Played with a metalring with a string rolled around it today - i.e. i played with an oversized yoyo. I had some trouble understanding the physics behind it all though. We assume that yoyo would roll giving us enough information to solve the equations of motions which gave us that the force of the string would be half the force of gravity during the phase, where the yoyo unrolled. That was all good, I have trouble however understanding how the string "knows" how to only exert an upwards force of ½mg!! Let me elaborate: Let's assume that the string is not stretched to start off with. The ring will then be acted on by the force of gravity only and accelerate down. But then after having fallen the necessary height for the string to get tight the ring will exert a downwards force on the string making the string exert an upwards force on the ring. But how does this become ½mg. I can see if from the equations but it seems like a mystery to me how it suddenly knows only to exert a force of ½mg when the reaction that causes this is actually a downwards force mg. I hope that made sense - probably didn't and if so please tell me where to elaborate, because this really pains me.