# Conservation of linear momentum, when is it conserved?

1. Will Linear momentum be conserved in a non-inertial frame of reference?

2. In other words what is the fundamental condition for linear momentum to be conserved?

3. Also which is more fundamental- Newton's Third Law or Conservation of Linear Momentum? (as in Newton's third law does not hold true when we go to the quantum/atomic level or at relativistic speeds whereas linear momentum to my knowledge holds true)