• You may wish to prepare a large table with glue guns if you do not have enough to go around.
• Designate floor space to measure the length of the launch. Make marks (with tape or chairs) every one-fifth (or one-tenth) of a meter. When they are finished building, the students will place their catapult on the designated floor space to measure the length the grape flies. Students catapult the grape three times and write down each value. Then they will sit down and analyze the data.
• Build an example of the catapults as shown above.

Part I: Preparing the Design

1. Arrange the students in groups of two.
2. Hand out or make available the necessary project materials.
3. Remind students that Mary Splendini, the client, wants the catapult to be as inexpensive as possible. Since less material implies less cost in this case, the constraint is that the engineering teams cannot use more than the allotted materials.

Part II: Building the Catapult

1. Stress to students that good engineering is characterized by well thought-out designs that are also attractive. An unnecessary amount of building material, such as glue, is not attractive and is considered wasteful. Just a daub of hot glue on each connection should suffice. Remind students that the hot glue dries quickly and becomes quite strong.
2. Build each upright and connect the two pieces together as shown in Figure 2.

Figure 2. The base of the catapult. click for copyright

3. Tie a rubber band to the stick that connects the two uprights as shown in Figure 3, leaving a loop extending upwards.

Figure 3. Securing the rubber band around the base of the catapult. click for copyright

4. Cut a short piece of straw and slide over wooden dowel.
5. Attach wooden dowel across top of side braces with two daubs of glue.
6. Glue rubber band to tongue depressor and the tongue depressor to the straw, as shown in Figure 4. Horizontal braces should be added to stiffen the structure.

Figure 4. Securing the tongue depressor to the wooden dowel/straw. click for copyright

7. The tongue depressor, which is the arm of the catapult, should rotate freely about the dowel allowing for the tension in the rubber band to be felt. See the Figure 5.

Figure 4. Stretching out the rubber band and testing the rotation around wooden dowel. click for copyright

8. Finish the catapult with a portion of straw glued to the arm to keep the grape in place. Also add two more sticks to the front (left side, see Figure 6) of the catapult to stabilize it after launch.

Figure 6. Front and rear stabilizers keep catapult from flipping. A straw is added to the end of the tongue depressor to prevent the grapes from sliding off during launch. click for copyright

Part III: Measuring the Launch

1. Have each engineering team take a turn to launch their grape. (Note: each grape should be about the same size.)
2. Each time their grape is launched, the team should write down the length of flight to the nearest tenth of a meter.
3. Have students calculate the average of their three measurements. Challenge them to do it in their head or longhand, without a calculator.
4. The client, Mary Splendini, said she can catch the grape if it's within one-fifth of a meter of its destination. Therefore, if each group's average length is within one-fifth meter of three meters, they have a successful catapult.
5. Have students make adjustments if necessary. The glue is strong but can be peeled away if changes need to be made. Try moving the arm back and forth along the dowel. Try adding a smaller rubber band.
6. Challenge students to convince our client the machine will perform to her satisfaction given our data (the three or more measured launches) and analysis (the calculated average and our judgment).

• Remind the teams that if they alter the catapult, not to include measurements from before it was altered, only take measurements from one catapult.

Brainstorming: As a class, have the students engage in open discussion. Remind students that in brainstorming, no idea or suggestion is "silly." All ideas should be respectfully heard. Take an uncritical position, encourage wild ideas and discourage criticism of ideas. Have them raise their hands to respond. Write their ideas on the board. Ask the students:

• What does a catapult look like? (Discuss where the students get their ideas. Perhaps from a film.)
• Does anyone have an idea on how to build one out of Popsicle sticks and a wooden dowel? Have a willing student draw it on the board.

Discussion Question: Solicit, integrate and summarize student responses.

• Ask students what units to use for measuring the grape-launch of our catapult. Since we wish to use SI units, which is appropriate: centimeters or meters or kilometers? (Answer: meters)
• Ask the students what simple machines are found in the catapult they are building? (Answer: The arm is a lever and the straw around the dowel forms a wheel-and-axel.)

Discussion Questions: Solicit, integrate and summarize student responses.

• How can you adjust the length of grape-flight?
• Why would the grape fly less distance if the dowel is closer to the other side of the arm, near the grape-projectile? Think of what we learned about levers.

Re-Engineering: Ask the students how they could improve the catapults and have them sketch or test their ideas.

Presentation: Have the team that got the grape closest to the target distance (and runner-up if time allows) present their design to the rest of the class. Ask them to explain why they think their design worked so well.