The Physics of Drag

As you stand in the shallow end of a pool, you slowly move your hand through the water. The resistance that you feel is called hydrodynamic drag. This type of drag is created by the force experienced between your body and the water. You exert a force on the water as you move through it in order to move forward and at the same time, the water exerts a force back on you to try and push you backward.

There are four forces in play while swimming:

1.     Gravitational Force: downward force dependent on the swimmer’s mass

2.     Buoyancy Force: the water pushes up on the swimmer with a proportional amount compared to the volume of the water being displaced by the swimmer. If the swimmer stays as the surface, that means that the buoyancy force is equal in magnitude to the gravitational force

3.     Thrust Force: the swimmer kicking and pulling to counteract the drag force

4.     Drag Force: as the swimmer moves forward, they are pushing the water and the water pushes back creating the drag force. This force depends on the shape and size of the swimmer and their speed relative to the water.

So what is the difference between good and bad drag? Good drag comes from the thrust force which is exerted by the swimmer kicking and pulling through the water. Bad drag is the force directly opposing the thrust force and making it difficult for them to move through the water.

So then, how do we minimize the bad drag? First, we must calculate the magnitude of the drag force through this equation:

                               

p= density of water

·      Fixed variable

A= cross-sectional area related to the underwater profile of the swimmer

·      Can be altered by changing how the swimmer holds their head or moves their armsà decreases drag

C= drag coefficient that takes into account how water interacts with an object, swimmer in this case

·      Depends on shape and surface of the object

·      Decreasing the drag coefficient even the smallest amount will make a difference because it will allow the swimmer to increase speed without increasing power output and therefore maintain stamina.

So, let’s look at power in relation to the drag force. We can look at both the equation for power and the equation for the force of drag to better understand this relationship.

                                                

In this case, Work is equal to the drag force multiplied by the distance traveled, but if you take the distance traveled and divide that by time, then you will get the velocity of the swimmer. Then by adding the equation for drag force, we get this new equation.

            From this equation, we can solve for velocity and manipulate the drag coefficient to find out how much speed increases or decreases with changes in the drag coefficient.

            For example, decreasing C by 1% increases the swimmer’s speed by 0.34%. While this seems like a negligible amount, every millisecond in swimming counts and this amount could be the difference between winning your event or losing. Swimmers decrease their drag coefficient by shaving their entire body and/or wearing full-body swimsuits. After that, it is up to them to push harder in order to win their event.