The collision only robs the ball of vertical momentum.  Since the normal 
force is (by definition) vertical, it cannot change the ball's 
horizontal momentum.  Any horizontal change comes from friction or 
conversion to rotational energy, which doesn't exist in a particle 
system anyway.

-- 
David Fontaine  <dav### [at] faricynet>  ICQ 55354965
My raytracing gallery:  http://davidf.faricy.net/

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