Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Hadrian on 16/09/2006 19:39:08
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Why when you are watching a propeller spinning it suddenly appears to be going the other way?
What you do speaks so loudly that I cannot hear what you say.
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Thats interesting!
Karen
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You mean that this happens even when you watch it in a real situation and not on TV/cinema/Videocamera-film?
If you refer to the second situation, the answer is quite simple: the videocamera used to make the film take a specific number of pictures every second; every time the propeller frequency of rotation is a multiple number of that previous number, you see the picture as if it were still; for a propeller's speed slightly larger, you see it rotating in its real sense, for a sligtly lower speed, the opposite.
Of course the same will happen for every rotating device (wheels ecc.)
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I have never noticed I should check it out!!
Karen
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I first noticed this effect watching a cup anemometer rotate against the sky. As a child it fascinated me. This has nothing to do with the stroboscopic effects one sees in film and videos where car and wagon wheels go backwards but it is in a vague way related to it.
The effect is a geometric one because you are looking at a repetitive structure in which you are unable to see clearly which part of the object is closer to you and the geometry is ambiguous. You can see two possible interpretations of the image and may switch between them. I can demonstrate this geometrical ambiguity effect to you statically if you wish to draw some lines on a piece of paper. Draw two squares of the same size with sides parallel to each other overlapping each other by about half the length of their sides join the corners of the squares together with lines to make a wire frame "cube" you can look at this cube in two ways accoding to whichever of the original squares you think is in front and the cube us either upwards or downwards according to which you chose.
Now return to your rotating propeller you will note that when you "see" it going the wrong way the rotation axis has shifted from when you see it the right way. You can tell which one is correct by noting which one is straight and in which one the shaft appears to have a kink in it.
Learn, create, test and tell
evolution rules in all things
God says so!
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It always looks so cool when it happens
What you do speaks so loudly that I cannot hear what you say.
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I have only been on two airplanes in my Life so I have never seen the propellers spin... on tv movies.
Karen
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quote:
Originally posted by Karen W.
I have only been on two airplanes in my Life so I have never seen the propellers spin... on tv movies.
Oh, you poor deprived thing [}:)]
I had not flown until I was about 16 or 17 years old, but I had been to a good number of airshows in my childhood. Still don't like flying in big jets – don't really like being squeezed into a large aluminium sausage with wings, but I like small aeroplanes, and at one stage even started learning to fly a single engined plane – never got my licence though – the story of my life, all the things I never finished.
George
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quote:
Originally posted by Karen W.
I have only been on two airplanes in my Life so I have never seen the propellers spin... on tv movies.
You could watch a fan's propeller. I have seen such an effect with it, under a neon lamp (electric current's frequency = 50 Hz).
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I done a bit of microlight flying so you get to see your share of propellers doing that
this is what they look like
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.basslake.co.za%2Fimages%2Fpic2microlight.jpg&hash=8640c90caf9a2a531b874fddef10c47a)
What you do speaks so loudly that I cannot hear what you say.
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I am 24 and I have never been on an airplane :-0
-Steven
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quote:
Originally posted by Hadrian
I done a bit of microlight flying so you get to see your share of propellers doing that
I thought about microlights as a cheap way to fly (and probably quite fun, even if maybe a bit exposed - but then if you drop out of the skies, the metal of a small aircraft wouldn't offer much protection anyway, and it's much easier to bring down something ultralight softly than a big metal box). The problem is that I never saw the likelihood of a microlight carrying my 18 stone frame.
Not that at the moment I can even afford cheap flight.
George
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Thats interesting!
Karen
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You mean that this happens even when you watch it in a real situation and not on TV/cinema/Videocamera-film?
If you refer to the second situation, the answer is quite simple: the videocamera used to make the film take a specific number of pictures every second; every time the propeller frequency of rotation is a multiple number of that previous number, you see the picture as if it were still; for a propeller's speed slightly larger, you see it rotating in its real sense, for a sligtly lower speed, the opposite.
Of course the same will happen for every rotating device (wheels ecc.)
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I have never noticed I should check it out!!
Karen
-
I first noticed this effect watching a cup anemometer rotate against the sky. As a child it fascinated me. This has nothing to do with the stroboscopic effects one sees in film and videos where car and wagon wheels go backwards but it is in a vague way related to it.
The effect is a geometric one because you are looking at a repetitive structure in which you are unable to see clearly which part of the object is closer to you and the geometry is ambiguous. You can see two possible interpretations of the image and may switch between them. I can demonstrate this geometrical ambiguity effect to you statically if you wish to draw some lines on a piece of paper. Draw two squares of the same size with sides parallel to each other overlapping each other by about half the length of their sides join the corners of the squares together with lines to make a wire frame "cube" you can look at this cube in two ways accoding to whichever of the original squares you think is in front and the cube us either upwards or downwards according to which you chose.
Now return to your rotating propeller you will note that when you "see" it going the wrong way the rotation axis has shifted from when you see it the right way. You can tell which one is correct by noting which one is straight and in which one the shaft appears to have a kink in it.
Learn, create, test and tell
evolution rules in all things
God says so!
-
It always looks so cool when it happens
What you do speaks so loudly that I cannot hear what you say.
-
I have only been on two airplanes in my Life so I have never seen the propellers spin... on tv movies.
Karen
-
quote:
Originally posted by Karen W.
I have only been on two airplanes in my Life so I have never seen the propellers spin... on tv movies.
Oh, you poor deprived thing [}:)]
I had not flown until I was about 16 or 17 years old, but I had been to a good number of airshows in my childhood. Still don't like flying in big jets – don't really like being squeezed into a large aluminium sausage with wings, but I like small aeroplanes, and at one stage even started learning to fly a single engined plane – never got my licence though – the story of my life, all the things I never finished.
George
-
quote:
Originally posted by Karen W.
I have only been on two airplanes in my Life so I have never seen the propellers spin... on tv movies.
You could watch a fan's propeller. I have seen such an effect with it, under a neon lamp (electric current's frequency = 50 Hz).
-
I done a bit of microlight flying so you get to see your share of propellers doing that
this is what they look like
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.basslake.co.za%2Fimages%2Fpic2microlight.jpg&hash=8640c90caf9a2a531b874fddef10c47a)
What you do speaks so loudly that I cannot hear what you say.
-
I am 24 and I have never been on an airplane :-0
-Steven
-
quote:
Originally posted by Hadrian
I done a bit of microlight flying so you get to see your share of propellers doing that
I thought about microlights as a cheap way to fly (and probably quite fun, even if maybe a bit exposed - but then if you drop out of the skies, the metal of a small aircraft wouldn't offer much protection anyway, and it's much easier to bring down something ultralight softly than a big metal box). The problem is that I never saw the likelihood of a microlight carrying my 18 stone frame.
Not that at the moment I can even afford cheap flight.
George