consider forces acting on
. It is the reaction N that is
responsible for accelerating of the ball in the horizontal direction.
is falling into the
well, it gains velocity, so, during this time it moves faster and get
ahead of the others. By similar reasons 
looses
the velocity while climbing the hill and get delayed.
N.B. We should consider not the absolute value of velocity, but
the component parallel to the X-axis. Actually, this does not change the
answer: consider forces acting on
. It is the reaction N that is
responsible for accelerating of the ball in the horizontal direction.
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