  move from P to Q, and hence the force varies as QR/t2. But the time is proportional to PQ divided by the velocity v, and in the limit as Q approaches P, PQ approaches PR, so that t2 becomes equal to PR2/v2. Proposition 36 of Book 3 of Euclid entails that in this limit PR2 is equal to the product of QR and twice the radius SP, and hence the force for uniform motion in a circle varies as v2/SP or  v2/r.[35]