09-19-2021, 12:52 AM (This post was last modified: 09-19-2021, 12:52 AM by Erik Josefsson.)
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)
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?
(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...
(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/ .
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.
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.
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...
