Radius The distance from the center of a circle to any point on the circle. M A point on the circle Center

Diameter AB The distance across a circle through the center. REMEMBER: The diameter is TWICE the radius d = 2r A B A circle Center

The distance around a circle is called its circumference. Centre A circle A

Each group will receive a ruler, string, and worksheet. In your groups measure the circumference and diameter of the 5 objects you choose. Everyone will record the measurements in the chart on their worksheet. Take turns measuring. Everyone must measure at least one item. Measure the circumference of each object by wrapping the string around the object and marking with your fingers. Then, measure the string with your ruler. Try to be as accurate as possible. In the last column, do the following calculation: Always round to the nearest hundredth. Activity Directions

What did you find out? Look at your answers in the last column. Do you get the same number every time? Have you seen this number before? If so, is there a symbol we can use to represent it? Why might you get a different number each time? Using this information, try to write a formula for the circumference of a circle.

When you take the circumference divided by the diameter you should always get the same number (3.14) is an approximation for π. π is considered an irrational number, so it never ends. We can use this ratio to find the formula for the circumference of a circle. What Did You Find Out? diameter

Circumference Formulas C = π ∙ d C = 2∙ π ∙ r When the diameter is given, use the following formula: When the radius is given, use the following formula:

Example C = πd C = (3.14)(0.5) C =1.57 miles Julia wants to find the distance around the circular track at her school. She knows it has a diameter of 0.5 miles. What is the circumference of the track? REMEMBER: We use 3.14 for π. Always round to the nearest hundredth.

Example C = 2 πr C = (2)(3.14)(35) C =219.8 ft John wants to build a fence around his circular lawn. He knows the distance from the center to the edge of the lawn is 35 feet. How much fencing does he need to buy? He needs to buy 220 feet of fencing.