This is the question:
When cars are equipped with flexible bumpers, they will bounce off each other during low-speed collisions, thus causing less damage. In one such accident, a 1850 kg car traveling to the right at 1.60 m/s collides with a 1450 kg car going to the left at 1.10 m/s . Measurements show that the heavier car's speed just after the collision was 0.270 m/s in its original direction. You can ignore any road friction during the collision.
I solved this problem correctly with conservation of momentum: (m1v1 + m2v2)i = (m1v1 + m2v2)f. The result was 0.597 m/s.
However, when I attempted to use the equations for elastic collisions, shown here: 
I ended up with v2,f = 1.927 m/s. Why is the second equation in the picture wrong? I thought that if a collision is elastic, these equations always apply.
Best Answer
I suspect the discrepancy arises because the collision was not elastic.