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Science project idea

Spinning paper-cup curve: the Magnus effect behind swing and spin bowling

A spinning object moving through the air curves sideways. A paper cup launched with a rubber band shows a curved flight, as a spinning cricket or football does.

Make it yourself8 numbered steps, the materials and the science, all free.Go to the stepsGet a ready-made kitReady-made models, kits and spare parts in the store under Physics.Browse PhysicsAsk on WhatsAppAsk the head office whether a model or kit for this idea is available.Send a message

What it shows

A spinning object moving through the air curves sideways. A paper cup launched with a rubber band shows a curved flight, as a spinning cricket or football does.

The science

The spinning surface drags a layer of air around with it. On one side the spin moves with the airflow and on the other side against it, so the air speeds are different. Faster air has lower pressure, so the ball is pushed toward the low pressure side. This sideways force is the Magnus force.

Materials

Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.

  • Two paper cups of the same size taped open ends together
  • A strong rubber band
  • A long string for a ruler
  • A tape measure
  • A phone to film overhead or from the side
  • A sheet of chalk marks on the floor

Safety

For this project: Launch in an open space away from people and put on eye protection. An adult must supervise.

Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.

All the safety rules

Step by step

8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.

  1. Tape the open ends of two paper cups together to make a spool-shaped body.

  2. Wind the rubber band around the middle, leaving a free tail of about 10 cm.

  3. Hold the free end and throw the spool forward so the band unwinds and spins the cup.

  4. Mark the start point and film from above as it flies.

  5. Note the sideways direction of the curve and measure its sideways drift.

  6. Throw with the band on the opposite side, so the spin reverses, and compare the drift.

  7. Repeat each direction five times and record the average drift.

  8. Draw a diagram with arrows showing the spin, the airflow and the sideways force.

What you should see

The cup curves to one side, and reversing the spin reverses the curve. The size of the curve depends on how fast it spins.

Viva questions

A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.

1What is the Magnus effect?

A sideways force on a spinning object moving through a fluid.

2Where is it used in sport?

Spin bowling, football free kicks and topspin in tennis.

3Why does reversing the spin reverse the curve?

The pressure difference now acts on the opposite side.

4What does faster air mean for pressure?

It means lower pressure, as in Bernoulli’s principle.

Free ebook: Viva Questions for Science Projects

Take it further

Count the number of spins and relate it to the sideways drift.

Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.

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