Lesson · Grade 7 · Mathematics
Radius, diameter and circumference
- Grade 7
- Mathematics
- 1 min 17 s
- British Columbia
- BC7-MA-46
A circle with a 5 cm radius shows that the radius goes from the centre to the edge and that two radii make the 10 cm diameter, then its circumference is rolled out into a straight line that fits just over three diameters, which is the number pi. Using circumference = pi × diameter, it works out 3.14 × 10 to find that the circumference is about 31.4 cm.
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In one sentence
The question this lesson answers
How far is it all the way around this circle?
The radius is a straight line from the centre of the circle to its edge.
Transcript
What the lesson says
How far is it all the way around this circle?
The radius is a straight line from the centre of the circle to its edge.
This circle has a radius of 5 centimetres.
The diameter goes straight across the circle, through the centre.
It is two radii long, so the diameter is twice the radius.
2 times 5 centimetres is 10 centimetres.
The circumference is the distance all the way around the circle.
Imagine cutting the circle open and unrolling it into a straight line.
How many diameters fit along this line?
Just over three diameters fit, and that is true for every circle.
This number is called pi, and it is about 3.14.
So the circumference equals pi times the diameter.
For our circle, that is 3.14 times 10 centimetres.
So the circumference is about 31.4 centimetres.
Remember, double the radius to get the diameter, then multiply by pi to get the circumference.
Curriculum
Where this fits
- British Columbia, Grade 7
Mathematics, Geometry and measurement: circumference and area of circlesBC7-MA-46
The outcomes and strands follow the British Columbia curriculum.
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