Two carts, Cart 1 of mass \(1.0 \text{ kg}\) and Cart 2 of mass \(3.0 \text{ kg}\), are placed on a horizontal, frictionless track. A compressed spring is positioned between them such that the carts are initially held at rest with a separation distance of \(0.10 \text{ m}\). The graph shows the potential energy \(U\) stored in the cart-spring system as a function of the separation distance \(d\) between the carts. When the carts are released, the spring expands until it reaches its relaxed length at \(d = 0.50 \text{ m}\) and detaches from the carts. What is the final speed of Cart 2 after the spring has completely relaxed?