1. The problem statement, all variables and given/known data 1.) A 1200 kg car travelling at 20 m/s collides with a stationary 1400 kg car. The two cars lock together. Determine the speed of the vehicles immediately after the collision if 80% of the initial kinetic energy is converted to heat and sound during the collision 2.) Must all kinetic energy be lost in a collision for the collision to be considered completely inelastic? Explain 2. Relevant equations Ek = 1/2mv^2 3. The attempt at a solution For both of these questions I have the answers to them in the book but I can't seem to arrive at/understand the solution. 1.) 80% loss so.. 4/5 Ekintial = Ekfinal (4/5) * (1/2)(1200 kg)(20 m/s)2 + 0 = (1/2)(1200 kg + 1400 kg)(v2) v^2 = 147.6923 m/s v = 12.15 m/s The answer should be 6.1 m/s according to my textbook though. 2. ) "No. As much as possible must be lost without violating the Law of Conversation of momentum" is the answer in the book. Does this mean the maximum energy lost while the momentum of the system is equal for both the initial and final? also, how do I know if an equation is completely inelastic rather than just inelastic?