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Czech originalArchimedes‘ Theorem Doesn’t Work
„Archimedes‘ theorem is false“ I hear this sentence time and time again from students after lecturing on Archimedes‘ theorem. The pupils don’t clearly understand how buoyancy works. And so we are, together, on a look out for experiments, which both impress and don’t always end the way one would expect them to.
The floating rubber ball
The assistant pours water inside a graduated cylinder and throws a rubber ball inside. This ball will float on the surface. We explain that the density of the ball is lower than that of water. Then, the assistant holds the ball close to the bottom of the cylinder with a magic wand and pours alcohol on top of the water. Then comes the magic formula and the wand is lifted. The ball starts to rise but stops on the water-alcohol boundary. We then explain that alcohol density is lower than the material from which the rubber ball was manufactured and therefore the ball doesn’t rise higher.
Disobedient egg
The assistant fills a glass with water. Then he puts in an egg, which will sink, because all the substances the egg is formed from is higher than the density of water. Then, the assistant pours another substance inside – salt water. The egg will rise to the surface, now the water has a higher density than the average density of the egg.
Dancing raisins
We place raisins in a glass and pour sparkling water on them. This experiment works best if the water is chilled. Carbon dioxide bubbles will “stick” to the raisins. The average density of such an object will be less dense than water and the raisin-bubble framework will rise to the surface. There the bubble will pop and the raisins will fall to the bottom.
Mix
We pour honey, water and oil inside a glass with a bead, a Lego brick, a wine berry and a piece of cork.
The objects will take place in the liquid according to their density. The whole framework can also be shaken and everything will return to its former position.