
Some particles bounce back too strongly
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Usually a particle bounces off the ground in the way that once it is
below the middle line it is mirrored to the other side of the line and
the y component of the direction vector is multiplied by (-1). Of
course it also looses a part of it's energy by bouncing. The same
happens with particles that enter the box from the two sides below the
line and not from the top. That could be prevented with another
if-statement but it would slow the simulation down again. Maybe I will
include that once I can save a bit of calculation time somewhere else.
