Yo-yos with Yo-yos

The intrepid AP Physics class had some fun with yo-yos this week, figuring out why it rolls one way when you pull on the string one direction and the other way when you pull the string in another direction, and the angle at which it'll just slide.  Even cooler: it doesn't depend on the coefficient of friction of the surface!

We're grateful to Rhett over at Dot Physics for this video.

They also figured out from what height that they could drop a yo-yo (while holding the string vertically) and have it hit the ground at the same time as a falling ball.  They did this for two idealized shapes of yo-yos (which wouldn't make good toys): a hoop and a uniform cylinder.  Their predictions were calculated as a fraction of the height of the ball, and they drew the locations on the board, set up their objects aligned with the drawings, and dropped!

The predictions were awesome; Kyle's timing was less so! :)  Video linked to the picture!