An object slides along the surface of the earth and slows down because of kinetic friction. if the object alone is considered as the system, the kinetic frictional force must be identified as an external force that decreases the momentum of the system.
a) if both the object and the earth are considered to be the system, is the force of kinetic friction still an external force?
b) can the friction force change the linear momentum of the two body system?

A) The friccion force can not be considered an external force

B)  can't change the moment

Explanation:

A) If the System is formed by the two bodies, the force of friction acts on each body separately, therefore by the law of action and reaction this is an internal force to the System that considered as a whole Cannot change the moment of the system.

The summary cannot be considered an external force

B) The linear momentum of the system is the moment of each body, in this case the object and the Earth, since the energy that one gains is lost by the other.

For this explicit case the difference in mass is so great that the change in the movement of the Earth is not appreciated

In short can't change the moment

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small = small current large mag large current