Re: [Phys-L] more prepping -> accelerometer

[John Denker <jsd@av8n.com>]

[sorry for duplicates -- somehow this thread got cloned onto
multiple listservs]


On 3/9/20 12:45 PM, in a message with a great many good
suggestions, Dan Beeker wrote:

> Perhaps there is an opportunity for PIRA or AAPT to provide a 
> location to post do at home activities and labs with some feedback 
> mechanism to rate how well the activity worked for those who used 
> it?

I suggest not waiting for PIRA or AAPT.  To borrow a slogan from
12 years ago:
	We are the ones we have been waiting for.
That is to say, we should just do what needs to be done.  PIRA and
AAPT can find out about it later.

Specifically:  If you discover or invent a good lab-at-home procedure,
stick the writeup on google docs (or on your personal web site if you
have one) and post a link here.  If there get to be a lot of them,
then somebody can collect and curate all the links.

==================

> • Just ask the question – can you accurately measure the
> acceleration of a falling object using a cell phone?

Yes, you can.

There are a number of excellent free accelerometer apps.  Some of
them come with a built in oscillograph function.  Some will save
all three vector components in .csv format. Available from the
usual app stores.

See additional accelerometer discussion below.

=================

> The things that work well are labs where students must assemble,
> measure and analyze something. Assuming people are not inclined to
> run down to the local hardware store the most challenging aspect is
> using materials found in an average dorm room or student apartment.
> Their furnishings will be quite different than what might be found in
> the typical [list-member's] home.

I'm willing to compromise on this point.  There are some things
they should have, even if they have to buy them.  That includes
a couple pair of pliers (basic slip-joint pliers plus needle-nose
pliers) and some baling wire or coathanger wire.  You can build
a *lot* of stuff out of baling wire, and if the students don't
know how, it's high time they learned.

I'd be willing to insist that they have an electric drill, too.
Wired ones are cheaper, but even cordless ones are available
for less than 20 bucks nowadays.  If students are quarantined
they can mail-order this kind of thing.

=================

Combining the three previous thoughts:

There are some experiments that involve dropping or tossing an
accelerometer, which is not something I plan on doing with my
phone.  However, you can build a nice robust simple accelerometer
out of baling wire plus a dowel rod and some rubber bands.  I
built one, and have enjoyed playing with it.

It teaches some really fundamental physics:

An ordinary accelerometer sitting upright on the shelf reads +1 g (not zero). If you turn it upside down, it reads −1 g. This is as it should be, even though the accelerometer is not being accelerated relative to the lab frame. This may come as a surprise to some folks, because in high school physics it is customary to measure everything relative to the lab frame. However, according to modern (post-1915) physics, there are many situations where you might wish to measure acceleration relative to a nearby freely-falling frame.

A writeup discussing the physics and the construction procedure
can be found here:
  https://www.av8n.com/physics/accelerometer.htm

===========================

Amusing anecdote:

 One day when we were very young, my brother and I were walking home
from school.  We came to a place where workers had been installing
power lines. They had thrown scraps of wire all over the place, pieces
of insulated 2-gauge solid copper wire, a few feet long. Not being
entirely stupid, we grabbed them and carried them home. My father was
a wiz at building stuff out of baling wire, and he could not resist
building stuff out of the wire scraps. In particular, he built
flashlights for us. Each one had a coil of wire to hold the batteries,
a crimp to hold the bulb, and a switch that doubled as momentary or
steady.  The nice thing was that unlike an off-the-shelf flashlight,
it was super-easy to trace the path of the current.

This made a huge impression on me.  You can build a *lot* of nifty
stuff out of wire.