Parents, Educators & Engineers

On Target

Materials (per zip line)

  • 9 feet (3m) of smooth line (e.g., fishing line or kite string)
  • index card
  • marble
  • masking tape
  • paper clip
  • 1 medium-sized paper cup
  • scissors
  • target drawn on a piece of paper

We Challenge You To...

...modify a paper cup so it can zip down a line and drop a marble onto a target.

Brainstorm and Design

Think about how you might design a way to carry and launch a marble:

  • How will you modify the cup so it can carry a marble down a zip line and also drop it onto a target?
  • How will you remotely release the marble from the cup?
  • When do you need to launch the marble so that it will hit the target?


  1. First, set up a zip line. Tie 6 feet (1.8 m) of the smooth line to two objects (e.g., two chairs or a table and chair). Make sure it's stretched tight and that one end is about 20 inches (50 cm) below the other.
  2. Next, figure out how to modify the cup to carry the marble down the zip line. Will it travel inside the cup? Outside the cup on a platform? Underneath?
  3. Then, add a remote release. Decide how you will tip the cup at just the right instant to launch the marble toward the target.
  4. Finally, clip the cup to the zip line. Figure out how to hook the cup onto the zip line so it slides easily.

Test, Evaluate, and Redesign

Ready for a test run? Place the target near the end of the zip line. Send down the cup and try to hit the target with the marble, using the remote release. How close did you get? See a way to improve your design? Engineers improve their designs by testing them. The steps they follow are called the design process. Try your idea and build an improved version. For example, if your cup:

  • goes slowly—Check that the zip line is steep enough. Also, make sure the cup slides freely.
  • can't keep the marble in—Roll a small tube of tape to keep the marble from falling out accidentally. Also, adjust the tilt of the cup so it doesn't tip the marble out.
  • doesn't let the marble out—Roll small tubes of tape and build a chute to funnel the marble toward the opening. If necessary, adjust the tilt of the cup so the marble can roll out more easily.
  • misses the target—Since the marble is already moving forward along the zip line, it keeps moving forward as it falls. Make sure to take this forward motion into account as you choose a release point.