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Build yours at home

Gears you can count

What we're doing

Pin two gears to cardboard so their teeth mesh. Mark a tooth on each and count the turns. Then add a string and find which way lifts a cup of coins more easily.

About 30 minutes.

Goes with these lessons

Get these parts

  • Two gears, one with twice as many teeth as the other (LEGO 12- and 24-tooth, or 3D-printed) (building kit or 3D printer)
  • Stiff cardboard and two split pins (brass paper fasteners) (household)
  • A marker, string, a paper cup and some coins (household)
  • A bottle cap, and strong tape or glue (household)
  • Tape (household)

Keep it safe

A grown-up pushes in the pins, does the gluing and checks the gears spin freely. Keep fingers out of meshing teeth.

Steps

  1. 1
    Meshing gears

    A grown-up pushes a split pin through each gear's centre into the cardboard, so the teeth mesh and both gears spin freely.

  2. 2

    Mark one tooth on each gear. Turn the big gear slowly. The two gears turn opposite ways.

  3. 3
    Gear swap

    Turn the big gear once, watching its marked tooth. Count the small gear's turns: two.

  4. 4

    Now turn the small gear once. The big gear turns only half way.

  5. 5
    Gears that lift

    A grown-up glues the bottle cap onto the big gear's face, over its pin, as a drum. Tie the string to the cap and the cup to the other end.

  6. 6

    Tape the cardboard upright at the edge of a table so the cup hangs down. Add coins. Turn the big gear by hand to lift the cup. Lower it, then turn the small gear instead. That way is slower, but easier.

  7. 7

    Untie the cup and put the coins away.

Talk while you build

  • How many turns does the small gear make?
  • Which way helps you lift the cup?

Spot it at home

  • An egg beater, a bike's chain and gears, a hand can opener, a wind-up toy.

Find a gear in the kitchen. Which way does each part turn?

  • An egg beater: one turn of the big handle wheel spins the beaters several times. A bike: one pedal turn spins the back wheel more than once.

One handle turn: how many beater turns?

  • A bike's easy gear on a hill, and a hand winch on a boat trailer: lots of easy turns lift something heavy, slowly.

Why does an easy gear make you pedal fast but go slowly up a hill?

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