What is the velocity of the center of mass of the carts after the massless spring is released (assume that since the spring is massless it cannot have a kinetic energy)? time for the kinetic energy of...What is the velocity of the center of mass of the carts after the massless spring is released (assume that since the spring is massless it cannot have a kinetic energy)? time for the kinetic energy of the projectile, the kinetic energy of the target, and the potential energy of the spring.