The workhorse of economics: how buyers and sellers set price and quantity in a single market.
Warming up the engine…
The workhorse of economics: buyers' willingness to pay (demand) and sellers' willingness to sell (supply) meet at one price where the quantity demanded equals the quantity supplied.
Qd(P) = Qs(P) at equilibrium price P*
If price sits above equilibrium, sellers can't find enough buyers and unsold stock pushes price down; below equilibrium, queues and shortages pull it up. The market clears not because anyone plans it, but because every mismatch creates pressure that removes itself. Every shock is just one curve shifting and a new intersection.
Movements ALONG a curve come only from the good's own price. Anything else (income, tastes, input costs, technology) SHIFTS a curve. Mixing these up is the most common first-year error.
Now prove you have it
Move the curve to where you think it lands, and get told exactly which part you got right.
Real-world scenarios on this model
Equilibrium at Q 50, P 40.
Equilibrium: Q* = 50.0, P* = 40.0
Current equations
One market, two forces. The demand curve shows how much buyers want at each price; the supply curve shows how much sellers offer. Where they cross is the equilibriumequilibriumThe point where opposing forces balance, quantity supplied equals quantity demanded, so there's no pressure for price to change., the price at which the market clears. Move the sliders to shift each curve and watch the equilibrium respond.
Source: Mankiw, Principles of Economics, ch. 4-6