Circles

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# Circles - PowerPoint PPT Presentation

Circles. Pi = . 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 14 th day March 14 th

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## PowerPoint Slideshow about 'Circles' - josiah

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Presentation Transcript

### Circles

Pi =
• 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 Circle
• 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
C =πd
• 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

Try these;
• 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
Working backwards
• 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

Now try these;
• 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
A =πr2
• 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

Try these;
• 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
Working backwards
• 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

Now try these;
• Find the radius if the area is
• 34 cm2
• 200mm2
• Find the circumference for both circles