Why do turning wheels look stopped in video?

[Stargazerfrank]

why in the video do turning wheels look stopped.

http://www.youtube.com/watch?v=CjAeMhuy5vs

[FlyingPhotog]

Camera Shutter is freezing the wheels in (nearly) the exact same orientation in each frame...

[cccc]

my favorite is when the wheels go backward then forward then backward then forward.

[basroil]

Called having too low of a sampling rate for the frequency you are capturing. The closer your sampling frequency is to the circular frequency (or an integer multiple of the sampling frequency to the circular frequency), the closer to being still you are to not seeing anything happening. Simply shoot at a higher frequency (if your camera supports 30fps and 60fps, use 60 rather than 30), or have a longer exposure (the more motion blur there is, the less you notice it). And as cccc said, just a bit past the sampling frequency, things will look like they are going backwards, then they will jump back and forth (if the wheel is symmetric, you won't notice it, but on five part rims you will), then speed up only to slow down again.

[RWatkins]

Why? Rotational frequency/# of spokes = Shutter Speed of Camera

Same principle as a timing light.

Actually, in lab we have a machine called a CentriVap and it has a strobe light on a 'gun' that will only flash once for every rotation of the spindle. Anyhow, the top is glass (but there is a mild vacuum internally). The point is, where the light is pointed, the tubes, though spinning several thousand RPM, looks suspended.

http://www.vwrsp.com/catjpg/070/070198.jpg

[nuffi]

It can happen just when you're looking at the wheels of cars driving down the freeway. No need for funky strobes or shutter speeds, just your eyes and the sunlight.

[RWatkins]

It can happen just when you're looking at the wheels of cars driving down the freeway. No need for funky strobes or shutter speeds, just your eyes and the sunlight.

Have you seen this? Could it be that your are actually an android but have not been told yet?

[Mu Eugene]

You can see it happen in propellers of a plane with naked eyes too.

Spinning top also - it's one of those fascinating phenomena that would be nice to have Dr. Michio Kaku explain to us in a simple, plain English in one of his shows on Science Channel.

[basroil]

You can see it happen in propellers of a plane with naked eyes too.

Spinning top also - it's one of those fascinating phenomena that would be nice to have Dr. Michio Kaku explain to us in a simple, plain English in one of his shows on Science Channel.

I prefer Dr. Hiro Mukai (electrical engineering professor) for explanations. For human eye, you CANNOT see the blade stopped, since your eyes work much differently than a camera. You can, however, see dark and light blurs, perhaps even slightly defined ones against direct sunlight, and you theoretically could see the blade stopped, but only under a pulsed light source (as mentioned above)

[gjl711]

It's called stroboscopic effect.
http://en.wikipedia.org/wiki/Wagon-wheel_effect
http://en.wikipedia.org/wiki/Stroboscope
http://www.youtube.com/watch?v=t8AOSymZNPQ

[basroil]

It's called stroboscopic effect.
http://en.wikipedia.org/wiki/Wagon-wheel_effect
http://en.wikipedia.org/wiki/Stroboscope
http://www.youtube.com/watch?v=t8AOSymZNPQ

It's technically called aliasing, stroboscopic effect is a type of aliasing
http://en.wikipedia.org/wiki/Aliasing

It's the same reason why dvds sound better than cds, or why 20MP cameras have much more detail than 10MP cameras, even taking extra resolution into account.