When a body moves with constant velocity in a direction parallel to its surface, the nature of its motion depends on the interaction between the body and the surface. Let's consider the cases of rolling and sliding:
Rolling: When a body moves with constant velocity and rotates smoothly without slipping, it is said to be rolling. Rolling motion occurs when there is sufficient friction between the body and the surface, allowing the body to rotate while maintaining forward motion. In this case, the body's motion is a combination of translational motion (moving forward) and rotational motion (rolling without slipping). An example of rolling motion is a ball rolling on the ground.
Sliding: If the body moves with constant velocity but does not rotate while in contact with the surface, it is sliding. Sliding occurs when the frictional force between the body and the surface is not sufficient to support rotation. The body slides without any rotational motion, only undergoing translational motion. For example, a box sliding along a smooth surface exhibits sliding motion.
Neither: There can also be situations where the body neither rolls nor slides. This typically happens when there is no friction or negligible friction between the body and the surface. In such cases, the body moves with constant velocity in a direction parallel to the surface without any rolling or sliding motion. A classic example is an object moving on a frictionless ice surface or in outer space.
In summary, when a body moves with constant velocity in a direction parallel to its surface, it can exhibit rolling motion, sliding motion, or neither, depending on the presence and nature of the interaction (friction) between the body and the surface.