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OBJECTIVES:
Have you ever wondered why a hot air or helium balloon rises into the sky and
floats through the air? Submarines can move on the water's surface or travel
underneath the surface completely submerged. How does the submarine sink
into the water and then rise back to the top?
Both the balloon and the submarine use the buoyancy of the air
and water -
the
upward force a liquid or gas exerts on an object.
In this lesson, you will use full soda cans and discover
how to make the soda cans float or submerge like a submarine!
STEPS TO FOLLOW:
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Your
container's shape will govern the
amount of water (and salt in Step 3) you will
need. The container should allow the can to float --
there should be some
distance between the bottom of the can and the bottom of the container.
Preferably, the container should be clear.
We used an 8 cup container. You may need to use more
or less water to observe buoyant behavior.
Put 5 cups of cold tap water into an 8 cup
container.
Immerse one can of soda and record its buoyancy behavior
on your worksheet.
Use the ruler and measure the height of the can from the bottom
of the container.
Label the type of
soda in the table. Immerse the second can of soda
and again record your results on your
worksheet. Did the soda cans behave differently?
If so, why?
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The salt content (salinity) of
sea water is approximately 2.5%.
Your container's shape will govern the amount of water
(and salt) you will need. Use the following information to
calculate the ratio of water and salt you will need.
There are 16 tablespoons in a cup. To determine the amount of salt
you need multiply the number of cups of water
you use by 16 tablespoons/cup. This equals the number of
tablespoons of water you used. Multiply that number by 0.025 (which is
equivalent to 2.5%).
This is the number of tablespoons of salt you need.
(Cups Of Water X 16 Tablespoons/Cup X 0.025 = Tablespoons of Salt Needed)
The
container we used needed 5 cups of
water to float the soda can. For the
water to have 2.5% salt, 2
tablespoons of salt were added.
Stir to dissolve
the salt. Again
immerse the cans of soda and record what you observe.
Did the behavior differ between the two cans? Did it
differ from what you observed in Step 2? What
might be the cause?
How much salt is required before all the sodas can be made to float?
Why do objects float more easily in salt water?
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MATERIALS:
8 Cup Container (8 Cups equals 2 quart or half gallon) *
Hot and Cold Tap Water
Ice Cubes
3 Cans Of Soda (Cola, Diet Soda and Orange)
1/4 Cup Of Salt
2 Balloons
2 12" Pieces Of String Or Ribbon
Spoon
Graduated Cylinder
Measuring Cup
Tablespoon
Ruler
Safety Scissors
Balance or Kitchen Food Scale
Modeling Clay - (Optional)
2 or 3 square inches of Styrofoam - (Optional)
(Note: You do not have to use an 8 cup container. You can use
any size container
you like, but you will need to follow the directions in Step 3
to adjust the amount of salt you use.
This demonstration can be a class or team activity.
If using student teams duplicate these supplies for each team.)
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Compute the density of one of the cans of soda as follows:
Density =
(Mass of Can)/(Volume of Can)
Use the balance or kitchen scale to determine the
mass (weight) of the can in grams.
Assume that the volume of the can is 385 ml.
Use the above formula to compute its
density. The density of water is about 1 g/ml. How does the
density of the can of soda compare to the water?
Record your results on your worksheet and on
the board
in class.
Based on these measurements, explain
the results you observed in Steps 2 and 3.
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Use a graduated cylinder to determine the exact volume of each can of soda. Does this value differ significantly from the approximation which was used? If so,
use this value to compute the density; how does this new calculation of density compare to that of water?
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Repeat Steps 2-3 using warm to hot tap water instead of cold. (Be careful
- do not use water over 100oF.) Does the can behave differently
in warm water?
Repeat Steps 2-3 placing ice cubes in the water. Does this make a difference?
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Optional - If you have a can which is floating, how could you make it
neutrally buoyant (neither floating nor sinking)? Attach objects to
the can to test out your ideas. (We used modeling clay.)
Why would you want to create an object which is neutrally buoyant?
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Optional - If you have a can which sinks to the bottom, how could you make it
neutrally buoyant? (We used one half of
a 1.5" styrofoam ball. Some of the styrofoam became saturated with water.)
Attach objects to the can to test out your ideas.
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Optional - Run the experiments using each can in a six pack of the
same kind of soda. Do you observe any differences? Record your
results on your worksheet.
If so, what might explain the difference?
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