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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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Presentation Transcript
slide2
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 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
slide5
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
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

answers
Answers
  • 39.6 cm
  • 27.0 cm
  • 10.1 cm
  • 21.4 cm
working backwards
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
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
answers1
Answers
  • 27.1 cm
  • 4 cm
  • 8 cm
  • 31.8 m
  • 63.7m
a r 2
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 these1
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

answers2
Answers
  • 498.8 cm2
  • 78.5 yards2
  • 111.3cm2
working backwards1
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 these1
Now try these;
  • Find the radius if the area is
  • 34 cm2
  • 200mm2
  • Find the circumference for both circles
answers3
Answers
  • 3.3 cm
  • 8 mm
  • 20.7 cm
  • 50.3 cm
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