# Frequency, Time and Calculations

Discussion in 'Mathematics and Physics' started by xheavenlyx, Oct 20, 2006.

1. ### xheavenlyxNew Member

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Since I could not post in the Theory section, I am threading it here.

I had some calculations and here goes the Q:

1. If I take a LED and while holding it, spin arround (yea its dizzing). What should be the frequency of flash so that I see a small line of dash of about 2 cm. Assuming my arm length to be about 90 cm and rate of spin 'normal'. Since I dont know at what rate I am spinning, It can be enough to calculate a frequency range of the LED flashing.

It could be helpful if someone can give a general way and formula of solving these kind of problems.

Regards!

2. ### PapabravoWell-Known Member

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If you are holding the LED, while both you and the LED are spinning then you see the same thing as if you were standing still. Oh...I get it, the wire from the LED has wrapped around your neck and cut off the flow of oxygen to your brain.

If you must be obscure, please try to be obscure clearly!

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Lets assume that you are spinning the LED around your stationary point of reference at 2 times per second(f=2Hz). Since 2*pi*radius = circumfrence and your radius is equal to 90cm, (C=180pi cm). Manipulate equations for motion to get the time required for the point of interest (the led) to complete the an arc length of 2 cm on the circular orbit.

t = arc length / (frequency * circumfrence)
t = (2cm) / (2Hz * 180pi)
t = 0.0017....s

-jacob

ps. dont let the wire wrap around your neck like earlier described

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5. ### xheavenlyxNew Member

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Lol @papabravo!! Sorry about that, i mean If I am spinning and someone else is watching the led. Or if I am looking at my brother take a LED and spin arround.(We both have to be careful about the wire chocking us! ) I should see a 2 cm line on every turn. Its like sticking a LED on a tire and driving your car...you should see it light up only on the top part for 2 cm aec asuming any speed of car for calculation as an example.

And jacob, thats exacly what I was looking for. Thanks a lot!!

Anyway, its interesting to see a 8 led combo stuck on your wheels. While driving, it can have different effects, if we program it in a certain way. (By calculations provided by jacob)

6. ### PapabravoWell-Known Member

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The apparent width of the line may vary depending the lens. IIRC the half angle of a typical LED is 30 degrees. This means that the apparent intensity falls off to about one half, if your angle with the centerline of the lens is 30 degrees.

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8. ### GastonMember

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i beleive that the human eye see's anything that flashes at 20Hz or more apears to be solid. so could we therfore conclude that 20revolutions per second would be the minumum that would appear to be a solid line?

9. ### RODALCOWell-Known Member

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run the LED from single rectified AC, The LED should pulse at 50 or 60 Hz. not sure what country you are from, and look from behind a fan. You may notice a drift in the LED ON and OFF states.

Look at the TV from behind a fan and you will notice the ON / OFF ratio of the TV screen.

Just a different approach.

Regards

Raymond

Similar effect as used in a movie film prjector where the shutter blades rotate in front of the film.

10. ### xheavenlyxNew Member

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@jacob.zurasky you movie was removed due to tearms violation? lol What did you put in there?

@RODALCO Thanks for the tip.

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