Two air carts of mass m_1 = 0.86 kg and m_2 = 0.45 kg are placed on a frictionless track. Cart 1 is at rest initially, and has a spring bumper with a force constant of 690 {rm N}/{rm m}. Cart 2 has a flat metal surface for a bumper, and moves toward the bumper of the stationary cart with an initial speed v = 0.61 m/s. Assume that positive x-axis is directed toward the direction of motion of cart 2. What is the speed of the two carts at the moment when their speeds are equal?