Two Experiments on Buoyancy
The goal of using the lab sheets is to introduce the student to the scientific method. Do not be discouraged if the hypothesis or even the interpretation is incorrect. The hypothesis should simply be an educated guess, and it is fun to see how one's guess was incorrect.
If the interpretation is incorrect, this is a good opportunity to explain how even the best scientists have some error in their methods which can lead to incorrect interpretations. For example, the interpretation from Archimedes' Experiment should be that the amount of water displaced weighs the same amount as the object that floats. If the cup was not filled with water, or if the displaced water was not poured carefully into the cup to be measured, there will be error. Tell the student the story of Archimedes and explain that the conclusion should be his famous principle:
"When a body is placed in a liquid, the weight it appears to lose is equal to the weight of the liquid it displaces." A floating object appears to have no weight so the amount of liquid it displaces is equal to its weight.
One of the famous stories of Archimedes is the basis for this experiment. Archimedes was a Greek scientist and mathematician. He worked for the king of Syracuse in Sicily. The king suspected that a goldsmith had cheated him by mixing silver with the gold for his new crown. He asked Archimedes to test the crown. Archimedes decided to compare the volume of the crown to a piece of pure gold that was the same weight as the crown. He knew that gold is heavier than silver, so if the goldsmith cheated, the identical weights would not have the same volume. The problem was how could he determine the volume of gold in the crown? One day he filled his tub with water to take a bath. When he entered the tub, water spilled over the sides. Archimedes realized that the water that spilled over was the same volume as his body! He was so excited that he jumped out of the bath and ran through the streets yelling "Eureka!" which means "I found it!" He measured the volume of the crown and an equal weight of pure gold and found that the goldsmith had cheated. His research led to more experiments, which in turn led to "Archimedes Principle." You can find the full story of Archimedes in your encyclopedia or through research at your library. This is an excellent research opportunity for students!
"Archimedes Principle" explains buoyancy, which is the subject of the Buoyancy Experiment. The student will find that different liquids have different buoyancy. Once your student sees the difference in buoyancy between different liquids, the natural question is to ask why. Ask your student: "Can you think of an experiment to help explain why?" One possibility would be to set up an experiment similar to the Archimedes Experiment and weigh the displaced water and the displaced salt water for identical floating boats with the same number of pennies. Use a blank lab form for your experiment.