I was just looking at an airplane video and was wondering why it looks like the propellers are moving relatively slowly when they infact have a RPM in the thousands probably.
Videos typically have a frame rate of ~25 frames per second. The 'movie' you see is a result of these snapshots. If you assume the propellers were moving at 9k RPM, that is 150 revolutions per second. If the camera recorded at 150 FPS, the propellers would appear stationary as they would be in the same position at each time a new frame was captured.
The relationship between these two speeds that causes the effect you are describing; given a camera with a near infinite shutter speed this would not happen - except for the fact human perception is also limited by a similar frame rate (that's why films look fluid at only 25 FPS)