Fly ball. On Earth (left), the force of gravity and the force of air resistance both act on falling objects, such as a baseball. But the Moon (right) has no atmosphere and therefore no air resistance; only the force of gravity acts on falling objects there. The importance of air resistance depends not only on the density of the atmosphere, but also on the shape and weight of the object and the distance it is dropped. Eventually all objects dropped on Earth will reach a terminal speed--air resistance prevents them from falling any faster. Diagram by Mary Urquhart.
Little do most children know that their playground is an ideal laboratory for physics. Simply by swinging, climbing, and jumping, they have already developed an intuitive grasp of many basic concepts in physics. The Playground Physics program relates this experience to scientific experiment and theory. Designed at the University of Colorado and tested in local schools, the program is geared toward fourth- through seventh- graders. For the younger children (grades four and five), the experience is mostly conceptual, but for the older children, there is a slightly more formal and mathematical approach.
The portion of Playground Physics presented here is a shortened version of "Jungle-Gym Drop," one of three modules in the complete program. The full program, available on the World Wide Web at http://lyra.colorado.edu/sbo/mary/play, includes introductory materials, student handouts, experiment reports, and teacher guides.
In "Jungle-Gym Drop," students drop objects from the top of a jungle gym to learn about gravity. The students choose objects of different shapes, sizes, and masses to drop. You may need to help your students to select appropriate objects, such as:
In a vacuum, such as that on the surface of the Moon, all objects fall with the same acceleration. This is a natural consequence of how the force of gravity acts. If you drop a feather and a bowling ball, they'll hit the ground at the same time. On Earth, too, this is usually true for objects dropped from small distances. But for some objects, another force besides gravity makes itself felt: air resistance. Feathers and paper, for example, have little mass and a high surface area, making air resistance important -- so they fall slower.
When you discuss this in class, your students may mention that some objects, such as hot air balloons, won't fall at all. Because they contain hot air, the air inside them is less dense than the surrounding air, and their buoyancy keeps them up. You can explain this to your students by saying that the balloon is supported by the air around it. The same principles apply in water, except that water resistance is even stronger than air resistance.
First, ask whether the students have ideas for the experiment. They may suggest just what I have in mind. If not, they can try their own experiments in addition to the ones I suggest here.
First, measure the mass of your objects with the scale and determine which are heaviest (the most massive) and which are lightest (the least massive). Have your students climb the jungle gym two at a time to drop their objects. By dropping two objects at a time, the students will get a subjective view of how fast objects fall.
To provide a more objective measurement, have the students measure the distance between each object and the ground before they drop it. Try to drop all of the objects from the same height the first time through. Then students can vary the height. While some students are dropping their objects, others should time the fall with the stopwatches.
Timing is tricky
Single measurements can be very misleading. Each student may see a slightly different number on the stopwatch. Use at least three stopwatches or repeat each drop at least three times. This shows that results in science need not be (and often aren't) identical.
This might be a good time to talk about uncertainty and error in science. Errors in measurements are normal and happen because people and equipment are never perfect. Uncertainty is another way of saying how trustworthy the measurements are. If, after several drops of a ball from the same height, the students all record times within a second of one another, then their measurements are believable to a second or two.
The problem in this experiment is that, because of the short distances involved, most objects only take a second or two to fall. So, these measurements aren't going to be very precise. The stopwatches are mainly a device to illustrate the idea of scientific measurement and error. You might try asking your students how the experiment could be changed to make the measurements more precise.
Measuring how long it takes a ball to fall is hard enough; measuring the momentum of an object as it hits the ground is nearly impossible. I have come up with a fun and messy solution.
Students should select only a few objects for this experiment. For each object, fill a pie pan with shaving cream (try to keep the amounts fairly consistent). After a practice drop to determine the best location of the target pie pans, let the students drop their objects two at a time into different pie pans. They might miss a time or two, but they'll probably hit it after that. To prevent an utter mess on the playground, try putting an old blanket under the pans. The bigger the splash, the more momentum the object has when it hits. No throwing allowed--students must simply drop their objects.
Before you leave the playground, ask the students about how their hypotheses held up. Would they change any of their answers to the questions? Did they learn anything new? If the experiments didn't work well, discuss what should have happened and have the students come up with reasons why it didn't. Discuss the responses and add your own suggestions if necessary.
For the older students, you can assign more formal reports and calculations using the time and distance measurements. For instance, you can give students the acceleration due to gravity and have them determine the time the objects would take to hit the ground if air resistance were neglected. This requires a calculator with a square-root key. The formulas are:
distance = 1/2 x acceleration x time x time
time = (2 x distance ÷ acceleration)1/2
On Earth, the acceleration due to gravity is 10 meters, or 32 feet, per second per second. On the Moon, it is 1.6 meters, or 5.3 feet, per second per second. For example, if you drop an object from 8 feet high on Earth, it should take 0.7 seconds to hit the ground; on the Moon, 1.7 seconds.
MARY URQUHART is a graduate student and research assistant at the Laboratory for Atmospheric and Space Physics of the University of Colorado in Boulder. She has written several activities for elementary and middle school students in addition to Playground Physics, including an award-winning Mars curriculum, Reaching for the Red Planet. She recently organized a teachers' workshop for comet Hale-Bopp and appeared on the national television show News for Kids to talk about comets. Urquhart's email address is firstname.lastname@example.org. Her educational materials are available at http://lyra.colorado.edu/sbo/mary. © 1997 Mary Urquhart. Used by permission.
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