Conservation of linear momentum in this system

[undividable]

A pet mouse sleeps near the eastern edge of a stationary, horizontal turntable that is supported by a frictionless,
vertical axle through its center. The mouse wakes up and starts to walk north on the turntable. Is the momentum of the system constant?
i understand that the initial momentum is zero, because the turntable and the mouse have v=0, and that the final momentum is not zero, since v≠0 for the mouse, and v=0 for the turntable, so linear momentum is not conserved, but since ∫∑Fext dt=dp, where is the external netforce (∑Fext) that causes the change in the system's momentum? It seems to me there are none. only internal forces (friction) between the mouse and the turntable

 

[Dale]

where is the external netforce (∑Fext) that causes the change in the system's momentum

The axle

 

[undividable]

The axle

so, as the mouse moves upward, the axle exerts and external force (normal downward force) on the turntable so it stays with v=0? and if the axle was a part of the system? the linear momentum should still change, but now the external net force would be is 0

 

[jbriggs444]

so, as the mouse moves upward, the axle exerts and external force (normal downward force) on the turntable so it stays with v=0? and if the axle was a part of the system? the linear momentum should still change, but now the external net force would be is 0

If the axle has finite mass, it will move south along with the wheel while the mouse moves north. Net linear momentum of the system (including axle) remains zero.

If the axle has infinite mass, it is an infinite momentum sink. Its momentum cannot be calculated by measuring zero velocity and crying "Eureka: zero momentum".

 

[Dale]

and if the axle was a part of the system?

Then identify the forces acting on the axle and apply Newton's laws.