4.3 Open and Closed Systems

Defining the system

A system is the object(s) you choose to analyze. A closed system has no matter entering or leaving, so its total mass stays constant. An open system exchanges matter with its surroundings. Choosing your system wisely simplifies applying conservation laws — momentum analysis usually requires defining a closed system.

Isolated vs. non-isolated systems

An isolated system experiences no net external force — no momentum is transferred in or out, so total momentum is conserved. A non-isolated system has external forces acting on it, causing its total momentum to change over time, per the impulse-momentum theorem (Δp = FΔt).

Why system choice matters

Internal forces (between objects inside the system) always cancel in pairs by Newton's third law, so they never change total system momentum. Only external forces can change a system's total momentum. Expanding your system's boundary to include an interacting object can turn an external force into an internal one, making momentum conserved.

Key terms

Closed system

A system where no matter enters or leaves, keeping total mass constant.

Open system

A system that exchanges matter with its surroundings.

Isolated system

A system with no net external force acting on it; total momentum is conserved.

Internal force

A force between objects within the same system; cancels in action-reaction pairs and doesn't change total momentum.

External force

A force from outside the system that can change the system's total momentum.

Try an AP-style question

When is the total momentum of a system conserved?

1. When there are no internal forces
2. When the net external force on it is zero
3. Only when objects are at rest
4. When kinetic energy is conserved