Isolated system
Also called Closed system
A system whose boundary nothing crosses: no net external force acts on it and no external work is done on it. Every conservation statement in AP Physics is a claim about a system chosen to meet one of those conditions.
Isolation is a decision about where the boundary goes, not a property the universe hands you. The same block on the same ramp is isolated or not depending on what you enclose.
The AP Physics 1 CED does not use the phrase. It states the condition each conservation law needs, and the two are not the same condition.
- For momentum: in the absence of net external forces, any change to the momentum of an object within a system must be balanced by an equivalent and opposite change of momentum elsewhere within the system. The CED adds that the velocity of a system's center of mass is constant in the absence of a net external force.
- For mechanical energy: if the work done on a selected system is zero and there are no nonconservative interactions within the system, the total mechanical energy of the system is constant.
So momentum cares about external forces, while energy cares about external work and about dissipation happening inside. A block sliding to a halt on a rough floor keeps constant momentum once the floor and Earth are inside the system, yet its mechanical energy still falls, because friction is an internal nonconservative interaction.
The CED also notes that systems may allow interactions between constituent parts of the system and the environment, which may result in the transfer of energy or mass. Picking a boundary that no such transfer crosses is what makes a conservation statement true in the first place.