Time to build a zip-mobile. The challenge: make a sturdy structure that will fly down the zip line in less than four seconds using a paperclip, 2 straws, 2 sticks, 2 washers, tape and scissors. To make a device that zips, consider what arrangement will avoid too much friction or drag and think about how your centre of gravity will affect the flight.
Most of the students easily figure out which material should be in contact with the zip line so that the zip-mobile slides quickly. The second challenge: now modify your zip-mobile to make a zip carrier that will transport a ping-pong ball down the zip line. You are given a cup as additional material.
Our third challenge was a complex design problem. The students are asked to modify their zip carrier to transport a marble down the line and to drop the marble on a target part way down. This has to be done without touching the carrier directly. There are several ways to tackle this challenge, but all involve some serious lateral thinking!
One team tries a remote string release mid flight. In multiple test runs they discover that the release needs to be before the cup is above the target. This helps because the marble is already moving forward along the zip line and moves in a curved trajectory (not a direct line down) once it is released from the cup
One student realizes that if she ties a string from the release point to the bottom of the zip-mobile it will tip the cup at a set point and release the marble towards the target. This is an elegant hands-off solution that needs some refining to make it reproducible.
And finally, it's race time. Which designs move faster and why? How does their shape and weight affect their speed? And how do friction and drag slow them down? But most of all, why can't you tell that was not a tie!
