I don't get this joke, so will you help me out?

Asked by troubleinharlem (7991) November 15th, 2010 from iPhone

Archimedes, Newton and Pascal are playing Hide&Seek. Archimedes is counting down, Pascal takes off and hides, but Newton takes few steps, turns around, comes behind Archimedes, pulls out a piece of chalk, makes a square 1 meter by 1 meter and steps in it.
Archimedes finishes counting, turns around and sees him immediately: “A-Ha, Newton!” Newton smiles and says: “No, this is Newton per sqare meter, and that’s Pascal.”