Density Demonstration: Floating Cans

Georgia Perimeter College 

Objectives

1.  To explore density and buoyancy.

Instructions

Materials:

10-gallon aquarium

Assorted soft drinks in 12 ounce cans

8 gallons of water

Note:  Be sure that the collection of soft drinks includes sugar sweetened, artificially sweetened, caffeinated, non-caffeinated, carbonated, non-carbonated, aluminum cans, and steel cans (Aluminum cans will not be attracted to a magnet, while steel cans will).

Procedure:

1. Place the aquarium where all students can see it and fill it with water.
2. Pick up the first soft drink can. Hold the can up for the entire class to see.  Ask for a prediction from the students as to whether the can will sink or float.  Submerge the can.  Be sure that, if the can has a concave bottom, you insert the can into the water in such a way that no air gets trapped under the bottom of the can.  Have students make a table with the names of the soft drinks in the first column, the student’s prediction for sinking or floating in the second column, and the actual result for whether each can sank or floated in the third column.  Students should also have a fourth column in their tables for the student activity that follows.  (see sample table below)
3. Before beginning the demonstration, think about the order in which you will add the cans to the aquarium.  Put in a few cans that will lead students to a particular conclusion (either sink or float) and then add one that will give a different result.  This discrepant event will hopefully lead students into a consideration of different variables that affect the buoyancy of the cans.
4. After testing all the soft drink cans, brainstorm a list of variables that could affect the buoyancy of the cans. 

Explanation:

When an object is submerged, two forces are acting on it.  One force is the downward pull of gravity; the other force is the upward push of the water.  This upward force is called the buoyant force.  If an object sinks, the force of gravity is greater than the buoyant force on the object.  If an object floats, the buoyant force is greater than the force of gravity.  The size of the buoyant force depends on the amount of liquid displaced or pushed out of the way by the submerged object.  As an object is lowered in the liquid, the buoyant force increases; it reaches a maximum when the object is completely submerged.  The volume of liquid displaced is equal to the volume of the object.  The buoyant force is equal to the weight of the volume of liquid displaced. The relationship between buoyant force and displaced liquid was described over 2000 years ago by the Greek philosopher Archimedes and is called Archimedes’ principle.  

How is this information related to the demonstration of floating cans?

The volume displaced is very nearly equal for all cans.  In other words, volume in this demonstration is a constant.  Therefore, the determining factor in sinking or floating is the mass (and therefore the weight) of the full cans.  The mass depends on three variables:  the amount of the soft drink in the can, the density of the soft drink, and the mass of the metal from which the can is made.  Soft drinks sweetened with sugar have a greater density than soft drinks sweetened with artificial sweeteners because sugar is denser than all currently used artificial sweeteners.  Steel cans are denser than aluminum cans, and therefore an aluminum can of the same volume as a steel can has a smaller mass.  Keep in mind that the density of soft drink cans are so nearly equal to the density of water that variations in the amount of soft drink in the can might cause unexpected results.  Obviously you will not be able to detect minor differences in the volume of liquid in the cans without opening them.

Student Activity:

Assign each of the cans used in the demonstration above to a student group.  Have students measure the mass and volume of the cans and calculate their densities.  Make a class summary table of the information and determine how the densities of the cans compare to the density of water (1 gram/cubic centimeter or 1 g/cm³).  Have students add this information to the table that they constructed in the demonstration portion of this activity.  Discuss whether the measured densities of the cans support the sink/float information collected earlier.   Obviously, a soft drink can density greater than that of water will cause the can to sink, while a soft drink can density less than that of water will cause the can to float.

Sample Table:

Extensions:

1.       Research how Archimedes discovered the principle of buoyancy that bears his name.

2.       Research the size and types of historic sailing ships.  How was the size of the ship related to the type (density) of its cargo?

Page created December 19, 2006