Grade Level: 4th Skill: Shapes and Geometry Topic: Radius and Diameter of a Circle Goal: Identify the radius and diameter of a circle Skill Description: Identify and calculate the radius (half point of a circle) and diameter (line that runs through he center of the circle) of a circle. Estimate the circumference (distance around a circle).

## Building Blocks/Prerequisites

### Sample Problems

 (1) Identify the line segment shown: ________ (chord) (2) Identify the line segment shown:__________ (radius) (3) Identify the line segment shown: ___________ (diameter) (4) Estimate the circumference of the circle: ______________ (about 21cm) (5) Use the interactive ruler to measure the diameter of the circle: (Measurement depends on type of ruler)

### Learning Tips

 (1) Math Vocabulary: Center point- the point from which every point on a circle is exactly the same distance. Radius- a line segment that connects the center of a circle with a point on the circle Chord- a line segment that has its endpoints on the circle Diameter- a chord that passes through the center of a circle to the other side Compass- a tool used to construct circles Circumference- the measure of the distance around the circle (2) The rule to estimate the circumference: Explain to the child that to measure the circumference the formula is: Circumference = 3.14 x diameter. To estimate the circumference they can multiply (3 x diameter). In fourth grade they are only required to estimate the circumference. For example: The diameter of the circle is 7 cm. Therefore, circumference = 3 x 7. Circumference = 21 cm If the radius is given, you can find the circumference by multiplying radius by 6. For example: The radius of the circle is 2 cm. Therefore, circumference = 6 x 2. Circumference = 12 cm The rule to calculate radius and diameter: Diameter = (2 x radius) Radius = (diameter ÷ 2) (3) Review how to measure lines (or items) using a ruler: Most kids begin the measurement at 1 on the ruler. Remind them to start at the tip of the ruler. Practice measuring various lines and remember to write the unit (cm, m, in. etc.) at the end. (4) Measure the circumference of circular objects in the house (i.e. cans, jars, cups, etc.) You can use a string to wrap around the objects, cut the string where it meets around the object. Then, measure the length of the string. You can also compare the circumference of the objects. Which circumference do you think is larger? After measuring, explain the results. (5) Practice drawing circles using a compass: Introduce how to use the compass with your child. Explain that the sharp end of the compass will be the center point of the circle. Practice with small-large circles and label parts of the circle (chord, center point, radius, circumference, and diameter).