Physics for Swimmers, Coaches and Parents

While there are many laws in physics that have some influence on the speed of a swimmer, the most significant are those pertaining to Newtonian and Fluid Mechanics. Sir Isaac Newton, a brilliant mathematician from the UK, defined the three laws of motion back in the 17th century, yet they are as important to a swimmer today as they were then.

Physics tends to get complicated and confusing very fast, so I will try to keep this as simple as I can. Newton’s laws applied to swimmers tells us that when a swimmer is at rest (not moving) or when the swimmer is moving at a constant speed, then the forces applied to him are balanced. If the swimmer is speeding up or slowing down, they are not balanced. The objective is to get the swimmer moving down the pool (and back) as fast as possible. What are the forces that act on a swimmer?

Since our weight in water ranges from zero (with lungs inflated with air) to about 8 pounds (full exhale), gravitational forces are not very significant, though they do come into play some. The most important forces for a swimmer are those that move us forward in our line of motion (propulsion) and those that slow us down (frontal drag). Because we are moving between air and water while swimming, and because there is a huge difference in density between air and water (784 times), lift forces that elevate our body position are also important. Like a boat, and assuming that we don’t tilt our bodies, the more of us that is in air rather than water, the less frontal drag we will encounter.

This last statement is complicated by the fact that when our bodies are in a relatively streamlined position, we can go slightly faster underwater than we can on the surface, similar to a submarine. By being underwater, we eliminate one of the three types of frontal drag, called surface or wave drag, that accounts for about 20-25% of the total drag forces at race speed. To do so also requires that we have a strong kick propulsion. While swimming, there are certain times in the stroke cycle when we are relatively streamlined. At those moments, we are better off being under water than on the surface. Swimming breaststroke is the most notable example of this. There are other times, when our body is not as streamlined, when we are better off as elevated at the surface as possible, with more of our body in air than in water.

The acceleration or deceleration of a swimmer at any given moment in a race is determined by the sum of the propulsion forces (+) minus the frontal drag forces (-). If the swimmer’s propulsion is greater than his drag forces, then he is accelerating. If the drag forces are greater than the propulsion, then he is decelerating. If the swimmer’s speed is constant, then the forces of frontal drag and propulsion are equal.

At The Race Club we use a technology to test swimmers called the Velocity Meter. With it, we can measure the velocity, acceleration and deceleration of the swimmer at each .02 seconds in the stroke cycle. These measurements are synchronized to the swimmer’s video. While knowing the swimmer’s velocity is important, knowing the acceleration and deceleration at any given moment is even more important. At the peak of acceleration, the propulsion is greatest and the amount of the peak acceleration is correlated with the amount of propulsion that was generated at that moment. That helps us determine roughly how much propulsion is coming from the kick or pull or both. At the peak of deceleration, the drag forces are greatest and the amount of deceleration is correlated with the amount of frontal drag that occurred at that moment. That helps us identify and quantify the mistakes in technique that were made that caused that frontal drag.

Lift forces simply help reduce frontal drag by elevating us higher in the water.  Since the forces we create are not just down or backward, motions of our kicking and pulling that create propulsion can also create lift at the same time.

In a series of articles, we will discuss what we have learned from Newton’s laws of motions; how to reduce frontal drag, how to increase propulsion, and how to use the law of inertia to our advantage. Finally, we will discuss how the law of conservation of energy can help us with our open and flip turns.

This week, in lanes 2, 3 and 4 ( you can see how important frontal drag is in swimming with our video that scientifically compares two of the most popular streamline positions from Olympic champion Jimmy Feigen.

Yours in Swimming,

Gary Sr.

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