Suppose there is a black box resting on a flat surface, which surface is rigidly attached to the earth. Suppose inside the black box there is a process initiated which causes the black box to lurch to the right 3 centimeters and lets say after one second the box comes to a complete stop. (No observable ejectile exits the black box, nor any energy or radiation. Also, there is no external force acting on the black box that causes it to lurch in one direction and the other direction) Now the only external forces acting on the black box after the internal process is triggered are friction forces, acting on the bottom of the box opposite to the direction of the motion of the box. Conservation of momentum would require that after the box comes to a stop that it should move to the left for the same amount of time before coming to a stop, if we assume the magnitude of the friction force remains constant. And this must be true even if the distance the black box travels to the left may be less than 3 centimeters. That is, the work done on the surface of the earth in the right direction may not be the same as the work down on the surface of the earth in the left direction. But the magnitude of the impulse must be the same. Put another way, the earth receives an impulse, friction force x time (1 second) in the right direction. In order for the earth to not have a net impulse, there must be an equal and opposite impulse on the earth in the opposite direction, friction force x time (1 second). Bottom line. If the box moves to the right for 1 second, it must move to the left for 1 second. Is this conclusion correct? Again, assume the coefficient of static and kinetic friction remain the same such as the case with polytetraflouroethelene(Teflon).