= Solution
Snow now leaves with zero horizontal <velocity> in the ice frame, the same as its incoming horizontal <velocity>. The net horizontal <momentum flux> is zero:
$$
M\dot v=0,\qquad\boxed{v_B(t)=v_0.}
$$
Any forward <momentum> given to the snow on capture is returned to the sled while sweeping it off. “Perpendicular” specifies a different outgoing <velocity> here than in case A; changing the reference frame without transforming that <velocity> would miss this difference.
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