3.1 Open and Closed Systems

Defining Systems

A system is the object or group of objects chosen for analysis; everything else is the surroundings. A closed system exchanges no matter (and often, no energy) with its surroundings, so quantities like total energy or momentum can be conserved within it. An open system exchanges matter and/or energy with the surroundings, so external influences must be tracked carefully.

Choosing Your System Wisely

How you define a system determines which forces are internal (occur between objects within the system, doing no net work on the system as a whole in the case of conservation laws) versus external (forces from outside the system, capable of changing the system's total energy or momentum). Enlarging a system can turn an external force, like gravity from an object outside, into an internal one, simplifying analysis.

Why It Matters for Energy/Momentum

Conservation laws (energy, momentum) strictly apply to closed, isolated systems — no external work or net external force. If a system is open, energy or momentum can flow in or out, so you must account for work done by external forces or impulses to correctly apply the work-energy theorem or conservation principles.

Key terms

System

The object or collection of objects chosen for analysis; everything else is the surroundings.

Closed system

A system that does not exchange matter with its surroundings.

Open system

A system that exchanges matter and/or energy with its surroundings.

Internal force

A force acting between two objects both within the defined system; does not change the system's total momentum.

External force

A force from outside the system acting on it, capable of changing the system's total energy or momentum.

Try an AP-style question

A ball falls from rest. For the ball + Earth system, where does its kinetic energy come from?

1. Gravitational potential energy of the system
2. The ball's own internal chemical energy
3. Thermal energy drawn from the air
4. Work done by an external force on the ball