Tony Alfrey - L. M. Nixon Elementary Parent
This project is suitable for all grade levels from 1st grade through college, depending on the mathematical and scientific sophistication of the students . All students build the same catapult, but the sophistication of the analysis of the catapult performance varies with grade level. We spend one session in assembling the catapult, followed by a session of testing, competition and analysis.
The heart of the assembly is a conventional wooden clothespin with a metal spring. Why build a catapult out of a clothespin when one could easily imagine some scheme using rubber bands? Because the clothespin contains all of the key components needed in one element, including an enclosed pivot, spring and gluing surfaces.
1. Traditional wooden, spring-loaded clothespin.
2. Base (random piece of thin wood about 2" x 5" x ¼". We used pre-cut, tapered cabinetry shims simply because they were readily available and required no cutting. Another suitable choice is a precut piece measuring 2-5/8in x 5-3/8in x 1/8in, part number ST5200 from Woodworks, Ltd., www.craftparts.com. See also www.larascrafts.com)
3. Support blocks, 2 each, ¾" wooden cubes are suitable. A possible part number is SQ0750, Woodworks, Ltd. www.craftparts.com. These blocks are used to raise the clothespin above the base, allowing a full range of motion of the clothespin.
4. Ice cream stick.
5. Small paper cup. We used ½ oz. "portion cups" from Genpack, #F050S (www.genpak.com). Genpak also sells the same cup under the name "soufle cup". We found them at a local Smart amd Final restaurant supply, but you can also make your own square "cup" with the template found at the end of the project.
6. Elmer's "School Glue" or "White Glue".
7. Three #30 rubber bands (2" x 1/8").
8. Sheet of #100 grit sandpaper.
Suggested Assembly Steps
1. Check the package that contains the clothespins. Some wooden clothespins are "weather resistant" (waxed) to improve their longevity outdoors. If you're using this type, sand the surfaces of each side of the clothespin to expose bare wood.
2. Examine the clothespins. Make sure that they are nice and strong and free of cracks or flaws. Open and close them all the way and snap them several times to make sure they are strong. Pick clothespins that are very "springy"; some clothespins are pretty wimpy and you will want to select some that have strong springs.
3. Set the clothespin on one end of the base and wrap three rubber bands around the base and clothespin.
4. Next, lift up each end of the clothespin and slide a block under each end of the clothespin. The blocks are used to elevate the clothespin off of the base so that the clothespin can be fully opened when the arm and cup are added. Don't use glue yet.
5. Put some glue between each block and the base.
6. Lift up the clothespin at each block and put a little glue between the clothespin and block.
7. Attach the ice cream stick "arm" to the top of the clothespin with a bit of glue. Place the rubber band over the ice cream stick to hold it in place while the glue dries.
8. Attach the paper cup to the end of the "arm" with a bit of glue. Make sure a little bit of the ice cream stick hangs exposed out of the back.
9. Set everything aside to dry for at least a full day.
Using your Catapult.
1. Remove the rubber bands (D'oh!).
2. First do an experiment. Place the catapult flat onto a table and load the cup with a jellybean or similar projectile. A small marshmallow makes a good projectile for primary grades.
3. Pull back the clothespin and release the catapult. Note how far the projectile goes (the "range").
4. Now tilt the catapult and try again. Is there some angle that yields the most range?
5. Have a contest! Who can launch their projectile the farthest? What things control the range?
Super Catapults (this section under construction)
Now that you've made a simple catapult, use your imagination to create Super Catapults.
Add some rubber bands for extra power.
Add extra clothespins.
Think of other ways to add more clothespins.
Maybe a longer arm will help?
More Sophisticated Stuff and Guided Discovery
(this section under construction)
Following are some possible exercises for different grade levels. These are not complete lesson plans, just sketches and ideas for possible activities to serve as starting points for lessons or science fair projects. We've included a list of possible California State Standards that the exercises might fulfill. We'll expand this list as time passes and we try out some of these exercises with real students.
Primary grades 1-2
1. Fashion a measuring stick with a paper strip of large-grid graph paper with a length equal to some convenient number of squares such as 10 or 20. Use the strip to measure the range of projectiles. Students will have to figure out how to measure the range expressed as the total number of squares of graph paper.
2. First collect projectiles such as a jelly bean, a marshmallow, a small wad of bond paper, and a small wad of tissue paper. Attempt to push these items across a table top by blowing on them. Which is easiest to blow across the table?
3. Now compare the range obtained with these items when launched with your catapult, always using a cardboard triangle as a gauge to hold the catapults at a fixed angle. Is there any correlation between the range obtained and the results observed when blowing the projectiles? Strictly speaking, each of these objects will have approximately the same wind resistance if they are the same diameter, but will have appreciably different mass. The least massive projectiles will experience the greatest change in velocity due to wind resistance after the projectile leaves the catapult.
1. Use pieces of cardboard cut into the shape of triangles to set the base at a fixed angle with respect to the table. Launch the projectiles and record the range obtained for each base angle. Do you see a correlation between the angle of the base and the range of your projectile?
2. Devise some way to measure the maximum height of the projectile and the range (measuring the height will require some ingenuity and is a good exercise for a directed discussion and a team exercise). Launch your projectile with the base tilted at several different angles and record the height and range at each base angle. A clear trend will develop: launching with the base nearly flat results in little range, but maximum height. Launching with severe tilt also yields little range (and lots of rolling after striking the ground) and little height. How can the information collected be turned into a drawing of the motion of the projectile?
1. Potential energy is stored in the clothespin spring when the clothespin is opened. The amount of potential energy stored depends on how hard it is to pull open the clothespin and how far the clothespin is opened. Before building catapults, sort clothespins by their springiness. This can be done by simply using a conventional bathroom scale. Place the clothespin on the scale and press down on the clothespin until the clothespin just begins to open. Record this bathroom scale reading for each clothespin. Now repeat the measurement, but this time push hard enough to fully open the clothespin, but no harder. Take an average of the two readings. This number will represent the potential energy that each clothespin can store (assuming that all clothespins open the same amount).
2. Now build identical catapults with the clothespins that you have "calibrated".
3. Set up your catapults at an angle such that the projectiles are launched perfectly straight up. Some thin wood blocks, cardboard or paper may be required to "shim" the base to achieve this result. Using a "C" clamp to hold the catapult to the edge of a table will make the job easier.
4. When the clothespin is fully open and ready for launch, potential energy is stored in the clothespin. When the clothespin is released, the cup and projectile on the catapult begins to move. Potential energy in the spring is converted into the kinetic energy of the moving projectile, and the moving parts of the catapult. When the clothespin finally closes and the projectile is released, there is kinetic energy in the projectile. Kinetic energy remaining in the moving parts of the catapult will be wasted and converted into some sound energy (hear the 'snap' of the clothespin) and some heat energy.
The kinetic energy provided to the projectile will determine how high the projectile will go. Measure the maximum height of the projectile and compare it to the amount of potential energy stored in the clothespin in step 1 (Caution: to do this properly, you will need a 'standard' projectile, that is, a projectile with the same mass for each of the different catapults.)