# What are the advantages of having a real time based physics? [duplicate]

Possible Duplicate:
Fixed time step vs Variable time step

Now just let me clarify what I mean by "real time based physics", which I will call RTBP from now on. Well, basically, it implies taking, you guessed it, actual time into calculations.

For example:

timePassed = time - lastTime

positionx += vx*timePassed
positiony += vy*timePassed

lastTime = time


This would be a simple RTBP model. What's the alternative? I call it "step based physics" (SBP from now on), which would just be this:

positionx += vx //v in SBP is obviously smaller than in RTBP
positiony += vy


I currently use SBP, but I see that many people actually include time in their calculations. Why is that? I of course limit the update time to 1000/60ms.

Once the update interval is limited, both models would behave the same. The only difference is when the FPS drops. I believe that here is where PBS excels over RTBP. In PBS the game would, yes, run slower, but would always produce the same results! In RTBP, the position might increase so much that an object actually passes through a wall!

Anyway, there must be something that I'm missing.