a strange, curvy cylinder thing made of skewers and sponges

This curvy shape is made from straight lines!

Skewers are straight and pretty tough. But put just the right twist on them, and you can make something surprisingly curvy!

You will need

  • Sponge cloth – ours was about 0.5 centimetres (cm) thick and about 20 cm wide
  • 16 bamboo skewers
  • Marker
  • Mug or other circular object to trace
  • Ruler (optional)

What to do

  1. someone tracing the outline of a mug on some spongeTrace 2 circles onto the sponge cloth.
  2. someone is cutting the circle out of the spongeLeave a gap around each circle and cut them out.
  3. someone is ruling a line across the circleDraw a vertical line through the centre of one circle, cutting it in half.
  4. someone is ruling a line across the circle at right angles to the firstDraw a horizontal line through the circle, dividing it into 4 equal quarters.
  5. a circle drawn on sponge is divided into 8 segmentsNow divide each quarter in half to make 8 equal pizza slice segments.
  6. a circle drawn on sponge is divided into 16 segmentsFinally, divide each of these slices in half once again. You will end up with 16 equal segments.
  7. someone is numbering the segments around the circleStarting at the top of the circle, number each point where a straight line meets the circle.
  8. there are 2 sponge circles with numbered segmentsRepeat steps 3–7 on the other circle.
  9. someone is pointing a skewer at the 1 written on the spongeLine up the point of a skewer with point 1 on one of the circles.
  10. a skewer is poking through a sponge. There are arrows suggesting the skewer needs to be spun.Roll the skewer between your fingers to make it spin a bit like a drill and gently poke it through the sponge cloth. Don’t force it! Keep going until most of the skewer has poked through.
  11. the skewer goes through 2 spongesTake your second circle, and do the same thing, gently drilling through the point marked 1.
  12. there are 2 sponge circles hanging one on each end of the skewer.Adjust the 2 sponge circles so they are about 3 cm from each end of the skewer.
  13. there are 2 skewers close together, and both go through the same 2 sponge circlesTake a new skewer and gently drill it through point 2 on the first circle, then the second circle. The 2 skewers should be parallel.
  14. A whole cylinder of skewers has been pushed through 2 spongesRepeat this process for each number around your circles. You will end up with a cylinder shape made of skewers.
  15. the cylinder of skewers has been twisted and is now narrow in the middle and flares out at each end.Hold one sponge in each hand and twist them in different directions. What shape do the skewers make?


What’s happening?

You may be surprised to make a curvy shape out of straight skewers. But such shapes are not as rare as you might think.

A 3D shape that can be made out of straight lines is called a ruled surface. There are many different types of ruled surface, including cylinders and helixes – like the underside of a spiral staircase.

Centrepoint tower from below. the cables make a curved shape

Sydney Tower has a hyperboloid shape

Image: ©

The shape in this activity is called a hyperboloid. It is a doubly ruled surface, because there are 2 ways of making a hyperboloid out of straight lines. You could twist the lines clockwise, or you could twist them anticlockwise. Sydney Tower, or Centrepoint, is supported by cables that make a hyperboloid shape. These cables run both clockwise and anticlockwise – showing the doubly ruled surface.

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