because dt is still based on a millisecond clock. C2 uses dt based on a 60fps millisecond clock. So when you use dt your trying to create a universal math and multiplied value. The reason and it's just suspect reason that the 30fps dt isn't the same due to the idea that yoru also only processing logic at 30fps rather than continuing to handle logic at 60fps.
Not that I would associate logic to FPS at all. I'm just using that as an anology.
PC, box.ImpulseAngle(0, 100 * dt)
iPAd, box.ImpulseAngle(0, 100 * dt)
In theory tries to unify the the force based on the clock. So results should be similar.
How physics works is that Box2D uses a time step.
Each time step consists of 2 primary values.*
Step is the amount of px the objects moves per itteration. Large steps allow for farther movement. but this reduces accuracy for collision detection. This can lead bullets to shoot through objects.
The number of times the physics shuold take 1 step(above). The more itterations a Physics.Render will call step.
Now when a programmer makes the basic timestep. it tries to take into account the difference of time from last frame render and how many Itterations shuold be called. However this usually works best for FASTER computers and often the basic time step doesn't do very well on slower than the primary development computer.
This is the problem that's happening hear. Your slower computer can't keep up with out something to fix the speed value.
Now how does dt come into this.
Well keep in mind that the dt is a reference time between. 0.0 to 1.0. On a faster computer the dt is much smaller due to the fact that it does everything faster. Where as a slower computer dt is a larger number to compensate for the difference in time.
So when you set your mobile game to run at 29fps vsync it falls out of sync. And the reason I think this is happening is because yoru logic is now running at 29fps(speculation based on what I'm hearing. In theory with all the exrta time your logic should be able to more than keep up... unless your game logic is throttled.
So this is why I think the current C2 use of 30fps isn't to give developers more logic time for physics and game processing, but instead to slow down the game to run on older devices. So instead of giving you access to all the yummy CPU time. C2 drops everything.
That's all speculation of course. but it's one reason why your game is having troubles with the lower 30fps.. you don't have more CPU time you have less.