For a person swinging a ball on a string, is there any speed at which the string would be horizontal? If so, what is that speed? 3) Explain how hanging a mass on the apparatus is able to directly measure the centripetal force, even if the apparatus is not rotating at that time. 4) How does centripetal force vary with mass? 5) How does centripetal force vary with radius of revolution? 6) How does centripetal force vary with period of revolutions? 7) In the centripetal force apparatus, why is it important for the spring to be horizontal and the vertical string to be, in fact, vertical