-- When the man at the North end throws the ball, the canoe
accelerates to the North.
-- While the ball is in flight, traveling south by the length of the
canoe, the canoe moves northward at a constant speed.
-- If the man at the south end misses the ball and fails to catch it,
then the canoe continues moving south at a constant speed, even
after the ball has passed him.
-- If the man at the south end successfully catches the ball, then the
canoe accelerates south at that moment, and comes to rest in the water.