Math Mission: Find the Secret of the Circle

Want a great activity that uses materials around your home? One that teaches math vocabulary? Introduces advanced math concepts simply? And is hands-on?

If you answered yes to any of those questions, this activity is a great one for your child. My third-grade son loved it!

What you need:

A ruler and/or a tape measure

pencil

paper

calculator

string

scissors

Circular objects (we used four)

This activity teaches:

• The difference between circumference (measurement AROUND the circle) and diameter (measurement THROUGH the exact center of the circle) .
• That the circumference of a circle is always in the same ratio to its diameter.
• How no matter the size of the circle, the diameter is always approximately 3 times smaller than the circumference.
• Math vocabulary (e.g. ratio, diameter, circumference).

Skills used: measuring, translating fractions into decimals, deductive reasoning, and division.

Before my son got home from school, I raided the kitchen cabinets and grabbed a glass, bowl small plastic food storage container, and a saucer. All of these objects were round and their shape could easily be traced onto paper.

After he dropped his backpack at the door and was sufficiently nourished with a tall glass  of milk and a homemade cookie, he got to work. One by one, he traced the shapes on blank pieces of paper. NOTE: all the objects I chose were small enough to trace onto standard office paper.

Next, he measured the diameter of each circle. To find the exact center, we folded the paper (making sure the edges of the circle lined up). Then he used yarn (I'd recommended a string that doesn't stretch; our yarn may have skewed some of the results slightly) to wrap around the base of each object. The yarn was the same size as the circles he'd drawn. Once it was measured, the circumference of each object recorded.

Now came the fun part, he divided the circumference by the diameter for each circle using a calculator. (This required figuring out what the decimal for 3/8, 5/8, 3/4, and 7/8 were.)

All of our answers were 3, plus a little. My son was able to deduce that no matter what the size of the circle, the ratio of the diameter to the circumference was basically the same. And THAT'S the secret of the circle!

If your measurements are precise enough, the number will  be PI (3.141592635). Even with our stretchy yarn, we ended up with 3.0163, 3.2, 3.11, and 3.1304. Not bad, I'd say!

This great activity came from Math Wizardry for Kids by Margaret Kenda and Phyllis S. Williams.