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Re: [Phys-L] spinner experiments; drag, et cetera.



This just posted online:

www.msn.com/en-us/video/wonder/astronauts-play-with-a-fidget-spinner-on-international-space-station/vi-AAtudIR

On Mon, Sep 18, 2017 at 1:57 PM, John Denker via Phys-l <
phys-l@mail.phys-l.org> wrote:

On 09/17/2017 01:01 PM, Daniel MacIsaac via Phys-l wrote:

Listening carefully we heard a pitched whir, hence a tuner was used.
I got data from a music tuner app on my iPhone called “Cleartune” that I
use for
trumpet /flugelhorn /cornet etc; it displays pitch and digital freq but
does not
graph same. There are many free iPhone FFT apps that graph.


This topic is interesting for several reasons:
*) Introductory physics classes are notorious for assuming
massless strings and frictionless pulleys. This is an
opportunity to get a fairly clean measurement of friction.
*) It is a nice way to make people aware that not every
decay is an exponential decay.
*) It is a nice way to make people aware that the notion
of "coefficient of friction" is not the whole story.
Reeeeally not.


Some suggestions:

1) If you have a photogate lying around, use that.
This is in contrast to the audio approach, which is OK
for "gee whiz" demonstrations, but not for serious data
taking ... especially at the low end of the speed range,
where the frequency of the audio signal is OK but the
amplitude is too small to be useful.

2) Similarly, the FFT is more suited for "gee whiz" demos
that serious data-taking. Instead, just record the signal
from the sensor. Stick the raw data in a file.
++ Looking at the interval between zero-crossings is much
simpler than taking the FFT, and introduces less bias.
++ If you want to get fancy, curve fitting is better than
taking the FFT. It uses more CPU time, but it's easier
on you, easier to understand, and less biased.
-- Taking the FFT is an unhappy medium, with nontrivial
CPU load, nontrivial conceptual issues, nontrivial
operational issues, and nontrivial bias.

3) If you don't have a photogate lying around, you can make
one from scratch. In particular, with a photodetector
plus a dollar-store laser you can make a photogate with
a large gap, so you can measure large things and/or measure
things from a distance.

3a) For a couple of bucks you can get a sensor with a
built-in amplifier:
http://www.ti.com/lit/ds/symlink/opt101.pdf

The alternative is to start with a bare phototransistor
or photodiode, but in that case I would strongly recommend
using an op-amp to boost the signal. The transimpedance
idea (i.e. hooking the collector to virtual ground) greatly
improves the frequency response, because of the Miller
effect, as discussed here:
https://gasstationwithoutpumps.wordpress.com/2015/07/05/
pullup-vs-transimpedance-amplifier/

The USB connector is a convenient source of 5 volt
power for the light source and the photodector circuit.
Some (but not all!) op-amps will run off 5 volts.

3b) You can feed the signal from the sensor to the audio
input of your computer. This is an easy way to record
the data. Use a high sampling rate.

3c) Beware that most of the phototransistors you find
lying around are sensitive in the IR only (because they
are designed around the properties of long-distance optical
fiber). So, either get one that comes with a matched light
source, or get one that is specified to work in the
visual. (Visual makes it a lot easier to align things.)
The so-called "ambient light detectors" that robotics
enthusiasts use are one possibility.

4) You don't have to use a fidget spinner. A bicycle wheel
spins nicely. It has a lot more mass, and (usually)
better bearings.

5) A spinner clamped in a vice will not spin nearly as long
as one in free air. Imagine that due to drag, it entrains
a large parcel of air that spins along with it. Energy
is dissipated when this parcel drags against the surrounding
environment.

Suggestion: Get a spinner that has a hole in the middle,
or at least one where you can snap off the covers so you
can get to the hole that is already there:
http://beecherbowers.com/wp-content/uploads/2017/01/
fidget-spinner-buttons.jpg

Then bolt it to the end of a long, moderately thin rod.
This should allow you to minimize drag.

I have a rod that I drilled and tapped so that I can
screw it directly onto a bicycle wheel.

6) Please put the raw data on the web someplace where people
can find it, so they can play with it. A situation where
people have to screen-grab data points from youtube videos
is kinda unprofessional.
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