I know that momentum conserves when no external forces interfere with a system. But I don’t completely understand it, here’s two examples that I don’t fully get:
Two objects collide on a plane that has friction- therefore momentum doesn’t conserve because there’s an external force applying of the system> the friction force.
Two objects collide on a frictionless plane as described: first object has a spring attached to its side and the second object collide into the spring that is attached to the first one changing the spring’s length (causing to elastic force) >the momentum still remains the same, the elastic force of the spring doesn’t count as an external force.
How can I know unambiguously what an external force is, what counts as one so the conservation is canceled?