A swimming pool with chlorine water has a density of about 1000gm/liter, compared to the 1.2gm/liter of air, which statistically means that pool water is about 800 times denser than air. Seems crazy, right? But, professional swimmers are still able to achieve a speed of around 10 km/h in water. Whereas, the maximum speed that humans can achieve on land is roughly around 45km/h.
These numbers say that the densities of air and water are almost incomparable; however, the difference between maximum achievable speeds in a swimming pool and on land is comparatively negligible (keeping in mind the ratio of densities of water and air).
You might be wondering that what special technique do the swimmers use, which enables them to cut through the dense water so quickly, that they are able to swim at an average speed of about 9.4 km/h. We will get to the swimming posture in a minute, but let’s go through the difference in the type of friction in both the mediums we are considering.
WHAT IS FRICTION?
Well, the frictional force is a force that supports, as well as, resists the motion of a body. Let’s consider a path that has no friction. Would you be able to even walk on a particular path? Well, the answer is NO. The results of such a hypothetical situation are rather uncertain, but going by the scientific researches, you would keep sliding and falling, having no balance in your body.
Now, we know how important role frictional force plays in motion, in general.
In swimming, the water rather resists the motion of a body, than supports it. The swimmers will have to face the frictional drag in the water, which is essentially similar to the drag that a person walking or running would experience. But, it is important for the swimmers to have a streamlined body. Fitness does help to overcome the drag, up to an extent, but a professional swimmer will have to use proper swimming costume, helping his body posture to remain streamline.
Now, let us consider that you and your friend decided to have a race, but there is a twist. One of you will have a small disadvantage i.e. he will have to swim, while the other person can walk/run on land. The results of the race were not shocking at all and the person running was easily able to win. The interesting part about the race was that after the race, the person who was swimming was much more tired than the person who was running. What this means, is that due to the higher density of water, the swimmer had to use up a huge amount of energy in order to overcome the drag force.