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Time interval with three phones - Printable Version

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Time interval with three phones - Erik Josefsson - 09-19-2021

Time interval with three phones

If phone one takes a photo (with flash) of phones two and three while both measure light you should get two stopwatches starting at the same time.

See gif below (sorry for quality and that only one phone is triggered in the gif).
   
Is there a simple way of assessing how accurate such a measurement can be?

(for example, as in the gif, when measuring the speed of a passing LED-light mounted on a toy car)

Best regards.

//Erik


RE: Time interval with three phones - Jens Noritzsch - 09-21-2021

On the technical side, the sensitivity and rate of light sensors heavily vary between smartphones. As far as I know, we have not seen rates beyond 10 Hz, more likely lower.

For a start, I would check how precisely the time between to flashs could be measured and if you observe differences when switching the two stopwatches. I wonder if the smartphone torch would work better as a signal?


RE: Time interval with three phones - Erik Josefsson - 09-22-2021

(09-21-2021, 10:54 AM)Jens Noritzsch Wrote: On the technical side, the sensitivity and rate of light sensors heavily vary between smartphones. As far as I know, we have not seen rates beyond 10 Hz, more likely lower.

For a start, I would check how precisely the time between to flashs could be measured and if you observe differences when switching the two stopwatches. I wonder if the smartphone torch would work better as a signal?

Thanks for feedback Jens!

Turned out about half of the class have iPhones where the light sensor seems to be disabled (or blocked so that phyphox cannot talk to it).

The mounted light source in the gif is strong enough, see attached jpg of signals from flash and passing light source. A phone torch would do fine too, but after measuring the horizontal velocity, the toy car is supposed to continue over the edge of a table (free fall). For this experiment, mounting a phone on the toy car would probably work, but maybe just once...

Best regards.

//Erik


RE: Time interval with three phones - solid - 09-22-2021

By the way the acoustic stopwatch is faster...


RE: Time interval with three phones - Erik Josefsson - 09-22-2021

(09-22-2021, 08:54 AM)solid Wrote: By the way the acoustic stopwatch is faster...

Thanks solid. Yes it is. Just realised that most students have cameras that can make slow motion films. Just filming anything in slow motion with a timer in the background solves the problem, see e.g. https://onlineklocka.se/stoppur/ .


RE: Time interval with three phones - Jens Noritzsch - 09-22-2021

Ad light sensor on iDevices: https://phyphox.org/faq/#faqIosLight

I have not put it clearly: I thought of replacing the synchronising flash with the torch, however, judging by your graph, flash appears to work quite fine. Smile


RE: Time interval with three phones - Erik Josefsson - 09-23-2021

(09-22-2021, 12:21 PM)Jens Noritzsch Wrote: Ad light sensor on iDevices: https://phyphox.org/faq/#faqIosLight

I have not put it clearly: I thought of replacing the synchronising flash with the torch, however, judging by your graph, flash appears to work quite fine. Smile

My question was also quite unclear but indeed, both camera flash and a passing maglite type torch work fine as triggers.

Just that I have to find another experiment for this type of measurement when it turns out most student have a high speed camera in their pocket...

Best regards.

//Erik


RE: Time interval with three phones - Jens Noritzsch - 09-23-2021

… and there is https://physlets.org/tracker/ Smile