# Energy(multiple different boxes)

A key feature of energy is that it’s a calculated quantity, and that there are many possible calculations to work out the energy shifted.

### Trade off examples

Machines are designed to make tasks easier, but there is always a trade-off: you can’t get something for nothing.

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Machines are often used as an introduction to energy, and this idea of a trade-off can be exploited and carried over.

### Filling stores

Weight and height are two compensated quantities — there is a trade-off between weight and height in shifting a quantity of energy.

Filling a kinetic store has different trade-offs.

As do others…

Much, much more...

Teaching energy

### Power, box different

Power in the electrical pathway is calculated from two compensated quantities: current and voltage.

Again, because it makes physical sense to compare powers, this is a natural fit for reasoning with boxes. An example: two circuits differing in current & potential difference, but switching the same power.