Kepler's third law states that the square of the period of any planet is proportional to the cube of its orbital radius.To derive it, two equations are required: F=GMm/r^2 and F = mv^2/r. These are Newtons law of gravity and the centripetal force for an object moving in circular motion respectively.By equating the two you can see that GMm/r^2 = mv^2/r. the m's and r's (mass and radius) then cancel to give: GM/r = v^2.v^2 can be shown to be equal to (2*pi*r/T)^2, which I would show in the video and this can be substituted in to finally show that: T^2 = 4*pi^2*r^3/GM. I can then expand on this question by asking the student to find the period of a planet with mass M and radius r.