Materials
Craft sticks, Paperclips, Propellers, Cardstock, Long rubberbands, Masking tape, Hot glue, Kite string, and scissors.

Learning Objective
Elementary school-aged students explore ideas like thrust, potential energy, and drag as they build and test propeller-powered zipline racers. Students are encouraged to experiment with their designs to improve their racer's performance - this allows students to apply their newfound experience and conceptual understanding to create more efficient (faster) designs.

How It Works
It's simple. Winding the propeller will store energy in a rubberband by twisting it. When released, the propeller rapidly unwinds and generates thrust, which pulls the craft along a low-friction zipline. However, the torque from the unwinding propeller can cause the entire racer to flip upside down. To counter this force, another force is introduced: drag. By attaching a paper cutout, drag perpendicular to the side of the racer is created, which helps prevent inversions.

The Lesson Plan
Prep work: 2/5
Difficulty: 2/5
Setup: 3/5
Cleanup: 2/5

Begin by demonstrating an example of a zipline racer, which helps focus the students at the start of the class. Show the students how to build my particular design, though I let them know that they're encouraged to try new things. Disassemble a racer and verbally define each part and it's significance: the racer's frame simply holds everything together; the propeller shaft is used to store energy provide thrust; the paperclips hook onto the zipline; and the paper cutout creates wind resistance, which prevents the racer from flipping upside down. Finally, demonstrate the technique of launching a zipline racer before allowing the kids to begin building.

Safety and Tips

• Pulling downward on the racer while it is hooked onto the zipline may cause it to be flung off the line upon being released. This can be dangerous when the propeller is rapidly unwinding.
• Make sure the zipline is either highly visible or above everyone's head.
• Young students (grades 1-3) may need extra guidance while bending the paperclips, attaching the rubberbands, and properly launching the racer.
• Encourage your students to experiment, especially if they're in grades 4 and up - I'm always amazed by their innovations and improvements.