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Rolling race

By David, 7 April 2015 Activity

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

Someone is cutting a section off a cardboard tube.

You may find cutting the tube is easier with a scalpel.

  • 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

What to do

A filled in circle of plasticine in one tube, and a ring in the second tube.

Put a ring of plasticine in one tube, and a disc in the other.

  1. Cut two, 2 cm long cylinders from the cardboard tube. Be careful and if you find it tricky, ask an adult to help.
  2. Make two balls of plasticine, each about half the diameter of the cylinder. Add and take away plasticine until the balls are the same size.
  3. Put 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.

    Someone is holding two plasticine filled tubes at the top of a slanty sheet of cardboard.

    Put both tubes at the top of a ramp.

  4. Roll 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.
  5. Set up a ramp by propping up one end of a board with some books.
  6. Put both cylinders at the top of the ramp, and get ready to start your rolling race.
  7. Let them go at the same time so they both roll down the ramp. Which one was faster? Was it the one you expected?

What’s happening?

Two tubes moving fast.

Which one will win the race?

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 two 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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