Idea 4of 5Builds onThe total never changes
Draw the box first
Before counting energy, what must you decide first?
Today you'll discover
- where a winch's joules came from
- why the same story can balance or not
- the word for what is inside the box
Think firstThe crate's bars grew from nothing to 240 J
The crate's ledger grew from nothing to 240 J. Where did that energy come from?
Decide first, then press Next. Ask of every bar whether it sits inside the line or crossed it.
An energy ledger for a winch lifting a crate, with a hollow bar for the work put in from outside. After a prediction, the hollow bar is named as energy crossing the line into the picture, then the scene changes to a pendulum whose small warmth bar sits inside or outside the line depending on whether the air is counted. The last step draws the line round a braking cart alone and asks where its energy went.
See the whole ideathe big idea and the key word, in one place
The big idea
Draw the box: the system. Inside it, totals hold.
Compare themPendulum alone: 2 J leave, and the ledger looks short. Pendulum plus air: nothing leaves, and the ledger balances. Same swing, different box.
Key word
- system
- the set of things whose energy you are counting; everything outside it can only add or remove energy by doing work across the boundary
About this modelThe bars move energy between stores; the total only changes when work is put in or taken out.
The bars move energy between stores; the total only changes when work is put in or taken out.
Ready to use it?
- Say what is inside a described system and whether energy crossed its boundary.