An isolated system is a system that exchanges neither matter nor energy with its surroundings, which means there is no net external force on it. On AP Physics 1, that's the condition that lets you say the system's total momentum (and total energy) stays constant.
An isolated system is a collection of objects that doesn't trade anything with the outside world. No mass enters or leaves, and no energy crosses the boundary. Since no outside influence pushes on it, the net external force on an isolated system is zero, and that's the part AP Physics 1 actually cares about.
Here's the move you'll make constantly in Unit 4. A collision is modeled as an interaction where the forces between objects inside the system are way bigger than any external force during the interaction. If you draw your system boundary around both colliding objects, the huge collision forces become internal, they cancel in pairs (Newton's third law), and the system behaves as isolated. Same logic for explosions, where internal forces push pieces of the system apart. In both cases, isolating the system is what justifies writing total momentum before = total momentum after. The label "isolated" isn't a fact about the universe; it's a choice you make when you decide where to draw the system boundary.
This term lives in Topic 4.1 (Open and Closed Systems) in Unit 4: Linear Momentum, supporting learning objective AP Physics 1 Revised 4.1.A, describing the linear momentum of an object or system. The essential knowledge for 4.1.A defines collisions and explosions as interactions where internal forces dwarf the net external force, and that's exactly the isolated-system condition in disguise. Every conservation-of-momentum problem you'll do, from carts sticking together to a firework bursting in midair, starts by establishing (or assuming) that the system is isolated. If you can't say "net external force is approximately zero," you can't legally conserve momentum, and graders check for that justification.
Keep studying AP Physics 1 Unit 4
Open and Closed Systems (Unit 4)
Isolated is the strictest category on the open/closed/isolated spectrum. An open system trades matter and energy, a closed system seals in matter but can still trade energy, and an isolated system trades nothing. Topic 4.1 asks you to pick the right label based on where you draw the boundary.
Collisions and Explosions (Unit 4)
Both are modeled by treating the interacting objects as one isolated system so the giant interaction forces become internal and cancel. That's why total momentum is the same before and after, even when kinetic energy isn't.
Total Mechanical Energy (Unit 3)
The same boundary-drawing logic shows up with energy. If no external force does work on the system and no energy crosses the boundary, total energy is constant. Mechanical energy specifically is only conserved if, on top of that, no internal friction converts it to thermal energy.
Mass (Unit 1)
Because an isolated system exchanges no matter, its total mass is fixed. That's a quiet but useful constraint, since p = mv means you can track momentum changes purely through velocity changes of the system's parts.
No released FRQ uses "isolated system" as the question itself, but the concept is the hidden first step of nearly every momentum problem. Multiple-choice stems test whether you can spot when momentum is conserved, for example "two carts collide on a frictionless track" (isolated, conserve momentum) versus "a cart hits a wall bolted to the Earth" (not isolated if your system is just the cart). On FRQs, the points come from your justification. Writing "momentum is conserved" earns nothing by itself; writing "the net external force on the two-cart system is approximately zero during the collision, so the system's total momentum is constant" is the kind of reasoning the rubric rewards. Also watch for questions that change the system boundary mid-problem, since the same situation can be isolated for one choice of system and not for another.
A closed system blocks matter but lets energy cross the boundary (a sealed pot on a stove still heats up). An isolated system blocks both matter and energy. The practical AP difference shows up in what you're allowed to conserve. A closed system can gain or lose energy through external work or heating, while an isolated system's total energy and total momentum are locked in. If a problem says "frictionless" and "no external forces," it's nudging you toward the isolated model.
An isolated system exchanges neither matter nor energy with its surroundings, so the net external force on it is zero.
Zero net external force is the condition that lets you conserve total momentum, which is the core skill of Unit 4 (learning objective AP Physics 1 Revised 4.1.A).
Collisions and explosions are modeled by drawing the system boundary around all the interacting objects, so the huge interaction forces become internal and cancel by Newton's third law.
Whether a system counts as isolated depends on where you draw the boundary; a cart hitting a wall is not isolated by itself, but cart-plus-Earth is.
In an isolated system total momentum is always conserved during a collision, but kinetic energy is only conserved if the collision is elastic.
On FRQs, earn the point by justifying conservation explicitly, for example "the net external force on the system is approximately zero, so total momentum is constant."
It's a system that exchanges no matter and no energy with its surroundings, meaning the net external force on it is zero. In Unit 4 it's the condition that justifies conserving the system's total momentum during collisions and explosions.
A closed system keeps matter in but can still exchange energy with its surroundings, like a sealed container being heated. An isolated system exchanges nothing, neither matter nor energy, so its total energy and momentum stay constant.
Yes. With zero net external force, the system's total momentum cannot change, no matter how messy the internal interactions are. Kinetic energy is a different story; it's only conserved in elastic collisions, while in inelastic collisions some of it converts to other forms.
Not really, but that's fine for the exam. Collisions are modeled as isolated because the internal forces between the objects are much larger than the net external force during the brief interaction, so external effects are negligible. AP Physics 1 wants you to apply the model, not debate its perfection.
Check the boundary you've drawn and look for external forces. Phrases like "frictionless surface" or "ignore air resistance" signal that the horizontal net external force is zero. If an external force still acts (like a wall or rope outside your system), redraw the boundary to include that object or don't conserve momentum.
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