Biomechanics and the Olympics: Part II

 There is little doubt about the greatness of Michael Phelps and it is truly remarkable all the Olympic medals he has won.  However, it is the race that he barely lost that caused him much frustration.  During the Men's 200 meter butterfly two days ago, South Africa's Chad le Clos beat Phelps by five hundredths of a second.  The race was very close and Phelps had a slight lead on le Clos coming into the wall at the end.  However, instead of taking another half stroke before touching the wall, Phelps decided to go ahead and reach for the wall, while le Clos did take a half stroke.  The announcer commented that Phelps lost the race because he did not have any momentum at the end.  This was only partially correct.  Momentum is the product of a person's (or objects) mass and velocity (think speed with a direction).  Any time a swimmer is moving, they have momentum.  When they are not moving, either before or after the race, they do not have any momentum.  The faster a person is moving, the more momentum he or she has.  In order to increase momentum, it requires a reactive force in the same direction you are moving in.  In order to change momentum to the opposite direction, it requires a reactive force in that direction.  When swimmers complete a turn, they push into the wall, and the wall pushes back in the opposite direction, thus giving them momentum in that direction.  Back to the finish.  When Phelps decided not to take another stroke or half stroke into the wall, he was not applying any force to the water to increase his momentum.  Due to the drag force from the water, which acts in the opposite direction that the swimmer is moving in, Phelps was losing momentum.  le Clos, by taking another half stroke, was able to produce more force and increase his momentum, thus allowing him to touch the wall right before Phelps.  If you remember the 2008 Beijing Olympics, Phelps won this same event by one hundredth of a second because he did take an additional half stroke.  These are incredible athletes, and often times the difference between winning a losing can be just a few hundredths of a second.  Phelps did not lose because he did not have any momentum, he lost because he was losing momentum while le Clos was gaining momentum.  The video of the race can be found at this link at around the 30 minute mark.