animations on R.U.B.E and my device. I suspected that the different time steps size leaded to those. So, I want to know the time step size you used in R.U.B.E, and any other reasons for the different results, thanks very much!!!

It's probably due to differences in floating point calculation between the two processor architectures. You cannot assume that you will get the same results on different devices, not even between different types of iPhone. Try googling "box2d determinism" for many discussions about this subject.