An Uplifting Project—The Buoyancy of Balloons
AbstractBalloons are a festive addition to many celebrations. You've probably noticed, though, that over a short period of time, helium-filled latex balloons start to lose their buoyancy. So when you're planning your next party, how soon can you buy the balloons in advance before they start deflating? In this science fair project, you will use a simple scale to measure the lift supplied by a set of balloons, and determine the rate of lift decay.
David Whyte, PhD, Science Buddies
The objective of this physics science fair project is to measure how the buoyancy of helium-filled latex balloons changes over time.
Helium-filled balloons float because the helium is lighter than the surrounding air. In other words, the weight of the air displaced by the balloon is greater than the weight of the balloon and the gas inside, so the balloon floats upward.
Figure 1. Ascending helium balloons. (Wikipedia, 2009.)
This force, or buoyancy, is exactly the difference in the weight of the balloon and its contents (plus a ribbon, if one is attached), versus the weight of the volume of air displaced.
Latex is a permeable membrane, which means it has very small holes that allow the helium atoms to escape. Helium escapes from latex balloons faster than air does, because of the small size of helium atoms. As the helium trapped inside of a latex balloon slowly escapes, the balloon starts to sink. There is a point at which the buoyancy supplied by the helium just equals the weight of the balloon and its contents (and any attached ribbon), and the balloon hovers in mid air, neither rising nor falling. This is called neutral buoyancy.
In this science fair project, you will use a simple scale and some balloons to investigate the following questions. Does the balloon lose its buoyancy at a constant rate, or does the rate change over time? What is the rate of lift decay?
Terms and Concepts
- Permeable membrane
- Neutral buoyancy
- How many latex balloons would it take to lift you off the ground? Hint: See the Bibliography.
- What other lighter-than-air gases can be used for balloons? What are their benefits and drawbacks?
- Why do latex balloons lose their buoyancy faster than Mylar® balloons do?
- University of Hawaii. (2002, May). Balloon Lift with Lighter than Air Gases. Retrieved February 27, 2009.
- Neer, K. (2008). How many regular-sized helium-filled balloons would it take to lift someone?. Retrieved February 27, 2009.
Materials and Equipment
- Metal spoons (3)
- Masking tape
- Permanent marker
- Digital scale, accurate to at least 1 g, such as the Fast Weigh MS-500-BLK Digital Pocket Scale, 500 by 0.1 G, available from Amazon.com
- Latex balloons, filled with helium and ribbon attached so you can hold them (9); available at party supply stores and some grocery stores
- Lab notebook
- Graph paper
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Performing the Experiment
- To start, label the three spoons using the masking tape and the permanent marker. Label them A, B, and C.
- Weigh each spoon and record the values in your lab notebook.
Attach three balloons to each spoon.
- Attach the balloons near the end of the ribbon, so that extra ribbon is not hanging from the spoon.
- Trim any extra ribbon with the scissors, as needed.
Weigh each spoon with the balloons attached.
- The balloons should be free to float above the scale.
- Note the weights in your lab notebook for spoons A, B, and C.
Repeat step 4 every 8 hours, until the balloons have lost most of their buoyancy.
- 8 hours is just a suggestion. Use your own judgment about how often to weigh the spoons.
- Poke a hole in each balloon to let any remaining helium escape.
- Weigh the balloons from the three sets by removing the spoons from the balloons and then weighing each balloon-and-ribbon set (a set is made up of three balloons and three ribbons). Record all data in your lab notebook.
Analyzing Your Data
Calculate the buoyancy of the balloons.
- Calculate the difference in weight between each spoon alone and each spoon with the balloons and ribbon attached.
- Add the weight of the three empty balloons and their ribbons to get the buoyancy. For example, say the weight of the spoon is 20 g, the weight of the spoon with the balloons and ribbons attached is 15 g, and the weight of the balloons and ribbons is 3 g. Then the total lift supplied by the helium gas equals 20 - 15 + 3 = 8 g. See Equation 1.
|Buoyancy =||weight of spoon − weight of spoon with balloons attached + weight of balloons and ribbon|
- Graph your data, with time on the x-axis and buoyancy on the y-axis.
- Was the rate of buoyancy loss (lift decay) linear over time, or was the rate non-constant? Explain your results.
- Repeat the experiment at least two more time so that you have three sets of data.
Ask an Expert
- Measure the volume of the balloons during the experiment. Include these measurements in your graphs. (The equation for the volume of a balloon can be found in Katherine Neer's article in the Bibliography).
- Compare the rates at which helium and air escape from the balloons by measuring the volumes over time. Try other gases.
- Use a spreadsheet program to add a trend line to your data. Use the program to determine the equation for the trend line.
- Experiment with the effect of temperature on lift decay.
- Measure the atmospheric pressure over the course of your experiments to see if it affects the buoyancy.
- Experiment with different material for the balloon, such as mylar.
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