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Harness a powerful simple machine to launch some tasty treats.
Safety: food safety hazard iconWhen dealing with food, use clean hands and clean equipment. Be careful with your catapult and make sure no one is standing where you’re aiming it.

You will need

  • Toilet paper roll (or a paper towel roll cut in half)
  • Masking or duct tape
  • Ruler
  • Plastic cap (like a milk cap)
  • Marshmallows (or cotton balls or balled up paper)

What to do

  1. Toilet roll taped to a flat surface.Tape the toilet paper roll to a flat surface, like a table or the floor. Make sure it’s secure and doesn’t roll.
  2. Ruler taped across a toilet roll which is taped to the flat surface.Tape the middle part of the ruler to the toilet paper roll. Again, make sure it’s secure and that the ruler doesn’t slip.
  3. A plastic milk bottle cap is taped to the end of the ruler.Tape the plastic cap to one end of the ruler. We used a loop of tape to do this.
  4. Marshmallow sitting in the cap taped to the end of the ruler.Load a marshmallow in the plastic cap.
  5. Hitting the opposite end of the ruler and launching the marshmallows.Launch the marshmallow by hitting the opposite end of the ruler! (You can even do this with the help of a plushie octopus friend.)
  6. Marshmallow in the cap attached to the shorter end of the ruler.Try changing the location of the ruler so the length of the marshmallow arm and the length of the opposite end vary. What position launches the marshmallow furthest?

What’s happening?

Catapults seem simple, but there’s some tricky science that makes them work. When you launch a marshmallow, you hit downwards – so why does the marshmallow go up?

The upwards force actually comes from the cardboard tube. As you push on the ruler, the tube gets compressed. It pushes back against the ruler, rotating it and launching the other end, along with the marshmallow, into the air.

Catapults are a type of lever, a simple machine that makes it easier to lift things. Levers are composed of two basic parts: a sturdy board that sits on a pivot point called a fulcrum. In our case, the ruler provides the board, and the toilet paper roll provides the fulcrum. Together, these two parts create leverage – also known as mechanical advantage – which changes the amount of force you need to lift things. Or in the case of your catapult, the force needed to fling a marshmallow!

Diagram of lever arm on fulcrum.

We can calculate the mechanical advantage by measuring how long the 2 sides of the lever are, starting from the cardboard tube. On the side you’re pushing, this is called the ‘effort arm’, and on the marshmallow side, it’s called the ‘load arm’. You can calculate how much this increases the force with the following equation:

Mechanical advantage = Effort arm ÷ Load arm
 

Moving the cardboard tube will make one side of your catapult longer and the other one shorter. So you can change the mechanical advantage of your catapult without lengthening or shortening your ruler!

Increasing the length of the effort arm increases the mechanical advantage. Does that make your catapult better? Maybe, but your lever doesn’t multiply force for free.

When the effort arm is long, you get lots of force, but your effort arm moves a long way to lift the marshmallow a short distance. If you go the other way and make the effort arm short, you can make the marshmallow move a long way – but with not much force. Marshmallows are very light – so do you need lots of force to fling them, or is it better to have a longer throwing arm? There’s only one way to find out!

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