Two plasticene rolls one solid and one hollow

Can you guess which one will be faster?

Two tubes, both the same weight and the same size, and yet slightly different. Can you guess which one will win the rolling race?

sharp hazard iconSafety: This activity uses a sharp scalpel or scissors to cut a cardboard tube. Ask an adult to help.

You will need

  • Cardboard tube such as the inside of a paper towel roll
  • Plasticine
  • Flat board or large, flat sheet of corrugated cardboard
  • Some books
  • Scissors or scalpel
  • Ruler
  • Kitchen scales

What to do

  1. Cutting a cardboard roll with a scalpelCut 2 cylinders from the cardboard tube, each 2 centimetres (cm) long. Be careful and if you find it tricky, ask an adult to help.
  2. Two balls of plasticene and 2 cardboard rollsMake 2 balls of plasticine, each about half the diameter of the cylinder.
  3. Weighing the plasticene balls on a kitchen scales Weigh the balls using the kitchen scales, add and take away plasticine until the balls are the same weight.
  4. Plasticene disc inside a cardboard rollPut one ball of plasticine inside one of the cylinders. Flatten the plasticine into a disc so that it touches the inside of the tube. Make sure the plasticine is evenly distributed and that you don’t change the shape of the tube as you squish, it should stay circular.
  5. Plasticene roll lining a cardboard rollRoll the other ball of plasticine out into a long rectangle, about 2 cm wide and 12 cm long. Wrap the rectangle into a ring and fit it inside the second cylinder. Spread the plasticine to make sure it is even all the way around the inside rim of the cardboard, and the cardboard stays circular.
  6. Cardboard ramp leaning on some booksSet up a ramp by propping up one end of a board with some books.
  7. Plasticene rolls racing down the rampPut both cylinders at the top of the ramp, and get ready to start your rolling race. Let them go at the same time so they both roll down the ramp. Guess which one will roll faster? Was it the one you expected?


What’s happening?

Both cylinders in this activity are made of the same materials and should weigh the same. But the disc will beat the ring every time in this rolling race.

When the cylinders go down the ramp, they accelerate and start rotating (or rolling). Even though the 2 tubes weigh the same, one is easier to roll than the other. It is easier to make something rotate when its mass is close to the middle.

The open ring of plasticine has all its mass near the outside of the cylinder while the disc has some mass near the rim and some near the middle. It’s harder for the cylinder with the plasticine ring to get spinning. They both get the same push from the ramp, so the disc will end up going faster than the ring.

A similar effect can be seen on a merry-go-round or playground roundabout. If you move toward the middle while the merry-go-round is spinning, it will spin faster. Ice skaters spin faster when they bring their arms closer to their body.

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