Why does the wheel of a fast moving car looks like it twirls back for moment and back to the front again?
In: Physics
This is called the stroboscopic effect, which is essentially an “error” of interpretation when continuous motion is represented by instantaneous pictures (so basically flickering)
If a wheel rotates in one direction and accelerates, it will reach a point where, if we consider the “frames” one by one, could be interpreted as moving backwards aswell.
For example, if you see two pictures of a wheel and the wheel in the second picture is rotated by 350°. How do you know it’s rotated by 350° in one direction and not by 10° in the other? Well without further background knowledge you simply don’t.
The human brain is prone to such illusions and some of them depend on said background knowledge, like these illusions of the rotating face or ballerina where some people see it rotating clockwise, some counterclockwise and some alternating between the two.
We detect motion by comparing the position of two objects within a frame. In rotational motion, our brains checks to see how close the spokes are to their old positions to determine the direction of motion. However, when a wheel is spinning fast enough, the position of the next spoke is closer to where the previous spoke tricking the brain into thinking that it is the previous spoke and it move backwards a little bit. As the wheel’s rotational speed increase, the next spoken moves past the position of the previous spoke, restoring the perception of forward motion.
Destin from smarter every day does an awesome (https://youtu.be/dNVtMmLlnoE) on the topic
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