1. The problem statement, all variables and given/known data A 90-kg halfback running north with a speed of 9 m/s is tackled by a 120-kg opponent running south with a speed of 3 m/s. If the collision is perfectly inelastic and head-on, calculate. (a) the velocity of the players just after the tackle and (b) the total energy lost as a result of the collision. 2. Relevant equations m1v1i + m2v2i = (m1 + m2)vf 3. The attempt at a solution I plugged the masses and velocities into the equation for perfectly inelastic collisions and solved for vf. I got 5.57 m/s and the answer is 2.14 m/s.