1. The problem statement, all variables and given/known data A 150 kg cart moving at 13 m/s east collided with a 420 kg wagon moving at 5.0 m/s east. The cart rebounded westward with a speed of 3.0 m/s . What was the speed of the wagon after the collision? Observe standard Cartesian coordinates. 2. Relevant equations KE1+KE2=KE1'+KE2' conservation of momentum conservation of kinetic energy in elastic collisions 3. The attempt at a solution (0.5)(150)(13)^{2}+(0.5)(420)(5)^{2}=(0.5)(150)(-3)^{2}+(0.5)(420)v^{2} v=9.0m/s Answer as given is 11m/s
the problem itself doesn't say that the collision is elastic. so just use conservation of momentum here.
issacnewton is right !!! Conservation of kinetic energy is valid only when coefficient of restitution(e) is 1 here its not specified, so you must use momentum conservation. ALSO: if e is given and is ≠1 then you may use: (v_{2} - v_{1}) = e(u_{1} - u_{2})