I was never big on economics, taking only a simple survey course on it in college. But I distinctly remember my prof say that around 5% is the ideal, with a higher rate bad for obvious reasons and a lower rate fueling inflation or something like that.
I taught econ.
5% is ideal because it basically means that everyone who wants to be working is. Maybe not at their ideal job, but they have a job. If the unemployment rate goes below that, it means that people have more cash to spend, which means the Fed has to find a way to take cash out of circulation to prevent inflation-so interest rates go up. In other words, there is more money than goods/services available, and the price of things increases. Supply low, demand high. Too much money makes the money worth less.
When the unemployment rate is over 5%, it means that there isn't enough cash in circulation, so the Fed has to find a way to get more cash circulating in the economy to keep it stable. There are more goods/services than there are buyers, which means you have an excess of supply. In order to keep cash in the economy and keep the supply and demand at equilibrium, the Fed will do things to increase the amount of disposable cash people have-lowering interest rates is the best example. In this case, supply is high, but demand is low.
It's all supply and demand.