1. The problem statement, all variables and given/known data A bullet of mass m and speed v passes completely through a pendulum bob of mass M. The bullet emerges on the other side of the pendulum bob with half its original speed. Assume that the pendulum bob is suspended by a stiff rod of length L and negligible mass. What is the minimum value of v such that the pendulum bob will barely swing through a complete vertical cycle 2. Relevant equations m1v1i2 + m2v2i2 = m1v1f2 + m2v2f2 w=Δk+Δp k = 1/2mv2 p = mgh 3. The attempt at a solution initial kinetic energy of bullet = potential energy of bob @ max height + final kinetic energy of bullet 1/2mv2 = Mg2L + 1/2 m(1/2v)2 1/2mv2 - 1/8 mv2 = 2MgL 3/8mv2=2MgL v2 = (16MgL)/(3m) v = 4[(MgL)/(3m)]^(1/2) Answer is [4M(gL)^(1/2)]/m The key sets 1/2MVb2 = Mg2L if Vb = velocity of the bob, then at max height the bob is not moving hence velocity is 0 and all of the energy that the bob carried at the bottom is now potential energy. and then solves via the equation: mv = MVb + mv/2 initial momentum = momentum of bob + momentum of bullet after going through bob If everything that I have said about the professors method is true, then I feel like I am beginning to understand why the professors way works, but why doesn't my method come to the same conclusion.