Why do spinning mechanisms (such as helicopter rotor blades) appear to be moving very slowly once they spin fast enough?


What is the actual physical process happening here?

In: 0

That’s only when recorded on camera and played back.

Because both the recording device and the display device will have a set frequency of capturing/displaying still images.

And those frquencies will not be the same as the speed of rotation of the rotors/wheels they’re re ording, especially as the rotors/wheels are changing speed.

So your eye is going not going to see the smooth speed of the spinning wheel that it would in real life. Instead it’s going to be seeing the frames captured/displayed by the devices, and those will appear to your eye to be out of order from what’s expected at certain speeds.

(Your eyes have a refresh rate too, but it’s faster and fuzzier than typical recording devices, and also your brain automatically conpensates for that!)

This “effect” only happens on recorded video, not when observing with your own eyes.

This is because video cameras have frame rates (typically 24, 30 or 60 fps), meaning they’re capturing an image every 1/24, 1/30 or 1/60th of a second.

Say a camera is shooting at 30 fps, there are a few things that can happen:

1) If an object is rotating at the exact same speed as the framerate (i.e. it makes one complete rotation every 1/30s) the rotating object [will appear stationary](https://www.youtube.com/watch?v=yr3ngmRuGUc).

2) If the object is rotating slightly faster than the framerate (i.e. slightly more than one complete rotation every 1/30s), it will appear to be moving in slow motion. Because every time the camera captures an image, the object will appear to be rotating just slightly clockwise, but in realty has rotated a full 360-degrees clockwise *and a bit*.

3) Similar to #2, if the object is rotating slower than the framerate (just under one revolution per 1/30s), it will appear to be rotating backwards because at each frame capture, the object will have almost *but not quite* completed a 360-degree rotation. Thus it will appear to be slowly rotating counterclockwise, relative to its position in the previous frame.

If you could stare perfectly still at a perfectly still object, you’d go blind as your cells overstimulate (think of the after image from looking at a bright light). This is your retnal cells getting tired. To keep them from getting tired, your eyes saccade, make tiny jumps from one point to the next. This gives them something new to see and a chance to rest a tiny bit.

Your brain is lazy. It hates how there is so much motion blur when your eyes move around so much and so quickly. Since the image is lost in the blur anyway, it ignores any signal from them while your eyes saccade. You go blind for a tiny fraction of a second. Your brain just assumes whatever was there is still there until your eyes get to the next spot and you get another look. That’s why you never see it.

(Try it out. Look at one eye in a mirror. Then shift your gaze to the other eye. You wont see your eyes move from one to the next)

This gives us a sort of frames per second. If an object can spin around and appear in the same spot by the time our eyes transition from one saccade to the next, our brain see’s that as the object not moving at all, or slowly in a direction, depending on how that matches up.

Final fun fact, when you’re in fight or flight mode, your brain super charges and wants as much information as possible, so speeds up how often it gets information from your eyes. This can give the feeling the world is slowing down, because you’re processing it faster.

In person they just disappear, but on camera they can slow, stop, or even reverse.

Cool 5 year old way to understand if your teenage sibling has a strobe light. Go in a room with a ceiling fan, turn off the lights and turn on the strobe. As you adjust the speed of the strobe, you can make the fan seem to stop, reverse, or go slower or faster. If the strobe fires when the blades are in the same place, your eyes see them as not moving. If they fire when the blades are slightly turned more each time, your eyes see them as turing slowly. The same concept is true with camera that basically take pictures every 24 or 30 or 60th of a second to create the video imagge.