Re: Swimmer's buoyancy greater on surface of 12-foot pool than 4-foot pool?

Hello Bob. Fortunately buoyancy does not decrease as depth decreases. If this were the case, ships would float lower in the water when they come into shallow harbors. If the buoyancy decreased enough they would sink. Buoyancy is a force. F=pVg. The force of buoyancy equals the density of the fluid multiplied by the volume of the fluid displaced multiplied by the acceleration of gravity. An object will float if the buoyant force is greater than its weight, which is also a force. Weight=massxgravity. Gravity is about 9.8meter/(second squared) on the surface of the earth.

[note added by MadSci Admin: It is the fact that the density at the surface is the same no matter what the water depth is that makes the buoyant force the same.]

If you take a steel ball and drop it in a pool, it will sink. If you take the ball and shape it into a bowl, it will float. This is because it has increased its overall volume. The density has decreased because of the air inside the bowl. It may be confusing to you to count the air inside the bowl. Think about putting a lid on it. Another way to think about it is to turn the bowl upside down and let all the air trapped in the bowl out. The bowl will then have the same density as the steel ball. The total volume of the covered bowl is the volume of the air plus the volume of the bowl. Take the weight of the covered bowl and divide it by the volume. This is the density. Remember gravity is a constant (relatively) at the earth’s surface. Therefore if the density of the covered bowl is less than the water, it will float.

We just looked at how shape can affect the buoyant force. However, you can change the buoyant force in other ways. Heating the water will expand it, which decreases the density. Since the density decreases, so does the buoyant force. If you decreased the temperature, the opposite would happen. Adding salt would increase the buoyant force by increasing density. A swimmer could increase his or her buoyant force by gaining fat. The swimmer could decrease the force by losing fat or gaining muscle mass.

This information and more can be found in a physics book. I could not find any good resources on general buoyancy. There were many specific examples, but they did not apply here. If you have any more questions, please write back to MadSci.

I hope this answer was helpful.

Kevin