Physics / Lesson 03
Energy and conservation
Track change with a ledger that remains stable across different mechanisms.
orient
Why this idea had to exist
Forces explain local changes; conservation laws reveal what remains accountable across an entire process. Energy is powerful because one ledger can connect motion, height, deformation, heat, and radiation.
intuition
Build a picture you can reason with
Choose a boundary around the system and keep a ledger. Energy can move between storage accounts inside or cross the boundary by work, heating, or radiation. A missing amount signals an omitted account or transfer—not vanished energy.
formalize
Give the intuition a precise edge
For a defined system, ΔE_system equals net energy transferred in. Kinetic energy is ½mv²; near Earth's surface gravitational potential differences are mgΔh. Mechanical energy is conserved only when transfers to thermal or other stores are negligible or included.
work through
Follow the decisions, not just the symbols
A descending block loses gravitational potential energy. With negligible friction, kinetic energy rises by the same amount. With friction, some energy enters thermal stores; writing only K+U would make the ledger appear to leak.
experiment
Change one thing and watch the model answer
Move a system boundary around a block and ramp. Watch which terms count as internal storage and which appear as transfers across the boundary.
retrieve
Close the page and reconstruct it
Answer before opening the explanation. Retrieval is evidence only when the answer is produced without a hint.
Why must an energy argument state the system boundary?
The boundary determines which energy changes are internal storage and which are transfers into or out of the system.
transfer
Move the idea into a new setting
Construct an energy ledger for a battery-powered fan slowing after power is disconnected, including where energy ultimately goes.
reflect
Leave with a diagnostic habit
When conservation seems broken, inspect the boundary, neglected stores, and transfer channels before questioning the law.
Source record
Follow the idea back.
- Reference
- University Physics
- Publisher
- OpenStax
- License
- CC BY 4.0
- Accessed
- 2026-08-12
- URL
- https://openstax.org/details/books/university-physics-volume-1