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World of Tan 19 - A Circular Problem

Age 7 to 11
Challenge Level

This activity follows on from World of Tan 18 - Soup.

It is almost the end of the tea break. The cups have been cleared away and the chairs are being stacked neatly in the corner, when Wu Ming poses his question...

Wu Ming: How do you find the centre of a circle?

After some discussion, it was agreed that all you had to do was fold it in half and then in half again. Where the folds crossed was the centre!

Chi Wing: Yes, that works for a circle of paper.

Wai Ping: It'll also work for a tablecloth, or even a small rug!

Chi Wing: But what about that wooden stage we have to move next week? We can't fold that!

Wu Ming: Let's ask Granma T - I've heard she enjoyed geometry at school. I'll leave a note asking for her help.

The workers went back to their tasks for the afternoon, and the next morning they were called into the office by Granma T...

Granma T: Imagine that you can walk round the edge of the circle - of any size, large or small.

Everyone seemed to understand this instruction.

Granma T: Now imagine that Mah Ling is standing still on the edge, and she is feeding out some rope attached to your waist. As you walk around the circle, the rope is kept tight at all times.

Chi Wing: The rope will get longer as I walk further around.

Wu Ming: Yes, until you get to that point when the rope starts to get shorter again.

Granma T: That is when you have gone as far out as possible and you are about to begin the journey back to where you started.

Wu Ming, Wai Ping and Chi Wing all voiced their agreement.

Granma T: Now, where is the point when the rope starts to get shorter again?

Wai Ping: At the end of the longest line across the circle - the diameter.

Granma T: Your problem about finding the centre should be easy now. Good morning!

And off she went, leaving the workers scratching their heads in confusion.

Chi Wing: Well, let's get back to furniture moving. We can all have a think and then discuss this at our tea break.

In the meantime, complete the silhouette of the teacup.

Extra activity:

  • Have a look at Granma T's method of finding the centre of a circle. How does it work? Can you finish it? 

The story continues in World of Tan 20 - Fractions.