Set the system boundary
Identify what belongs to the system and which interactions transfer energy across its boundary.
AP Physics 1 · Unit 3 · Work, Energy, and Power
Conservation of Energy with External Work
Add external work and friction to an energy model. Track where energy enters, leaves, or changes form during motion.
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Learn by doing
Three ideas you’ll put into motion
Account for work
Compare positive and negative work with changes in the system’s energy.
Include dissipative effects
Explain why mechanical energy can decrease while total energy is still accounted for.
The big idea
The key ideas, at a glance.
Work transfers energy
An external force can transfer energy into or out of a system. Its work sign depends on the force and displacement directions.
Mechanical energy can change form
Friction can transfer mechanical energy into thermal energy. Choose the system before deciding whether to describe that transfer as external work or an internal change.
A quick comparison
An external force opposes an object’s displacement.
Its work on the object is negative.
Teaching this topic?Classroom fit, AP connections, and support
- Classroom fit
- Use for a concept introduction, guided independent practice, or a projected class discussion.
- Before students begin
- Work, kinetic and potential energy, and mechanical-energy conservation.
- Built-in support
- Prepared scenes, guided prompts, and feedback on lesson questions.
- AP Physics 1 connections
- Unit 3: energy accounting, external work, and system boundaries.
- Check for understanding
- Ask students to account for the mechanical energy lost by a sliding object.
Ready to test your thinking?
The first scene is set up. Start the lesson and explore.