In a certain physics experiment, carts run along a frictionless, level, one-dimensional track. Cart A is 1 kg and cart B is 2 kg. Initially cart A is moving to the right at 0.2 m/s, and cart B is moving to the left at 0.7 m/s. After they collide, they stick together. (See figure.)
What is the magnitude of the final velocity of the two carts?