What defines an isolated system and the role of external forces in momentum?

[qwertymeister42]

This is with regard to 2 boxes of different masses that are stacked on top of each other and pushed at a constant velocity across a table that has friction. One of the sentences is right:

"When the boxes move at a constant speed, there is no change in momentum. This means that the boxes are an isolated system."

My initial thought was that the mistake was in saying "constant speed" when it should be "constant velocity". But is there a problem in how the system is defined as only the two boxes? Should it include the table, for example?

EDIT: Also, could someone clear up exactly what the threshold for "isolated" is? This always confused me because nothing is isolated unless the system is literally everything.

 

[Let'sthink]

Any system can continue to have the same momentum under two conditions:
1. System is isolated meaning nothing can act on it from its outside no connection no interaction.
2. System is not really isolated but the forces acting on them as a whole or on its parts are balanced to give net zero force.
System does not include table but table and pushing agency interact with the blocks and also also blocks interact with each other through the touching surfaces.

 

[Let'sthink]

Also I would like to add although Newton's laws define force the force that is talked about in first two laws is different from the force that is spelled as action reaction in the third law!