4. A 1500 kg car moving east at 17 m/s collides with a 1800 kg car moving south at 15 m/s and the two cars stick together. What is the speed of the cars after collision? http://img223.imageshack.us/img223/3373/untitledao7.png [Broken] I understand all but the last part; why does Vf = sqrt (x^2+y^2)? Why are the x and y squared?