The Punika submarine

The test tube floats at the top of the bottle. If you press firmly and continuously on the lid, the test tube lowers. The test tube can be moved up and down or made to "float" in the bottle by applying different amounts of pressure.

The experiment only works if the bottle is really filled to the brim with water. It is not easy to balance the test tube correctly. To avoid having to empty the bottle every time the glass sinks to the bottom, it is advisable to use a container that you can reach into with your hand.

What is technically behind the experiment

Static buoyancy refers to the effect of a force on a body surrounded by liquid or gas. This force is called buoyant force_FA. It is directed vertically upwards and is caused by the fact that a lower gravitational pressure acts from above than from below. (The forces acting laterally cancel each other out.) If a body is immersed in a liquid or gas, it experiences a buoyant force that is just as great as the weight of the volume of liquid or gas displaced by the body. This principle is also known as Archimedes' principle. The pressure on the lid of the punica bottle is transferred to the water in the bottle. However, the water can only escape onto the rubber membrane of the test tube. This causes the rubber membrane to indent. This compresses the air in the test tube. This reduces the amount of water displaced by the test tube and consequently also the buoyancy. Previously, the test tube could float straight, i.e. the buoyancy force and the weight force were approximately equal. Now the buoyancy force has decreased, while the weight force has not changed. This means that the test tube is sinking.

What the experiment has to do with everyday life

The buoyant force is often encountered in everyday life. Children who cannot swim are given water wings. Only when the water wings are filled with air do the children not sink. Ships float thanks to buoyancy.

You can introduce the topic with the experiment: "What floats, what doesn't float." A tub of water is placed in the classroom and the children are allowed to throw various objects made of wood, metal, rubber etc. into the tub. An alternative experiment is the Cartesian diver experiment

Jennifer Heise

The Punika bottle is completely filled with water. There must be no air left in the bottle. Then fill the test tube with a few lead pellets. The test tube is now placed in the bottle and enough water is poured into the test tube so that it just floats in the bottle. Now take the test tube out again and close it with a piece of the balloon and the thread. Then put the test tube back into the bottle. The bottle must be filled to the brim with water before it is closed with the lid.

• 1 empty Punika bottle,

• 1 test tube (or other small glass container),

• 1 balloon,

• Several lead pellets,

• thread,

• water,