system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does dust manage to settle on a constantly moving fan without being pushed off?
gpt
A fan moves air by pushing it over the angled blades. As the air moves over the blades it gets pushed faster and faster until it's moving as fast as the blades. At the trailing edge of the blade, the air and fan blade are moving at almost the same speed. The air is moving fast, and the blade is moving fast in the same direction so the relative speed is near zero. If you were an ant at that point, you feel as if there was almost no wind. The dust particles in the air at this point would settle just as they would on a desk. That's why dust collects on the trailing edge of each blade, because the relative wind speed is zero just like it is above a desk.