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[Václav Šmilauer <eudoxos@xxxxxxxx>]

>     at the begining the clumps have both rotational and translational
> motion, but later the rotational motion finish and just have
> translational motion. at this time I found the clump slip on the plane
> in a curve. I think its trajectory should slip in a line, because
> there is no rotational motion and just have translational motion.

At the beginning they have no motion at all. Gravity, since it is
uniform, gives translational motion. At impact, it causes rotation of
the clump. There is nothing wrong with that, it is just sum of kinetic
energy that should be preserved and one can transform in another.

If you take a stick and let it fall, then if one of the ends touches
ground the first, the still will get rotational motion. I just tried
with a pen on table and it is true ;-)

V