Why is kinetic friction present when two objects are slipping across each other my mechanics sir was telling us the other day that when the relative velocity is zero between the surface and the object then the object starts sliding, eg in rolling motion without slipping, the v rel is zero at the bottom most point .how does kinetic friction come into play here. I understand that since the object has started moving then kinetic friction will come into play. Is this the one and only reason?
he, secondly, mentioned that slipping and sliding are essentially the same phenomena. I don't quite agree with this statement. can someone pls explain this as well?
Also, this is not really a question but a few thoughts that came to my mind that I wanted to discuss.
 A: When a wheel is rolling without slipping or sliding, it has static friction with the ground not kinetic friction. When a wheel is slipping or sliding, such as on ice or mud, it has kinetic friction not static friction. The words slipping and sliding are fairly synonymous, but we might be more inclined to say that when a wheel has lost static friction and has kinetic friction it is slipping, and a block moving with kinetic friction on a surface is sliding. However either word could work in either situation.
A: When you send a bowling ball down the alley, it starts by sliding. The kinetic friction force (predicted by the coefficient of kinetic friction) on the bottom of the ball produces a torque which causes an angular acceleration.  When the backward tangential velocity of the bottom of the ball (measured relative to the center of mass), matches the forward velocity of the center of mass (measured relative to the alley), the ball is rolling, and the bottom is in static contact with the alley.  There may be a small “rolling friction” due to the deformation of the two surfaces.  If the ball then rolls up an incline, there can be a static friction force acting up the incline (which matches the loss of angular velocity to the loss of linear velocity). The static force has an upper limit predicted by the coefficient of static friction.  Static and kinetic friction originate at the microscopic level where the peaks and valleys of the two surfaces inter-mesh. Generally, kinetic friction is a little smaller than the maximum allowed static friction (the moving surfaces are bouncing peaks off of peaks).
