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Define the focus and the eccentricity of an ellipse?
A circle is perfectly round, and has one center. An ellipse is like a circle with TWO "centers", and each "center" is called a "focus". The plural of "focus" is "foci". Take a piece of string and tie a loop in each end. Put a pin through the loops, and hold it still in the center of the circle. Place the tip of your pencil at the center of the string, and you can draw a circle by keeping the string taut. Now take TWO pins, and put one pin at each end of the string; place the pins at some short distance apart, and hold them there. Place your pencil and draw, and the shape you draw will be an ellipse. The two pinpoints are the focuses, or foci, of the ellipse. Eccentricity is a measure of how far the ellipse varies from a circle. An ellipse with an eccentricity of zero _IS_ a circle, while an eccentricity of 1.0 is a straight line, with that string stretched out straight. In astronomy, every natural orbit is an ellipse.
How do you draw a circle with a pencil and ruler?
Use the ruler to measure the size of the circle you want - this will be a line all the way across the whole circle. Then, measure a piece of string with the ruler -- make this half the size of the original line. Use a thumbtack to tack the string in the spot you want the middle of your circle. Use a bit of tape to tape the other end onto your pencil (don't wrap it around or you will end up with a smaller circle!) Gently move the pencil around the thumbtack to make your circle.
How could you use a piece of string thumbtack and a pencil to construct a circle with radius AB?
Loop the string around the thumbtack and the pencil so that the distance from the tack to the pencil point is AB units. Fix the thumbtack to where you want the centre of the circle. Using the pencil, and with the string taut, mark its positions. These will lie on a circle with radius AB.
How long does a piece of string need to be to make a circle that is 32 inches in diameter?
pi*32 = 100.53 inches (approx)
How can one draw a perfect ellipse?
One can create a perfect ellipse using two pins on a piece of string and a pencil. Place the pins at opposite sides and then loop the string around them. Finally place the pencil in the loops and pull it around the pins to draw the ellipse.
