Adjustment of buoyancy and sinking, unexpectedly discovered.?

Question

I used to create mini submarines and place them inside clear glass bottles with water to make them float. Adding some weight carefully to achieve balanced buoyancy to make it stay at the middle is a challenge. To my surprise, to calibrate its buoyancy inside the bottle, I just twist the bottle cap tighter then it sunk slowly, and slacking the bottle cap twist makes it float, so i used the cap to float or sink the object accordingly. What is the real principle for this? I'm wondering of relative density or pressure makes it sink or float accordingly.

Answer 1

My guess is that your submarines were made of a slightly compressible material (e.g. plastic) and when you screwed the cap in, the pressure inside the bottle would increase, compressing the submarine and making its volume smaller, buoyancy would then decrease and the sub would sink.

Answer 2

Observe what happens to the level/quantity of water in your mini submarine as you tighten or loosen the bottle cap. When you tighten the cap, the air trapped between the cap and the upper surface of the water is compressed. This results in increased pressure on the upper surface of the water. This pressure is transmitted equally in all directions (Pascal's principle). This pushes more water into your submarine, total mass of submarine increases (consequently, density of the submarine also) and submarine sinks. When you loosen the cap the the pressure decreases and water comes out of your submarine and its mass (as well as its density) decreases & submarine rises. The rat at which submarine sinks / rises can be controled by varying the pressure exerted on the water (i.e. how much you tighten or loosen the cap). You can make your submarine rwmain stationary at the desired height also.

Answer 3

This is a well known experiment in physics. It is called a cartesian diver. Look it up or

http://en.wikipedia.org/wiki/Cartesian_diver

Answer 4

This is credited to Archemedes.