• 1

    Here are some of the materials you can use

    • cardboard (from a cereal box or back of a notepad) 
    • paper clips
    • Ping-Pong ball
    • 4 plastic straws or skewers
    • scissors
    • single-hole punch 
    • 2–4 small paper cups (3-oz. [89 ml])
    • smooth line (4 ft. [1.2 m]) (fishing line or unwaxed dental floss) 
    • tape (duct or masking) 
    • weights (10 pennies or 5 flat steel washers [1-in. (2.5 cm)])
  • 2

    Prepare ahead of time

    • Have paper and a pencil ready to write down ideas and sketches as you design.
    • Set up a zip line. Run a 4-foot (1.2 m) length of fishing line between two objects, such as the back of a chair and a stack of books on the floor. Be sure the zip line is about 2 feet (0.6 m) higher at one end than the other.
    • Think about things that move along a zip line. (People, containers, heavy materials)
    • TIP: A zip line is a cable that starts at a higher point and ends at a lower point.
  • 3

    Think about the challenge

    • Why do people use zip lines? (To move between steep points, to carry supplies across dangerous areas, for fun)
    • How will the ball carrier stay on the line and not fall off as it goes from top to bottom?
    • How will you make the carrier travel quickly down the zip line?
    • TIP: When two things rub together, it causes friction, which is the force that resists motion. You will need to find ways to reduce the friction so your ball carrier can speed up.
  • 4

    Think about and write down your ideas

    • How will you keep the ball inside the carrier as it speeds down the zip line? 
    • How will you help the carrier stay balanced on the line? 
    • TIP: You can use weights to balance your carrier. The weights will pull the carrier down and keep it firmly on the line.
    • TIP: Inventors’ and engineers’ first ideas rarely solve a problem. They brainstorm ideas, try different ideas, learn from mistakes, and try again––this is part of the design process.
  • 5

    Look at the materials

    • What materials will you use to make the ball carrier?
    • How will you attach your carrier to the zip line so it’s easy to put on and take off? 
    • What will you use to weigh your carrier down?
    • What materials will you use to be in contact with the zip line so the carrier slides quickly?
    • TIP: If there is too little weight holding the carrier down, it can be hard to keep the carrier balanced once the ball is inside. Try adding weight a little at a time.
  • 6

    Design and build your carrier

    • Decide what materials you will use to build the ball carrier. 
    • Build the carrier.
    • TIP: If your carrier is not big enough to hold the ball, the ball will keep dropping out. Try modifying your carrier to hold the ball in place.
  • 7

    Design and build your carrier (continued)

    • Think about how your carrier will travel down the zip line.
    • Choose the materials that will allow it to slide smoothly on the line.
    • Attach them to the carrier.
    • TIP: if you need to reduce friction to allow your carrier to travel smoothly, try making the part of the carrier touching the line as slippery as possible by using a smooth, hard material like plastic. Or adjust how hard the carrier will press on the zip line by designing it to hook on the zip line at two or more points.
  • 8

    Weigh down your carrier 

    • Decide how you will weigh down the carrier so it can stay balanced on the line.
    • Add the weights.
    • TIP: If your carrier doesn’t balance well, try placing the weights so they are below the zip line. Doing this will change the carrier’s center of gravity, the point within an object where all parts are in balance with one another. Or, try changing the number or position of the weights. 
    • TIP: If the ball carrier is not stable, try adding the same amount of weight on each side. If the weights on each side are balanced, it will help keep the carrier stable.
  • 9

    Attach the carrier to the zip line

    • Put the ball in the carrier.
    • Place the carrier at the top of the zip line.
  • 10

    Let it zip 

    • Remember you have four seconds to get your carrier from the top to the bottom.
    • Let it go and start counting!
    • TIP: If your carrier stops partway down, check to see that nothing is blocking your carrier where it touches the line.
    • TIP: If it takes longer than 4 seconds for your carrier to travel the length of the zip line, try to reduce the friction and speed up the carrier by experimenting with different materials where the carrier comes in contact with the line. 
    • TIP: If the zip line sags, try checking the tension of the line and tighten if needed. If it still sags, your carrier may be too heavy. You may need to revise your design.
  • 11

    Did you know?

    • Real-Life Superheroes
      Have you ever dreamed of zipping up the side of a building like Batman or Spiderman? Engineer Nate Ball, host of Design Squad, and his friends made it possible. For a contest, they designed and built a climbing device that could carry a person 50 feet (15 m) up the side of a building in less than 5 seconds. After months of work, the team tested their climber by lifting a 150-pound (68-kg) load of tires—and the climber exploded and crashed! But after analyzing and redesigning the ruined climber, they ended up winning third prize in the contest. Ultimately, they patented the climber and started a company to sell it. Today, soldiers, firefighters, and rescue workers around the world use the team’s climber to fly up buildings. Now, those are real superheroes.
  • 12

    Try this next!

    • Slow down! Build a ball carrier that takes 10 seconds to travel the length of the zip line.
    • On your mark, get set, go! Set up two zip lines side-by-side and race different ball carriers.
    • Increase the load. Build a ball carrier that can carry several Ping-Pong balls down the zip line.