Hi, I just don't really understand rotational motion very well, and I don't know how to proceed with this problem. 1. The problem statement, all variables and given/known data You throw a 235 g ball 26 m/s at the apparatus shown in the figure. http://volta.byu.edu/ph121/homework/hw18f3.png [Broken] The apparatus catches the ball, causing it to rotate. There is a kinetic frictional force on the bearing of the ball of 17 Newtons. The bearing has a radius of a=2.5 cm and the apparatus has a distance of r=1.2 m between the catcher and the axis of rotation. The catcher has a mass of 408 g and the rest of the aparatus has negligible mass. How many revolutions does the apparatus rotate before it stops? 2. Relevant equations L = mvr [tex]\Sigma[/tex][tex]\tau[/tex] = I[tex]\alpha[/tex] [tex]\omega[/tex]=v/r 3. The attempt at a solution Here is what I have tried so far... First, I need to find the velocity after the ball and the apparatus collide. So, mball*vball = (mball + mcatcher)*vf .235 * 26 = (.235 + .408)*vf solving for vf yields 9.5 m/s then I can find the angular velocity using [tex]\omega[/tex] = vf/r 9.5/2.5 = 7.92 rad/s I can also get the angular momentum from this. My problem is, is that I have no idea how to factor gravity into the whole equation. I figure if I could find [tex]\alpha[/tex], then I could use a rotational kinematic equation to find the change in [tex]\vartheta[/tex]. My other thought was that I could somehow use the friction and rotational kinetic energy to find when it stops. Any help would be greatly appreciated.