Geometry - part circle inside another circle?

A farmer attaches a rope to a goats neck and the other end to a post situated on the circumference of a circular field. How long must the rope be such that the goat is only able to eat half the grass in the field? Give your answer as a multiple of the radius (r) of the field and keep your answer exact (no decimal equivalents)Most of you are not reading the question - it is tied to the circumference of the circular field, draw it out if unsure

Why would a farmer do that anyway