Buoyancy and Archimedes Principle
This material is replicated on a number of sites
as part of the
SERC Pedagogic Service Project
Initial Publication Date: April 27, 2010
This Starting Point page was organized by R.M. MacKay. Clark College, Physics and Meteorology.
Summary
Buoyancy is based on Archimedes' Principle which states that the buoyant force acting upward on an object completely or partially immersed in a fluid equals the weight of the fluid displaced by the object. Here are five ideas for demonstrating buoyancy to your students.
I. Measure the mass of a rock in air and then when completely submerged in water. II. Find the maximum load that can be supported by a boat. III. Make a Cartesian diver and have students explain how it works. IV. Compare cans of diet and regular soda in a tank of water. V. Float an egg in salt water.How to set up these demonstrations
I. Measure the mass of a rock in air and then when completely submerged in water. Use this data and the density of water to estimate the rock's density. You will need a rock, some string, a spring scale, and a container of water big enough to accommodate the rock. A suggested Activity Sheet (Acrobat (PDF) 191kB Aug16 05) Reading through this 1-page activity sheet can help you better understand the demonstration setup. 
II. Find the maximum load that can be supported by a boat. Use a small rectangular shaped plastic container floating in a larger container of water. Measure the dimensions of the container to calculate its outside volume. A suggested Activity Sheet (Acrobat (PDF) 34kB Aug16 05). This activity sheet describes the demonstration setup.
III. Make a Cartesian diver and have students explain how it works. A suggested Activity Sheet (Acrobat (PDF) 258kB Aug16 05). The appendix of this activity sheet describes the demonstration setup. 
IV. Compare cans of diet and regular soda in a tank of water. All you need is a can of diet and regular soda and a tank for water. 
V. Float an egg in salt water. You will need an egg, a beaker or clear glass so the class can easily see the egg, some salt, a stirring spoon, and some warm and cold water. How to do these demonstrations
I. Measure the mass of a rock in air and then when completely submerged in water. A suggested Activity Sheet (Acrobat (PDF) 191kB Aug16 05) Reading through this activity sheet before class can help you better understand how to do this demonstration. Remember, as with any demonstration, ask students to predict as many things as possible before actually doing the demonstration.II. Find the maximum load that can be supported by a boat. A suggested Activity Sheet (Acrobat (PDF) 34kB Aug16 05). This activity sheet describes how to do the demonstration.
III. Make a Cartesian diver and have students explain how it works. A suggested Activity Sheet (Acrobat (PDF) 258kB Aug16 05). This activity sheet describes how to do the demonstration. For larger lectures the ketchup diver is by far the most visible. All ketchup packages are not equal. Some demonstrators find a package that just barely floats so only a slight squeeze of the bottle makes it sink. This makes squeezing the bottle unnoticeable so you can pretend that the ketchup package level is being controlled by magic.
IV. Compare cans of diet and regular soda in a tank of water. A clear fish tank works best so the whole class can easily see the cans. V. Float an egg in salt water. A clear glass beaker works best for this so the whole class can see. A 400-ml beaker is good since it will not require that much water or salt to see results. Start with about 300 ml of fresh water and show the class that the egg sinks. Now use 150 ml of hot water to dissolve as much salt as possible (about 5 or 6 tablespoons). Dump half of the fresh water from the beaker and replace with the newly created salty brine. The egg should now be floating.
Ideas for discussing these demonstrations in class
I.Measure the mass of a rock in air and then when completely submerged in water. A suggested Activity Sheet (Acrobat (PDF) 191kB Aug16 05) Working through this 1-page activity sheet with your students during the interactive lecture demonstration can help them better understand key concepts.II. Find the maximum load that can be supported by a boat. A suggested Activity Sheet (Acrobat (PDF) 34kB Aug16 05). Working through this 1-page activity sheet with your students during the interactive lecture demonstration can help them better understand key concepts.
III. Make a Cartesian diver and have students explain how it works. A suggested Activity Sheet (Acrobat (PDF) 258kB Aug16 05). This activity sheet provides ideas for student questions. You may want to give copies of the first page of this activity so students can work through it in class. The link Diver provides an online applet which clearly shows why the Cartesian diver dives. Just move the red bar up or down with the mouse. This is a helpful supplement to the actual demonstration. IV. Comparing cans of diet and regular soda. Ask students about: the volume, weight, and density of each can. Ask them if there is a way to test to see if they are correct. A balance or scale on hand can make it easy to check the weight (or mass) of each can. It should also be clear that the volume of the cans are the same.
III. Make a Cartesian diver and have students explain how it works.V. Float an egg in salt water. Why does the egg float in salt water and not fresh water? Ask students to design a method to measure the egg's density. Measure the density of both fresh and salt water with a graduated cylinder and balance (i.e. measure the mass of 100 cm3 of each fluid and divide by 100). Ask students to estimate the egg's density.
References and resources
Click on this image to go to an online JAVA applet that simulates the rock in water experiment. The buoyant applet has a mass attached to a spring scale. A student worksheet using this applet environment is also provided.
