[Phys-L] angular momentum demos; was: bike wheel handles

[John Denker <jsd@av8n.com>]

On 05/26/2013 10:08 AM, Dr. Holly Priestley wrote:

> > One uses these on a piano stool, no?

On 05/26/2013 10:36 PM, I replied:

> Yes ... and no.  There are a lot of demos you can do with a 
> bicycle wheel, some of which involve a stool ... and some of
> which don't.

Let's take a step back and look at the larger picture:

Strictly speaking, you don't want a stool but rather an 
office chair that swivels.  People can easily fall off 
a stool, but it is harder to fall out of an office chair.
Also a typical stool is too tippy, because it is too tall
in comparison to the width of its base.  (Even an office
chair can be tipped over, so you still have to be a little
bit careful.)

A) To demonstrate conservation of momentum, you /could/ 
use a chair and a heavy wheel ... but it is simpler to 
just use a beanbag on a string.
  http://www.av8n.com/how/htm/motion.html#fig-angular-pull

I recognize that the chair + wheel provides an entertaining 
spectacle.  It is however not the only way -- nor even the
easiest way -- to illustrate conservation of angular momentum.

B) As mentioned yesterday, to demonstrate gyroscopic stability 
and precession, you don't need a chair, and you don't need 
a heavy wheel.  A lightweight, rapidly-spinning wheel works 
great.

C) To demonstrate vertigo, you don't need a chair or any
other apparatus;  it suffices to ask people to stand and
spin themselves in place.

D) You can demonstrate the Coriolis effect by sitting in
a chair, spinning, and moving you arm radially.  Extend
your arm in front of your face, and then pull it in, as
if you were going to almost punch yourself in the nose.
In this situation the Coriolis effect is not huge, but 
it is detectable.

===

To summarize:  There are experiments that involve a swivel
chair and experiments that involve a wheel, or both, or
neither ... but the one experiment in the "both" category
is nowhere near being my favorite experiment.  I haven't 
done that one in many years.

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

One more thing:  For the chair + wheel demo and some of 
the other demos, rather than having small handles on both 
sides of the wheel, you would rather have a large handle 
on one side of the wheel, so you can use both hands without
reaching around the wheel.  Use a socket-type axle peg and
insert a broomstick.  Secure it with a set-screw, clevis
pin, or duct tape.

  Start with the wheel in your 3:00 position, spinning in
  a vertical plane.  Then raise it to the overhead position,
  spinning in a horizontal plane.

If you do this, there's no longer any advantage to using
a small wheel.  Remember that the angular momentum goes
like the square of the radius, so in fact there is a 
noticeable advantage to using a full-sized bike wheel,
rather than a BMX wheel.

On 05/25/2013 09:47 AM, John Caranci wrote:
> > I just bought a threaded rod with 2 nuts of the correct diam. Then I
> > coated it threads using silicone. It works great but you might sleeve
> > it so the bearings slide better.

I don't recommend using ersatz bearings.  There is a premium 
on having really good bearings.  Many of these demos work best
at high rotational speeds ... and precession can put a huge
stress on the bearings.  A quality bike wheel comes with
amazingly good bearings:  low mass, high strength, and low
friction.