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Once when I was in college, a fellow physics major stormed into the
physics library all fired up about a question about a water sprinkler and
which way would it spin if you ran it backwards. Later I found that the
question was popularized in one of Feynman's books (I think it was _Surely
You're Joking_) in which Feynman makes a mess in someone's lab trying to
answer he question. At the time I remember it as a curious but not
particularly important puzzle. Now one of my students has put that puzzle
back on the front burner for me.
If you don't know the puzzle, here it is:
A popular kind of lawn sprinkler has a head which is free to rotate and
arms that are crooked in the horizontal plane. With the sprinkler
connected to a hose, water jets out the ends of the arms, and the head and
arms rotate. The tangential velocity of the arms is in the opposite
direction of the tangential velocity of the water.
Suppose, however, that the sprinkler were placed under water, say, at the
bottom of a swimming pool. Suppose further that a pump were connected to
the hose so that water was drawn into the arms instead of expelled from
them. Which way would the sprinkler rotate? Would it rotate at all? Why?
David Strasburger
Noble & Greenough School
Dedham MA
PS: in the wake of the tennis ball discussion, let me assure you that the
observations described herein have been recorded faithfully, carefully,
and repeatedly! If anyone is truly in doubt we can probably figure out
how to post a video clip to the web!