How Does a Submarine Sink and Rise? Soda Bottle Diver [video]

Abstract

How Can a Submarine Sink and Rise? Cartesian Diver Hello. I’m Dr. Bruce Denardo here in the Physics Lecture Demonstrations Laboratory at the Naval Postgraduate School in Monterey, California. How can a submarine sink or rise? One way is to change the amount of water in the ballast tanks. But how does this cause sinking or rising? We can understand the behavior by considering a simple physics demonstration called a Cartesian diver. Submarine showing air vents, water intake, and double hull which serves as the ballast tank. Demo Here is our Cartesian diver apparatus. It is a flexible clear 2- liter soda bottle with an inverted glass test tube inside. The test tube is open, and it contains some air. The yellow tape around the tube and cork at the top of the bottle allow the test tube to be clearly seen in a classroom. The bottle is filled with water and securely capped. There is enough air to make the test tube float. Without the air, it will sink. There are different ways that the diver can be made to sink and rise. One is by telekinesis: using only your mind to move objects. This takes a lot of practice and concentration. The Cartesian diver is a well-known physics demonstration and toy. There are many versions of it, and you can even make one at home! Here is an old version. Another type is a simple medicine dropper. Some condiment packages of ketchup, mustard, soy sauce, or whatever can also work! Googling “cartesian diver” yields tens of thousands of results, and there are also many discussions on it in educational scientific journals. The apparatus may have been discovered by René Descartes. However, the first thorough and printed account was in 1648 by a contemporary of Descartes named Raffaello Magiotti, who claimed to have discovered it. Magiotti was a favorite student of Galileo. To understand the Cartesian diver, we first need to recognize something about fluids, which are liquids or gases. In gravity, a body in a fluid experiences not only a downward gravitational force but also an upward force. This force is called the buoyant force. The buoyant force occurs because the pressure in the fluid increases with depth, due to a greater amount of fluid that is supported above. For example, if you go deep enough in a pool, your ears will hurt due to the increased pressure. This increase causes the upward force on the bottom of a body to be greater than the downward force on the top. The net effect is an upward buoyant force. The buoyant force is quantitatively given by Archimedes’ law: the buoyant force on a body in a fluid equals the weight of the fluid displaced by the body. You can find a proof in an introductory physics textbook. From Archimedes’ law, the condition for sinking is easy to remember: Sinking occurs when the average density of the body (total mass divided by total volume) is greater than the density of the fluid. Demo Let me set up a simple demonstration. Consider the test tube with no air. The density of glass is greater than the density of water, so the test tube sinks. We can now explain the Cartesian diver demonstration. Demo Look carefully at the air in the test tube. When the bottle is squeezed, water moves into the tube, causing the air compress. This happens because the water is incompressible. Let’s consider the diver to be the glass, air, and water inside the test tube. The diver then has constant volume, but its mass increases when the bottle is squeezed because water enters the test tube. So its average density increases. Once this density is greater than the density of water, Archimedes’ law tells us that the body will sink! In submarines, the ballast tanks are used to increase or decrease the mass while the volume is constant, just as we have seen with the Cartesian diver. This is one way that submarines can be made to sink or rise. As the container is squeezed, the volume of air in the test tube decreases. The body sinks when the buoyant force becomes less than the gravitational force. water gravity Test Tube Cartesian Diver System The Cartesian diver is an old popular physics demonstration and toy. The apparatus is simple, and you can make one at home. We use an inverted glass test tube. Squeezing the container causes water to flow into the tube, which compresses the air, so the average density of the diver increases. By Archimedes’ law, once the average density is greater than the density of water, the diver sinks. This explains one way that submarines can or sink or rise. Pumping enough water into the ballast tanks can cause sinking, and pumping enough water out of these tanks can cause rising. On camera Can the Cartesian diver effect be used to understand how scuba divers and most fish sink or rise? Yes! We consider this in our second video on the Cartesian diver.