Both are wrong . It ' s not about the difference in kinetic energy between car A and car B . It ' s about the difference in kinetic energy before and after the collision . Let ' s assume the collision is perfectly inelastic . If car A is going 60 kmh and rearends car B who is going 50 kms , the sum of the speed 2 is 3600 2500 6100 km 2 s 2 . After the collision , they ' re each going 55 mph , and the sum of the speed 2 is 23025 6050 km 2 s 2 . The difference is 50 km 2 s 2 . Now consider it from the reference frame of car B . Before the speed 2 is 100 0 100 km 2 s 2 . After it ' s 225 50 km 2 s 2 . The difference is 50 km 2 s 2 . Or if you really want to simplify the math , use the reference frame they end at . Before both cars are moving at 5 kms , and the total speed 2 is 225 50 km 2 s 2 . After they ' re both at rest , and it ' s zero . The difference is 50 km 2 s 2 .