The Physics of Swimming

         In a normal year I spend upwards of 20 hours per week swimming thousands of laps in a pool. However, this year I have a new appreciation for the science behind what makes me move through the water. When a swimmer is moving horizontally though the water, there are multiple forces acting on them. The force of drag is opposite of the direction of motion, as it arises from the swimmer’s collision with the water. The thrust force is in the direction of the motion because it originates from the swimmer’s arm and leg movement to pull the water.  Although they don’t affect the swimmer’s speed because they are in the vertical direction, the force of gravity pulls the swimmer down, while the upwards force from buoyancy keeps them afloat. 

 

            In swimming, where the goal is to swim as fast as possible, the drag force is the biggest enemy. The drag force can be represented with this expression: F_drag = ½ pACv². Many of the variables that affect drag can’t be changed by the swimmer. For example, p is the density of water and C is the drag coefficient which represents how the object interacts with water. A is the cross-sectional area of the swimmer which can only slightly be altered with tweaks to body position. Decreasing the drag coefficient is the best way a swimmer can increase their velocity. For example, according to the below equation, by decreasing the drag coefficient by 1%, you would increase your speed by 0.34% assuming all other variables are held constant. 

            Although this may not seem like a lot, in a sport that comes down to winning by a hundredth of a second, it may win you a race. Due to this, before a big meet, swimmers do everything possible to reduce their drag. This includes shaving, wearing skintight hydrophobic technical swimwear, and swim caps. 

References

https://www.wired.com/2012/08/olympics-physics-swimming/

 

https://www.wired.com/2016/08/wanna-swim-like-ledecky-take-dive-physics-drag/