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Circular pasture

  1. Nov 5, 2009 #1
    My maths teacher recently gave me a weird question which really got my attention, but I have not managed to solve it so far. It can't exactly be described as homework, although it would give me a huge bonus if I managed to hand in the results.

    There is a pasture in the shape of a circle. A cow is tied to a point on the circle. How long does the rope have to be, so the cow manages to pasture exactly 1/2 of the grassland?

    I'm looking forward to everybody's opinions, and thanks in advance!

  2. jcsd
  3. Nov 5, 2009 #2
    To start, assume the pasture has radius R, then the area of the pasture is [tex]\pi R^2[/tex].

    It may help to assume that the center of the pasture lies on the positive x-axis, and that edge of the pasture passes through the origin.

    Now, assume that the cow's rope has radius C and is tied to the origin. This is the center of a second circle having area [tex]\pi C^2[/tex].

    You are looking for the intersection of these two circles.

    Note that you can assume C>R if it is to cover half of the pasture.
  4. Nov 5, 2009 #3
    Also, you know that [tex]C < R\sqrt{2}[/tex], since having [tex]C = R\sqrt{2}[/tex] would mean that circle C (the cow's circle) would intersect circle R (the pasture circle) at (R,R) and (R,-R). This intersection is obviously greater than 1/2 the area of circle R.

    So now we have [tex]R < C < R\sqrt{2}[/tex].
  5. Nov 5, 2009 #4
    Assume the intersecting points between the two circles are A (at the top) and B (at the bottom). Line segment AB is a chord of both circles, and thus we have 2 circle segments.

    This should give you enough information to get you on the right track...
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