# Circles - PowerPoint PPT Presentation

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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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Circles

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## 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