I teach 7th grade science. During our physics unit, I have students build craft stick catapults to help demonstrate Newton's Laws of Motion. They also use their catapults to create a claim, develop an experiment to test their claim, and use evidence from the experiment in their reasoning. For more information about the catapult lab, you can skip to the last step. If you're no longer a student, don't let that stop you!
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Now let's get building!
Step 1: Materials
For this project, you'll need:
- craft sticks (8)
- duct tape (about 24")
- rubber bands
- index card
- writing utensil (optional)
Step 2: The Base
- Start with about 24"-30" of duct tape.
- Rip the tape in half lengthwise.
- Rip the tape into about twenty, 1.5" strips.
- Create an equilateral triangle. Make sure the craft stick closest to you is on top of the others. The corner of the triangle facing away from you should have the edges of the craft sticks touching.
- Add duct tape to each corner to hold the shape.
- Double the duct tape to double the strength.
*The bottom stick is on top because when you pull back on the throwing arm (after the build is complete), there will be more tension on those two corners (compared to the rest of the joints).
Step 3: The "A" Frame
- Lay two craft sticks on top of the equilateral triangle from the first step. Make sure the ends of the sticks closest to you are nestled into the groove (created when we laid the craft stick on top of the other two in our first step).
- Lay duct tape over the craft sticks at each corner to keep them in place.
- Lift the "A" and wrap the tape around the three corners. The "A" will naturally stay up after the tape is wrapped.
Step 4: The Front Post
When taping the front post, you will notice the top of the "A" can be taped in a variety of positions. Depending on the angle of the front post will influence the angle in which the throwing arm stops, therefore impacting the release point of your projectile. A more vertical front post will result in a lower trajectory. If the top of the front post is taped to the top of the "A" (creating a pyramid shape), the catapult will have a higher trajectory. I tend to tape my catapult somewhere in the middle.
Step 5: The Throwing Arm
Take your two remaining craft sticks and duct tape them together as shown. The length of the throwing arm is another variable in the distance, accuracy, and angle of your projectile trajectory.
Tape the end of the throwing arm where the base meets the front post. The throwing arm should have the ability to move freely up and down, without falling off. I'm sure there is more than one way to affix the throwing arm, but my picture (and the video) is worth 1000 words.
Make sure to add an extra layer of tape around the front post, around the throwing arm, and around the base to keep the throwing arm secure.
Step 6: Projectile Basket and Rubber Band
- Create a square on an index card.
- Cut out your square and imagine where your projectile will need to fit (we launch marbles).
- I like to draw out the shape of a tic-tac-toe grid with the center square just the right size for my projectile. Make sure the bottom of your basket will fit the projectile snugly. Too much space and the projectile can roll around and won't be as accurate, too little space and the projectile won't fit. It's okay to go back and make a new basket if the original isn't working like you planned.
- Fold on the dotted lines.
- Cut to the crease on the solid lines.
- Fold up the sides and tape into place.
- Tape the basket to your throwing arm leaving a small amount of the craft stick at the top (to pull back on).
- Place a rubber band (or two, or three) so that it stretches behind the throwing arm and over the front of the front post.
- Ready... Aim.... FIRE!!!
Step 7: Modifications and Lab
Mod 1: Some students decide to tape an extra craft stick to the back "A" frame (horizontally) to influence the stopping point of the throwing arm for a higher trajectory.
Mod 2: Some students decide to tape two extra craft sticks to the back "A" frame (vertically) on either side of the throwing arm to prevent it from wiggling to the left or right.
Lab: What all teachers should be striving to get their students to master...
Claim, Evidence, Reasoning
My students take one class period to test their catapults in a "beer pong" style catapult tournament. We push the lab tables together (end to end) to make a series of long tables. We set up three cups at the end of the table filled with sand. Students work in pairs to either hit the cups (one point) or sink the marble into the cups (3 points). First to 10 wins, but they have to win by 2. The team who shoots second at the start of the game gets the last shot.
Winners play winners, losers play losers.
After students have a chance to test their catapults, identify problems, and make adjustments they get to form their claim.
Claim: To launch a catapult 300 cm, students must be aware of ___________________ and __________________.
They are allowed to fill in the variables (including distance, but that's the length of our lab tables pushed together). Possible variables include but are not limited to:
- the distance the throwing arm gets pulled back in cm.
- the number of rubber bands
- the mass of the projectile
- the angle the throwing arm gets pulled back
- the angle of the throwing arm at it's stopping point
- the size of the basket
Evidence: Students then design an experiment to test their claim. They collect data and create a data table.
Reasoning: Students are then expected to be able to use the evidence they collected to support their claim. For example: To launch a catapult 300 cm, students must be aware of the distance the throwing arm gets pulled back in centimeters and the number of rubber bands. First, our team used a metric ruler to mark centimeters 1-6 on the "A" frame of our catapult. Next, we collected 4 rubber bands. To keep the mass of the projectile the same, we used the same marble with a mass of 4.5 grams for the entire experiment. According to our data, using 3 rubber bands and pulling the throwing arm back a distance of 4 cm resulted in a throwing distance of 300 cm. Our next closest trial was using 2 rubber bands and pulling the throwing arm back 6 cm because the marble landed 315 cm away. One possible variable we weren't able to account for was...