# A couple of examples

### Hot enough?

Here is a situation about which I would like to know something: how much gas I need to use to get a cup of hot tea using a small gas camping stove to boil water

The amount of gas used turns out to be crucial when planning a multiple-day expedition. You do not want to carry too much, but not having a cup of hot tea is a disaster of the first order.

What's the best I can do?

To get started, you need to describe what happens physically.
Decide on a pair of snapshots during the process (choose cunningly and describe carefully so you can calculate or reason about the energy shifted between the snapshots).

Use clues from the physical descriptions to identify the stores.

Do not include stores where there is no change in energy.

The energy shifted from the one store is shared between the remaining stores.

How much energy is shifted and how this quantity of energy is shared out determines what is possible or impossible.

The energy shifted from the chemical store is shared across the three thermal stores.

The more gas I burn, the more energy is shifted to be shared out. But to raise the temperature of the water, which is what is important to me, I want most of that energy to end up in the thermal store of the water and as little as possible in the thermal stores of a cup and other surroundings. This has implications for the cup that I choose and for minimising the energy shifted to the surroundings, for example, by using a windshield.

So for this process the energy description does provide some insights.

### How fast will I be going?

I freewheel down a favourite hill on my bike. SymbolParaBreak What's the maximum speed I could achieve by freewheeling down this hill?

To get started, you need to describe what happens physically.
Decide on a pair of snapshots during the process (choose cunningly and describe carefully so you can calculate or reason about the energy shifted between the snapshots).

Use clues from the physical descriptions to identify the stores.

Do not include stores where there is no change in energy.

The energy shifted from the one store is shared between the remaining stores.

How much energy is shifted and how this quantity of energy is shared out determines what is possible or impossible.

Energy is shifted from the gravity store to the kinetic store and to the thermal store of the surroundings.

The better oiled the bike and the less the surrounding air is stirred (so, the more streamlined the bike), the smaller the quantity of energy shifted to the thermal store.

The greater the quantity of energy shifted to the kinetic store: the faster I go down this hill.

Change hills, to one where the gravity store is emptied more, and the energy available to be shared between the kinetic and thermal stores increases, and this changes the maximum possible speed.