Physics for Swimmers, Coaches, and Parents – Inertia

Part IV: Inertia

Newton’s law of inertia, which was originally defined by Galileo, is also important for swimmers to understand. Basically, inertia simply means that objects (swimmers) that are at rest tend to stay at rest and objects (swimmers) that are moving tend to stay moving, unless they are acted on by external forces.

In order for a swimmer to go from the rest state (taking your mark on the starting block or getting ready to push off the wall) to the moving state (gliding or swimming down the pool), external forces must be applied. Whether that force comes from our legs (feet) pushing us off the starting block or wall or our hands and feet propelling us down the pool, once we start moving, unless we are in a vacuum or outer space, frontal drag forces will start to slow us down. That means in order to keep moving, we must continue applying propulsion.

If the propulsion and drag forces are equal, our speed will remain constant. If the propulsion is greater than the drag forces, we will accelerate. If the drag forces are greater than the propulsion, we will decelerate. As difficult as it is for us to maintain a constant speed in swimming, it requires more work or energy for us to reach our maximum speed from a rested position (dead stop) than it does to maintain that speed. Consider when you completely miss the wall on a flip turn in a race and come to a dead stop. The amount of energy required to get back up to race speed is overwhelming. The race is probably over. Similar to the difference in gas mileage we get in our car while driving in town (stop and go) compared to on the freeway (constant speed), the swimmer will use less energy maintaining a more constant speed than he or she will by repeatedly slowing down or stopping and then speeding up again. Swimming at a more constant speed is simply a more efficient way to swim.

The challenge of swimmers conforming to the law of inertia is that with the nature of our propulsion, coming from the hands and feet and at certain intervals of time, we cannot provide a constant propulsion. Only two of the four stroke, freestyle and backstroke, allow us to come close to maintaining constant speed. Breaststroke and butterfly, due to the longer down time (time between propulsion efforts) and the higher drag coefficients we must create at certain times in the stroke cycle, are fraught with a considerable variation in speed. Therefore, these two strokes are either slower (breaststroke) or require more energy to sustain a higher average speed (butterfly).

How do we conform more to the law of inertia while swimming and maintain a more constant speed? There are only three ways that I know of, regardless of the stroke. First, we can sustain a more constant kicking speed. Since the kick provides potentially more propulsive moments than the pull, using a six-beat kick, emphasizing both the down and up kicks, and creating a shorter kicking cycle time will help.

Second, we can increase our pulling stroke rate. In freestyle, fly, and backstroke, each hand spends about .35 seconds during the propulsion phase of the pull. If our stroke rate is 60 (cycle rate of 30 and cycle time 2.0 seconds), then in free and back, 35% of the cycle time is spent in propulsion (.70/2.0). The remaining time of the pull is either spent in lift, release, or recovery, so called down time. In fly, at a 2.0 second cycle time, only 18% of that time would be spent in propulsion. The propulsion is greater, however, since we are pulling with both hands simultaneously. At a stroke rate of 120 (60 cycle rate or 1.0 second cycle time), 70% of the time would be spent in propulsion. In fly at that cycle rate, 35% of the time is spent in propulsion. The higher the stroke rate, the more percentage of time is spent in propulsion. The less down time there is in the pull, the less time there is for the swimmer’s speed to drop. However, if the stroke rate becomes too fast, other factors may change, such as lower propulsion achieved with the pulling arm, increased frontal drag or diminished coupling motions, any of which can lead to lower velocity of the swimmer. Faster stroke rate is not always better.

Third, we can avoid any of the technical errors that lead to dramatically increased drag coefficients. The frontal drag of the human body at race speed is extremely sensitive to minute changes in our shape. Even the smallest mistakes can lead to significant drops in speed. For example, lifting the head too high, pulling too deep, overbending the knees on the kick, leaving a thumb out on the streamline off the wall, etc. can all lead to precipitous drops in speed.

In summary, by paying attention to the techniques that enable our speed to remain more constant, we will swim more efficiently in all four strokes. We will conform better to the law of inertia.

Yours in swimming,

Gary Sr.