Answer
a) Kinetic frictional force is no longer an external force.
b) The frictional force does not change the total linear momentum of the object-Earth system.
Work Step by Step
a) If both the object and the earth are in a system, kinetic frictional force is, in fact, an internal force.
Because, for example, let's say kinetic friction acts on the object to decrease its momentum. At the same time, there is a reaction force back onto earth that increases the earth's momentum. So kinetic friction, indeed, is exerted by 2 bodies in this system on each other and therefore, is an internal force.
b) Because kinetic friction is an internal force, it does not change the total linear momentum of the object-Earth system.
