1. The problem statement, all variables and given/known data A 2100kg van collides with a 1200kg car that is at rest. They lock together and move together at a speed of 4.5 m/s. What is the initial speed of the van? 2. Relevant equations Eki = Ekf 1/2mv^2 3. The attempt at a solution Okay, so I know that the energy before the collision must equal the energy after the collision. I know everything I need to figure out the energy after the collision: E = 1/2mv^2 (1/2)(2100kg + 1200kg)(4.5m/s)^2 =33412.5 J So now I need to figure out the other side of the equation using that number. So: 33412.5 = 1/2mv^2 v^2 = 2(33412.5J)/m =2(33412.5J)/2100kg Square Root of the result gives me: 5.6 m/s That looks right to me, but my worksheet says the answer should be 7.07 m/s. What am I doing wrong?