Transfer · stores · total
Energy
Treat energy as a scalar accounting quantity. Forces can transfer it across a boundary, interactions can store it, and conservation lets us compare states without reconstructing every instant in between.
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Unit throughline
Energy is useful because we can account for change without following every force at every instant.
We begin with transfer, identify important stores, then combine the stores into one conservation statement. Power comes later because it asks a different question: not how much energy changes, but how quickly.
system boundaryenergy accounting
work
transfer
kinetic
motion store
potential
interaction store
total
conserved
cross boundary → occupy stores → exchange between stores → reconcile total
Learning path
Follow energy across a system boundary and into its stores.
Energy is scalar
Energy amounts add without a direction sign convention. Directional information lives elsewhere in the model.
Choose the system first
Work and energy transfer depend on what crosses the boundary of the system you decided to analyze.
Conservation is accounting
Energy does not vanish when a familiar store decreases. Find the new store or the transfer across the boundary.