Answer by HM Wamboldt:
Ah. But the Earth doesn't rotate at the same speed every day. Ask any astronomer. They periodically have to adjust the computers which point their largest telescopes to accommodate the Earth rotating slower or faster.
The change is very very small, but it's measurable and affects the length of the day. It's caused by rainy seasons filling high altitude lakes like Lake Mead and the Yangtze River behind the Three Gorges Dam. Lakes and their watersheds around the globe are filled during the rainy season, this slows the Earth's rotation. The Earth then speeds up as the water drains into the oceans as the year progresses. Since most of these areas are in the northern hemisphere, there is an imbalance between the northern hemisphere's rainy season and the southern hemisphere's rainy season. The result is an annual speeding up and slowing down of the Earth's rotation.
As glaciers melt into the oceans the Earth speeds up.
When plate tectonics lift mountains the Earth slows down. When parts of the crust drop along rift valleys the Earth speeds up.
When the average global winds blows harder from the West to the East the Earth's rotation slows down. When the average global winds blows harder from the East to the West the Earth's rotation speeds up.
As for why planets don't just slow down and stop, yes, you answered your own question. It's because it's spinning in a vacuum, and since it's in a vacuum there is no friction to slow it down.