Why do rims in motion spin backwards when seen on tv?

[dangereuxjeux]

Beer has it right. Because the wheel is revolving at a rate that makes it closer in the backwards direction than in the forwards direction, the next time a frame is taken, either by your eyes or by the camera, it looks like it's going backwards. You sense motion based on your brain's interpretation of the series of pictures generated by your eyes.

Example: Take a four spoke rim. If it were to spin 330 degrees forwards in the time it took the camera to take the next frame, then the spokes would be closer to their previous position in the backwards direction than in the forward direction, and so when you viewed the footage with your eyes, your brain would interpret the wheel as spinning backwards.

Same thing with your eyes directly, except that your eyes themselves have a much higher refresh rate, you might say, so the math would be different.

Here, have fun: http://www.michaelbach.de/ot/mot_strob/

 

[her209]

So you have something to wonder about.

 

[CTho9305]

Originally posted by: bradruth

Originally posted by: rh71
Let's think about this... it's true it doesn't have to be on tv only... I've seen where fans spin the "correct" way at first but as it goes even faster, it appears to spin the other way with the naked eye. Do we see stuff at fps ? Something about us not being able to discern faster than 24fps (on a tv) anyway ?

I think the human eye can distinguish up to ~60fps.

Your eyes don't really detect discreet frames like the way a computer renders videogames.

 

[InverseOfNeo]

Originally posted by: CTho9305

Originally posted by: bradruth

Originally posted by: rh71
Let's think about this... it's true it doesn't have to be on tv only... I've seen where fans spin the "correct" way at first but as it goes even faster, it appears to spin the other way with the naked eye. Do we see stuff at fps ? Something about us not being able to discern faster than 24fps (on a tv) anyway ?

I think the human eye can distinguish up to ~60fps.

Your eyes don't really detect discreet frames like the way a computer renders videogames.

In a way you are right but on the other hand the human eye can only "see" at like ~30 fps. My electronic circuits class experimentally figured that out using ~250 students.

 

[AgaBoogaBoo]

Great thread! I learnwed something today, hehe

 

for the people who haven't seen it in real life, and not on TV - pay attention to ceiling fans and helicoptor rotors when they start up or take off