AP® Physics 1: Algebra-Based · Unit 3 · Topic 3.4
Choosing the System Boundary for Energy
Whether gravity counts as external work or as a change in stored potential energy depends entirely on whether Earth is inside or outside the system you chose. The physical outcome never changes.
Energy bookkeeping starts with a choice: what's inside your system, and what's outside? Anything outside can do work ON the system. Anything inside can store energy that converts between forms. Gravitational potential energy only makes sense if Earth is part of the system, because that energy comes from the gravitational interaction between the ball and Earth. Pull Earth inside the boundary, and gravity stops being an outside force doing work; it becomes an internal interaction, stored as potential energy that converts to kinetic energy as the ball falls.
Take one falling ball. System A: just the ball. Earth is external, so gravity is an external force doing work on the ball, and that work shows up directly as a gain in kinetic energy. System B: ball plus Earth together. Now gravity is internal, so there's no external work from gravity at all. Instead, gravitational potential energy stored in the ball-Earth system converts into kinetic energy as the ball falls. Both versions predict the exact same speed at the exact same height. The boundary choice changes the language and the energy bar chart, not the physics.
This is why potential energy never belongs to a single object. It's a property of a relationship, ball and Earth pulling on each other, so it can only be assigned to the system containing both. This connects directly to energy bar charts and conservation: pick your system first, then decide whether gravity belongs in the external work column or the potential energy column, but never double-count it in both.
Key terms
- System boundary
- The chosen dividing line between what's included in the analysis and what's treated as external.
- External work
- Energy transferred into or out of a system by a force from outside its boundary.
- Gravitational potential energy
- Energy stored in a system due to the gravitational interaction between its parts, such as a ball and Earth.
- Internal interaction
- A force acting between two objects that are both inside the chosen system, so it transforms energy rather than doing external work.
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End of topic.