Parents, Educators & Engineers

Zip Line

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Materials (per person)

  • chipboard (from a cereal box or back of a notepad)
  • 2-4 small paper cups (i.e., 3-ounce)
  • Ping-Pong ball
  • 4 plastic straws
  • scissors
  • single-hole hole punch
  • 4 feet of smooth line (e.g., fishing line or unwaxed dental floss)
  • tape (duct or masking)
  • 4 standard, flat steel washers (1 inch in diameter or larger)
  • 4 wooden skewers

Your Challenge

Design and build something that can carry a Ping-Pong ball from the top of a zip line string to the bottom in four seconds (or less!).

Brainstorm & Design

Look at your Materials and think about the questions below. Then sketch your ideas on a piece of paper or in your design notebook.

  1. Using these Materials, what can you design that can carry a Ping-Pong ball down a zip line?
  2. How will your Ping-Pong ball carrier stay on the zip line as it goes from the top to the bottom?
  3. What kinds of Materials should be in contact with the zip line so that the carrier slides quickly?

Build, Test, Evaluate & Redesign

Use the Materials to build your Ping-Pong ball carrier. Then make a zip line. Run the line between the back of a chair and a stack of books. Make sure the high end is about two feet above the low end. Test the carrier by putting it on the line. When you test, your design may not work as planned. The design process is all about "if at first you don't succeed, then try, try again." On Design Squad, we say, "Fail fast—succeed sooner!" Study the problems and then redesign. For example, if your Ping-Pong ball carrier:

  • keeps dropping the ball—Check that it has a big enough place to hold the ball.
  • stops partway down—Make sure there's nothing blocking your carrier where it touches the line.
  • doesn't balance well—Adjust the weights. Add weights or move them so they are farther below the zip line. Doing this changes the carrier's center of gravity, the point within an object where all parts are in balance with one another. See how changing the numbers and positions of washers affects the carrier's balance.
  • takes longer than four seconds to travel the zip line—Find ways to reduce friction. Yes, there's friction—the force that resists motion—even when you're dealing with something as smooth as fishing line. You'll find friction anytime things rub together. Experiment with different Materials to see if you can reduce friction and speed up the Ping-Pong ball carrier.
Zip Line