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Circles

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Circles

- Pi (π) is a Greek letter
- It is a number that goes on forever
- It is the ratio of a circles circumference to its diameter
- This number is shortened to 3.14
- It has it’s own button on the calculator
- Pi day is on the 3 month and 14th day
- March 14th
- It is extremely important to Physics and engineering

- The circumference is the whole way around the circle
- The diameter is the width of the circle going through the centre
- The radius is halfthe width

- I f you need to find the circumference of a circle you multiply the diameter by Pi
- E.g.
- Find the circumference of this circle (to 1 d.p.)
- C = π x 7
- C = 22 cm

7cm

- Find the circumference if:
- The diameter is 12.6 cm?
- The radius is 4.3 cm?
- The radius is 1.6 cm?
- Find the outer circumference of this roll of sticky tape:

3.4

1.7cm

- 39.6 cm
- 27.0 cm
- 10.1 cm
- 21.4 cm

- If C =πd,then we can work out the diameter if we are given the circumference
- E.g.
C =πd

52= πd

We need to divide 52 by π

D= 16.6 cm

Circumference= 52 cm

- Find the diameter if the circumference is;
- 85 cm
- 12.6 cm
- 25.3 cm
- Find the radius if the circumference is;
- 200m
- 400m

- 27.1 cm
- 4 cm
- 8 cm
- 31.8 m
- 63.7m

- Like most areas you need to multiply two sides together, this is where the r2 comes in, but for a circle you have to multiply this by pi
- E.g.
- Find the area of this circle:
- A =πr2
- A =π x 7.42
- A = 54.8 xπ
- A = 172.2cm2

7.4cm

- Find the area if;
- The diameter is 25.2 cm?
- The diameter is 10 yards?
- A CD is made from plastic. Find the area of plastic in this CD;

12 cm

1.5cm

- 498.8 cm2
- 78.5 yards2
- 111.3cm2

- If A =πr2 then you can find the radius if you have the area
- E.g.
- 500 = πr2
- Divide 500 byπ to find out what r2 is
- 159.2 = r2
- This is not the radius, but the radius squared, so we need to square root our answer
- R = 12.6 cm

Area = 500 cm2

- Find the radius if the area is
- 34 cm2
- 200mm2
- Find the circumference for both circles

- 3.3 cm
- 8 mm
- 20.7 cm
- 50.3 cm