Why is the supply curve for dollars in the foreign-currency exchange market vertical in an open economy?5 changes in the money supply the following graph represents the money market in a hypothetical economy as in the united states this economy has a central bank called the fed but unlike in the united states the economy is closed that is the economy does not interact with other economies in the world the money market is currently in equilibrium at an interest rate of 3 and a quantity of money equal to 04 trillion as indicated by the grey star