(The author of this puzzle is Prof. Evelyn Patterson, United States Air Force Academy. Used by permission) 

Little Sally is floating peacefully in her small red plastic boat, at the position marked "A" in the figure below. The lake is calm and the day is sunny. A motorboat speeds by, about 30 m off the shore, and the waves reach the shore about half a minute later. Suddenly, little Sally's older brother and sister decide to give Sally some fun. Each sibling is in a rowboat firmly tied to moorings out in the lake, as shown in the figure. They each jump up and down in their respective rowboats, bobbing up and down in synch together, about 15 times every half a minute. Soon large waves are emanating from each of the bobbing boats, and the older brother and sister are giggling fiendishly. 

Sally's father, seeing the situation, and having taken the equivalent of Honors Physics, does some quick thinking and moves Sally to the position marked "B" in the figure. 

Sally likes to float in calm water. Does Sally's father do Sally a favor by moving her?