1. The problem statement, all variables and given/known data A spring has mass Ms=120g and, if unstreched, length l=65cm. A boy attaches a point mass m=500g to one end of the spring. He holds the spring at the other end and spins it horizontally around his head, such that the point mass moves with speed v=12ms-1 and the spring is streched by 4.5cm. The effect of gravity can be neglected. A: Determine the force constant of the spring and list all forces that act upon the point mass. B: Find the total energy of the spring and point mass. 2. Relevant equations arad=V2/r F=kx 3. The attempt at a solution Forces on point mass: Tension from spring. I want to say centrifugal force pointing outwards, but I have a feeling I remember being told not to call it that? Gravity is told to be neglected, its a point mass and I have no info on friction so assume that can be neglected. Attempt at force on spring: Moving at constant velocity, so no linear acceleration. arad=v2/r. For the body as a whole, I'd think the r used here would be for the center of mass, which would be (rcm, spring*Mspring+rpoint*Mpoint)/Mtotal, which is 62.8cm from the middle. (Note: Rounded here, stored on calc for precise value in later calculations). (1/Mmass)*Fspring=0.5*0.045*k=v2/rcm k=(v2/rcm)/(lextention/Mmass) V=12, rcm=0.628, lext=0.045, mmass=0.5 This gives over 2500 as the answer, which is obviously extremely wrong, and I'm really not sure what to do. For part B: Etot=Klinear+Krot+Uspring =0.5*Mtotal*12^2+0.5*I*ω2+0.5*k*lextension^2 EDIT: Although thinking through this a bit more, I'm not sure I need both the Kinetic Energy terms... If that is correct: Not entirely sure on the moment of inertia - I know it's integrate of r^2 dm, but would I treat it as a line with an extremely high density at one end, sum the I of the spring with something? So, I'm stuck. Anyone able to point me in the right direction?