Buoyancy
Objectives
Upon completion of this lesson students will be able to:
1. Predict and observe the behavior of regular and diet soft drink cans when immersed in
water.
2. Calculate the density of solutions using mass-volume data.
3. Design and construct a neutrally buoyant vessel.
4. Relate density to neutral buoyancy.
Materials
Balance
Aquarium or other large, deep, straight-sided container filled with water
One sealed can each of regular and diet soda (same brand) at room temperature (Coke and
Diet Coke and Seven-Up and Diet 7-Up are recommended. Some other brands may not
exhibit significant differences between regular and diet versions.)
One empty can of each type of soda
Paper towels
Pencils and paper for student data tables
Ice (optional)
Plastic foam (cups, blocks, strips or poly puff "packing worms")
Finishing nails or metal washers, several sizes
Tape, preferably water resistant
Scissors
Procedure
(This activity takes one class period or less, depending on how much of it you decide to do.) 1. Ask students for examples of objects or substances that will sink in water float in water hover in water
2. Explain the concepts of buoyant force and weight. Buoyant force is a force that acts upon an object causing it to float. An object will float when the buoyant force is greater than its weight. An object will sink when its weight is greater than the buoyant force. When the buoyant force and weight are equal, the object will be neutrally buoyant.
3. Explain that engineers must design machines to be energy efficient. When it comes to exploring in water, an object that is neutrally buoyant will not have to use energy to remain at a constant depth. Explain that Bob Ballard and his crew use vehicles, like JASON, that are neutrally buoyant.
4. Perform this demonstration:
A. Show students unopened cans of regular and diet soda. Ask how they can tell the difference between the two cans. Write or summarize their responses on the board.
B. Ask the class to predict what would happen if the cans were placed in water.
C. Fully immerse the can of regular soda in water (it should sink and stay sunk), then the can of diet soda (it should rise to the surface and float). Compare observations with predictions. (Before the experiment, test several cans of sodas to be sure the desired results are obtained. Despite their labels, cans in a six-pack may have slightly different volumes, and this will affect their buoyancy. If the expected result does not occur at room temperature, try ice cold cans in ice water.)
D. Ask students to offer probable explanations for their observations. Refer them to the list of ingredients on the side of each can. Do the different solutions contain anything that might make them more or less dense? Discuss. (Regular soft drinks are sweetened with dissolved sugar and/or high-fructose corn syrup. A great quantity of this relatively heavy substance is required to sweeten regular soft drinks. The artificial sweeteners in so-called diet drinks are many times sweeter than sugar and so they are required in smaller quantities. Therefore, regular soft drinks are more dense than water, and diet drinks are less dense.)
E. Distribute plastic foam and nails or washers. Give these directions for a student
experiment:
1. Set up data table for recording mass, volume, and density.
2. Determine the mass of an empty, dry, regular drink can and a diet drink can. Determine
the mass of a full, unopened can of each solution and record. Read the volume of solution
from the side of each can. Record this information in your data table.
3. Determine the overall volume of your unopened cans by water displacement. How do
these volumes compare with the volume of the contents indicated on the side of the cans?
4. Calculate the density of the unopened cans by dividing the mass by the volume of each
can. (M V=D)
5. Modify your cans to make them hover in the middle of the container of water. Use plastic
foam materials and metal nails or washers to raise or lower the cans.
6. Use these questions as the basis for a class discussion:
How do the densities of the regular and diet drink solutions compare to the density of water,
which we will assume to be lg/mL? (Actually, at room temperature, the density of water is
slightly less than this. The regular drink should have a density greater than that of water,
while the diet drink's density should be less.)
Do the densities explain what students observed when the drink cans were placed in water? (Yes)
How does the density of something that will sink compare to that of the fluid surrounding it?
What about the density of something that floats? Something that hovers?
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Explain why modifications to the originally sinking and floating cans caused them to hover.
Why would an object that hovers be easier to maneuver than one that sinks or floats. (Extra
equipment to control sinking and floating is unnecessary. It is more maneuverable because
the operator does not have to spend time getting it up from the bottom or down from the
surface.)
Activity was provided courtesy of the National Science Teachers Association and the JASON Foundation for Education.