A 4000 kg truck is at rest at an intersection, waiting for a green light (with no other vehicles around). It is a snowy and icy day, with no friction on the road. A 1000 kg car traveling at 5 m/s strikes the stopped truck square on from behind. The bumpers of the two vehicles lock together as a result of the collision, so that they skid across the intersection together. How fast are they traveling?