/sci/ - Science & Math » Thread #14300738

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Can Light make an active Buzzsaw appear still?

Anonymous Sun 13 Mar 18:18:46 2022 No.14300738 View Reply Original Report

Quoted By: >>14300762 >>14300795 >>14300839 >>14300848

I have a question about the interaction between fast rotating objects and light reflecting off them.

If a typical Buzzsaw blade (Pic related) is turn on, and a specific frequency of light waves would synchronize with its rate of rotation, would the saw appear to be "still". Can anyone elaborate as to why or why not?

Anonymous

Anonymous Sun 13 Mar 2022 18:19:35 No.14300745 Report

Quoted By:

Turned On*

Anonymous

Anonymous Sun 13 Mar 2022 18:26:03 No.14300762 Report

Quoted By: >>14300790

>>14300738
Theoretically yeah if the frequency of the saw is a multiple of the frequency of the light
Practically, no. Both the saw and light changes frequency enough to make it impossible

Did you know aircraft hangars use two different frequencies of lighting for this exact reason?

Anonymous

Anonymous Sun 13 Mar 2022 18:28:30 No.14300768 Report

Quoted By: >>14300801

In principal, yes. You would have to pulse the light at the same frequency that the saw is spinning at, and you'd need a low duty cycle so that the blade isn't illuminated long enough to notice any motion blur

For a saw running at ~3600 RPM a 60 Hz light pulse with a 1-2% duty cycle would probably do the trick.

Anonymous

Anonymous Sun 13 Mar 2022 18:34:47 No.14300790 Report

Quoted By: >>14300831

>>14300762
No I didn't, I am assuming that they use two to prevent the propellers to appear motionless?

Anonymous

Anonymous Sun 13 Mar 2022 18:36:04 No.14300795 Report

Quoted By:

>>14300738
Yes. It's a known safety problem.

Anonymous

Anonymous Sun 13 Mar 2022 18:37:17 No.14300801 Report

Quoted By: >>14300901

>>14300768
Is Duty Cycling referring to frequency of light or is it the actual "turning on and off" of the light. This is how light dimmers work in homes yeah?

Anonymous

Anonymous Sun 13 Mar 2022 18:50:10 No.14300831 Report

Quoted By:

>>14300790
Exactly

Anonymous

Anonymous Sun 13 Mar 2022 18:52:16 No.14300839 Report

Quoted By: >>14300901

>>14300738
literally just point a strobe light at the saw and adjust the frequency of the strobe to be number of teeth*frequency of saw

Anonymous

Anonymous Sun 13 Mar 2022 18:55:50 No.14300848 Report

Quoted By:

>>14300738
This is a good idea, but what do you do about the sound it makes?

Anonymous

Anonymous Sun 13 Mar 2022 19:20:29 No.14300901 Report

Quoted By:

>>14300801
Frequency is how often you pulse the light.
Duty cycle is how much of each period the signal is "on" vs "off"

Ex. 50% duty cycle means the light is on for half of each period and off for half of each period, 10% means its on for 10% of each period and off for 90% of each period.

Frequency of the light signal in this case is matched with the saw to ensure the light turns on when the saw is in the same orientation.
Duty cycle is kept low to make sure the saw is only illuminated for a small amount of its motion, making motion blur less apparent.

If you're just trying to match the teeth as >>14300839 describes, you can get away with a much higher frequency and therefore a much less noticeable flickering (f = Fsaw * Nteeth)

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