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Aggregate Demand is the amount of spending that occurs in the economy. It is the total demand for goods and services in the economy, as comprised by our spending consumption, investment, government, and net exports.
Flimsy and fickle, this is the weird wild world of Aggregate Supply. It is the output we can produce given the resources that we have, which doesn't depend on the price level*.
Since Aggregate Demand is just spending, when spending changes so does aggregate demand. This change in spending can come in the form of consumption, investment, government spending, or an increase in exports.
Aggregate supply is how we use our resources, and so if how we use them is changing than aggregate supply will shift. Oh but also in those situations with sticky prices and wages, if wages or rents or interest change than short-run aggregate supply will change, but not long-run because those aren't sticky in the long-run and...ah geez... just watch the video.
Putting Aggregate Supply and Aggregate Demand together gives us an equilibrium for the price level and output. And where SRAS and AD cross relative to LRAS gives us an understanding of the state of the economy.
Answer each question on a piece of paper. Then watch the solution video. Trust me, I make the answer look easy. If you don't try it first, you won't build up your mechanism for answering Assessment questions.