A 4.5 kg object with a speed of 4.0 m/s collides head-on with a 11 kg object moving toward it with a speed of 3.0 m/s. The 11 kg object stops dead after the collision. (a) What is the final speed of the 4.5 kg object? (b) Is the collision elastic? so i tried initial momentum = final momentum but I get the wrong answer. (4.5 kg)(4.0 m/s) + (11 kg)(3.0 m/s)=(4.5 kg)(Vf) and solving for Vf = 11.333 m/s, but that is the wrong answer.