In order to understand why some people float motionless and others do not, you will need to know a little bit about about how Hydrodynamic Principles work.
- Buoyancy is the upward force that water exerts on an object or a body and the amount of buoyancy exerted on a body is primarily determined by that body’s specific gravity.
- Specific gravity is the ratio of the weight of a body to the weight of the water it displaces. Pure water has a specific gravity of 1.0. Therefore objects with a specific gravity of less than 1.0 will float, whereas objects with a specific gravity greater than 1.0 will sink. Specific gravity among humans varies greatly because people have different amounts of muscle mass, adipose tissue and bone density. Adipose tissue (body fat) has a specific gravity less than 1.0 and promotes floating. Bone and muscle tissue have specific gravity values that are slightly greater than 1.0, which causes sinking.
So this must be why there are some students that float easily and others sink like a rock!
Yes! Most people tend to float at or near the surface of the water, depending on their specific gravity however others (depending on their body composition) may never be able to float motionless! There is one easy trick you can try to assist with a better back float (of course a student has to be comfortable in the back float body position for any of this to work).
- Have a student deeply inhale and hold their breath while floating. This may allow the student to lower their specific gravity and be able to maintain a float longer.
In addition to specific gravity, two other factors affect the position a floating body takes in the water: the center of mass and the center of buoyancy. The center of mass is a is a theoretical point in a body where the entire mass of that body can be considered to be concentrated. The center of buoyancy is a theoretical point in a body where the entire buoyancy of that body can be considered to be concentrated. When the center of mass is directly below the center of buoyancy, a person is able to float in a stable position. If the center of mass is not directly below the center of buoyancy, the body will rotate until it achieves this alignment. The relationship of these two centers changes, however, as people change their body shape or position. Moving the center of mass and the center of buoyancy so that they are closer together increases stability during a horizontal float by minimizing the tendency of the body to rotate.
There are a variety of techniques you can try with your students to change their body position and enable them to float better. Experiment with a variety positions to see what works best for each individual.
- Float on the back with the arms outstretched at the sides (think Letter “T” position).
- Or move the arms above the head.
- Or flex the wrists so the hands are out of the water.
- Or bend the knees.
We are always looking for our students to be comfortable with buoyancy and balance and show us a motionless float and most students will be able to achieve this. However, some students may never be able to float motionless. We should always consider a “float” as maintaining their body position on the back with the face out (whether horizontal or diagonal) and recognize that a student may need a few kicks or under water arm movements to get their body to rise to the surface.