Momentum querie?

Question

I have a question:

If an object is sliding on the earths surface:

and slows down due to kinetic friction, and only the object is
taken as the system, the kinetic force is taken as the external force.

But if both the aerth and the object are taken as a system, is the kinetic force an external or internal force?

Can the frictional force change the momentum of the two body system?

Answer 1

But if both the earth and the object are taken as a system, is the kinetic force an external or internal force?

internal

Can the frictional force change the momentum of the two body system?

Yes, friction converts kinetic energy into heat , which leaves the system by radiation, and causes a net loss in both system energy and momentum. (The energy lost is no longer considered part of the system)