Brief Description:

Students will use gear ratios to design a tug-o-war machine

Grades: 5, 6, 7, 8

Time: 1-2 hours

Keywords: gears, gear ratios, NXT robotics, cars, torque, speed

Lesson Objectives

  • Students will use their knowledge of gears ratios to determine whether “gearing up” or “gearing down” is better for tug-o-war


  • NXT robotics kits or Simple Machines kits
  • String
  • Scissors
  • Tape


  1. Review gear ratios. Define gearing up and gearing down.
    1. Gearing up means that for every revolution of the drive gear the follower gear turns more than one revolution. This is useful when you want to make something move faster and give less power to whatever is attached to the follower gear.
    2. Gearing down means that for every revolution of the drive gear, the follower gear turns less than one revolution. This is useful for when you want to give more power to whatever is attached to the follower gear but it moves slower.
    3. If the drive gear is bigger than the driven gear, the car will be geared for speed. If the other way around, it will be geared for torque.
  2. In this challenge students will be playing tug-o-war by building a machine that can provide enough torque to pull on another machine doing the same thing. String will be tied between the two, and a machine will have to pull the other over a line of tape in order to win.
    1. Their machine must use gears
    2. Their machine should run on simple NXT on-brick programming
  3. Allow the class to brainstorm different ideas for their machines. Have them plan out and draw their design before giving them materials to build.
  4. Distribute materials and have students start building. You may have to assist students with tying string to their machine.
  5. After the students finish building and programming, pair up machines and tie them to either end of a string, making sure the middle of the string is right above the tape on the ground, and that both machines are equidistant from the tape.
  6. Have the students start their machines. Whoever’s machine gets pulled over the line first, loses.
  7. After the students finish, review the activity with the class. Have them share their ideas, ask groups to explain what the hardest part of the challenge was, etc.

Extensions and Modifications:

  • Have students use more than 2 gears
  • Create a class-wide Tug-o-War bracket so they can compete to see whose machine was the “strongest”

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