This is an example from Morin's Classical Mechanics text, in the section on momentum:
You are riding on a sled that is given an initial push and slides across frictionless ice. Snow is falling vertically (in the frame of the ice) on the sled. Assume that the sled travels in tracks that constrain it to move in a straight line. Which of the following three strategies causes the sled to move the fastest? The slowest?
A: You sweep the snow off the sled so that it leaves the sled in the direction perpendicular to the sled’s tracks, as seen by you in the frame of the sled.
B: You sweep the snow off the sled so that it leaves the sled in the direction perpendicular to the sled’s tracks, as seen by someone in the frame of the ice.
C: You do nothing.
The text indicates that we are to implicitly make the following assumptions:
Horizontal force due to sweeping snow can be neglected, as the normal force from the track is plenty to keep the sled from sliding off.
Vertical motion of the snow is also irrelevant, as the vertical normal force from the tracks keeps the sled from falling through the ice.
I am having difficulty understanding the solution in the text, which argues that in strategy B the snow is moving slower than the sled, and in strategy A the snow is moving faster than the sled.
My misunderstanding probably arises from not really knowing how to interpret the phrases "perpendicular in frame of the sled" and "perpendicular in frame of the ice." Presumably, the former means "as measured by someone on the sled" and latter means "as measured by someone still on the ice," but I'm struggling to visualize how these directions are different. If someone could clarify this and provide a more clearly written solution than the one in Morin's text, I'd really appreciate it!