I'm doing this without a teacher, so this isn't really a homework question, but this still seemed the most appropriate forum. I have the following question: A square board, of side 2a and mass M, is to be used to estimate the speed of bullets. It is freely hinged about one horizontal edge and hangs at rest in a vertical plane. A bullet of mass m, travelling horizontally with speed V hits the borad at its centre and becomes embedded in it. The board then rotates through an angle x before coming to rest. i) Show that the initial angular speed of the board is 3mv/(4M+3m)a My attempt: Using Conservation of Energy: 1/2mV^2 = 1/2Iw^2 (w = angular speed) =1/2(4/3Ma^2 + ma^2)w^2 w^2 = (3mV^2)/(4M+3m)a^2 This almost gives the solution, but with some 'squarings' that I don't want. Any idea where I've gone wrong? Thanks!