Posted below is Quiz 7. You may, as always, submit corrections to any questions on the quiz. In addition, the question for revision using the IDEA format is #2.
A very popular incorrect answer to #2 involved treating the problem as if it were in one dimension. It is crucial to remember that momentum is a vector quantity, and that your equations must treat the conservation of x and y components of momentum independently. You cannot write a single equation like m_A v_A + m_B v_B = (m_A + m_B) v_f and expect to obtain a correct answer. It should also concern you that nothing about such an analysis tells you in what direction the cars will be moving after the collision. If, by contrast, you have the x and y components of velocity, you have fully specified the velocity vector, and could obtain a speed and direction by applying right triangle trigonometry using the components.