A common demonstration, illustrated in Figure P10.72, consists of a ball resting at one end of a uniform board of length l, hinged at the other end, and elevated at an angle θ. A light cup is attached to the board at rc so that it will catch the ball when the support stick is suddenly removed.
(a) Show that the ball will lag behind the falling board when θ is less than 35.3°.
(b) If the board is 1.00 m long and is supported at this limiting angle, show that the cup must be 18.4 cm from the moving end.

Cup Support stick Hinged end Figure P10.72