<Mentor's note: moved from a technical forum and thus no template.> A pendulum consists of a bob of mass M and a rigid rod of length ℓ with negligible mass. The bob is struck by a bullet of mass m. The bullet passes through the bob and emerges with a speed half that of impact. The shot causes the pendulum to swing through a complete circle, but the bob only barely passes the top of the circle. Find v, the impact speed of the bullet. v1 is speed of bullet before impact, v2 is speed of bob This is what I done so far: (at beginning position h=0 therefore potential energy =0) 1/2mv11 = 1/2m(v2/2)2+Mg2l (since h at top = 2l) For pendulum to go in a verticular circular motion at min velocity Fc = Fg then: mv2/l = mg => v2 = gl so plugging this in makes: 1/2mv12 = mv12/8+M2v22 cleaning this up we have v12 = 2(mv12 + 16Mv22)/8m Am I right in my thinking?. If not can someone explain where I went wrong and how to fix it?