1. "Consider a minor parking-lot accident. Car A backs out at 30 cm/s toward the west, and Car B looks for a place to park, driving north at 40 cm/s. Both cars mass 1,000 kg. What is the total system momentum before the collision? Remember that momentum is a vector quantity. Also, be careful with your units." 2. Momentum= Mass * Velocity 3. First, I convert 30 cm/s and 40 cm/s to m/s, which gets me .3 m/s and .4 m/s. Then, I multiply mass and velocity together and I get 400 kg*m/s for Car B and 300 kg*m/s for Car A. I combine the momentums to get 700 kg*m/s in a NW direction. The book, however, says 500 kg*m/s is the correct answer (without explanation). Can anyone help? I'm just starting out, trying to learn basic physics on my own.