1. The problem statement, all variables and given/known data A bullet hits balk of 5kg on a rope of 2m lenths. Bullets mass 2gr, velocity unknown. Plastic collission, balk goes upat 45 degrees. 2. Relevant equations When i solve it with energy equations i get h=2-2cos45=0.58m V(of balk and bullet) = sqrt(2gh)=3.38 m/s. (potential energy of total mass) Now i need to find a velocity of bullet at t(0). Here is the trick when i solve it using momentum i get: mv=(m+M)V v=8450 m/s But!! when i solve it using energy i got m*sqr(v)/2=(m+M)*g*h v=168.5 m/s. Thinking about that we used the potential energy to calculate a balk speed and taking into account an absolute plastic collision i dont know why is the differense? Thanks for any help.