2
$\begingroup$

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.

$\endgroup$
  • 2
    $\begingroup$ It is a well known illusion, see the link en.wikipedia.org/wiki/Wagon-wheel_effect $\endgroup$ – Wolphram jonny Oct 17 '14 at 22:30
  • 1
    $\begingroup$ This question appears to be off-topic because it is about human perception and not physics. $\endgroup$ – Kyle Kanos Oct 17 '14 at 22:35
  • 3
    $\begingroup$ The one word answer is "Aliasing": en.wikipedia.org/wiki/Aliasing $\endgroup$ – Alfred Centauri Oct 17 '14 at 22:40
  • 3
    $\begingroup$ @KyleKanos Its a question about optics. Seems perfectly on topic to me. $\endgroup$ – David Grinberg Oct 17 '14 at 22:50
  • $\begingroup$ This seems a perfectly fair question. Actually I'm surprised it hasn't been asked before, but I can't find a duplicate - a search for aliasing turns up related questions but no duplicates. $\endgroup$ – John Rennie Oct 18 '14 at 5:53
2
$\begingroup$

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)

$\endgroup$
  • $\begingroup$ 25 fps or higher. $\endgroup$ – Gimelist Oct 19 '14 at 3:40

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.