A $2.0 kg$ block is attached to one end of a spring with a spring constant of $100 N/m$ and a $4.0 kg$ block is attached to the other end. The blocks are placed on a horizontal frictionless surface and set into motion. At one instant the $2.0 kg$ block is observed to be traveling to the right with a speed of $0.5 m/s$ and the $4.0 kg$ block is observed to be traveling to the left with a speed of $0.30 m/s$. Since the only forces on the blocks are the force of gravity, the normal force of the surface, and the force of the spring, we conclude that :