What Is That Floating There?
The above image shows a tall narrow class cylinder siting on the table in a lab. The cylinder is completely filled with a clear liquid and sealed tightly with a flexible membrane, which was done using a neoprene glove and a rubber band. Inside the cylinder, there is second glass container that appears to be floating upside down (opening facing downward) and resting against the rubber membrane that is covering the top of the cylinder.
What is the likely explanation for this setup?
This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.
1 solution
Container floating at top with no pressure on the membrane.
Container floating part ways down with a bit of pressure on the membrane.
Container sitting at bottom of the cylinder with a bit more pressure on the membrane.
This is a simple apparatus that can be used to demonstrate the basic principles of buoyancy. The cylinder is completely filled with water. Glass is heavier than water. Normally a glass container would sink to the bottom of a cylinder filled with water. The inverted glass container is floating at the top of the apparatus because it had a volume of air trapped inside the container. The volume of air would be sufficient to float the inverted container but not much more. The rubber membrane is used to contain the water in the cylinder. Water cannot be compressed. Within the membrane sealed cylinder, only the air that is trapped at the top of the floating inverted container can be compress. By pressing down on the flexible rubber membrane, the total volume inside the cylinder would be reduced by compressing the trapped air in the inverted container. By compressing the air in the glass container the buoyancy of the container would be reduced. Adjusting the amount of downward pressure on the membrane adjusts the buoyancy of the inverted container so that the position where the container is floating within the cylinder can be easily controlled.
My answer could be considered somewhat subjective and challenged in the following ways.
If the inverted container had been floating at some position other than at the top of the cylinder than both a type of barometer and apparatus to demensrate the effects of thermal expantion of air could also be considered more likely. Changes in the ambient atmospheric pressure or air temp inside the inverted container would have an effect on the volume of the air trapped in the container. Any change in the volume of the trapped air would have a direct effect on the buoyancy of the container and therefore the position where the container would float. The principles of buoyancy will be demonstrated as well as other possible considerations.
It has just accord to me that the inverted container will never float at any mid position within the cylinder, and therefore be useful for demonstrating changes in ambient air pressure or changes in the air temp in the inverted container, without constant adjustment to balance it there. The natural feedback in this system will work against a stable balancing point. As the container goes deeper in the column of water the pressure on the air in the inverted container increases do to added water pressure resulting from the water above the container. The higher water pressure will compress the air and therefore reduce the buoyancy of the container thus contributing to a downward force.
Someone may possibiliy be atemping to contain some fumes while cleaning the containers but it seems unlikely. For one thing the container inside is not even completely full of the liquid.
I do not see any way that is set up could be used to demonstrate anything with regard to thermal expansion of water.