### High Schoolers rough out a version of Cavendish's Experiment

Faced with students that didn't quite believe Newton's law of gravitation, Anthony Rennekamp
of Bishop O'Connell High School in Arlington, Virginia, he did what any physicist would do; he had his students build an experiment.  They created a version of Cavendish's experiment using two yard sticks, two one kilogram weights, and a few 50 pound dumbbells.  You can find the write-up of the experiment on the Physics Today website[1].  Mr. Rennekamp and his students also made a video of the experiment.  Given the slow velocities with which the masses move in this experiment, the class reasoned that speeding up a video was the only practical way to view the results.  They published their experimental results video on youtube, (you can watch it below).  There was some online disbelief that the experiment could work as well as it apparently did.  I thought it would be fun to just run a few back-of-the-envelope calculations here and see if their results are reasonable.

The equation for the gravitational attraction between two masses is

$F_g = G\dfrac{m_1m_2}{r^2}$

Where $G = 6.67 \times 10^{-11} m^3/kg\cdot s^2$ is the gravitational constant, $m_1$ is the mass of the first object in kg, $m_2$ is the mass of the second object, and $r$ is the distance between the objects in meters.  As you can see above, as the objects move closer to each other the force due to gravity will increase as $r$ decreases.  Let's ignore that though, and just make an estimate with the force calculated using the initial distance between the two masses, (the mass suspended on the end of the yardstick and the dumbbell).  The distance looks like 7 cm on the video, so we get:

$F_g = 6.67 \times 10^{-11} m^3/kg\cdot s^2 \dfrac{1 kg \cdot 22.68 kg}{0.07m^2} = 3.08 \times 10^{-7} N$

Then, using that force, we can get the time it should take the $1 kg$ mass to travel the $7 cm$:

$a = F/m = 1.51\times 10^{-7} m/s^2$

$distance = 1/2 a t^2$
$t = \sqrt{\dfrac{2*distance}{a}}$
$t = 673 seconds = 11.22 minutes$

While this is only an estimate, (it ignores the dependence of the force on distance; it also ignores the second set of masses), it comes out in the same ballpark as the effect measured by Mr. Rennekamp's class who found that it took 10 minutes for the hanging mass to swing to the dumbbell.  As an extra test, the class placed the dumbbells on the opposite sides of the yardstick and found that the yardstick twisted in the opposite direction as expected.  Pretty Cool!

References:
1.  Write-up of the experiment
http://scitation.aip.org/content/aip/magazine/physicstoday/news/10.1063/PT.5.2025

### Cool Math Tricks: Deriving the Divergence, (Del or Nabla) into New (Cylindrical) Coordinate Systems

The following is a pretty lengthy procedure, but converting the divergence, (nabla, del) operator between coordinate systems comes up pretty often. While there are tables for converting between common coordinate systems, there seem to be fewer explanations of the procedure for deriving the conversion, so here goes!

What do we actually want?

To convert the Cartesian nabla

to the nabla for another coordinate system, say… cylindrical coordinates.

What we’ll need:

1. The Cartesian Nabla:

2. A set of equations relating the Cartesian coordinates to cylindrical coordinates:

3. A set of equations relating the Cartesian basis vectors to the basis vectors of the new coordinate system:

How to do it:

Use the chain rule for differentiation to convert the derivatives with respect to the Cartesian variables to derivatives with respect to the cylindrical variables.

The chain rule can be used to convert a differential operator in terms of one variable into a series of differential operators in terms of othe…

### Lost Phone

We were incredibly lucky to have both been in university settings when our kids were born.  When No. 1 arrived, we were both still grad students.  Not long after No. 2 arrived, (about 10 days to be exact), mom-person defended her dissertation and gained the appellation prependage Dr.

While there are lots of perks attendant to grad school, not the least of them phenomenal health insurance, that’s not the one that’s come to mind for me just now.  The one I’m most grateful for at the moment with respect to our kids was the opportunities for sheer independence.  Most days, we’d meet for lunch on the quad of whatever university we were hanging out at at the time, (physics research requires a bit of travel), to eat lunch.  During those lunches, the kids could crawl, toddle, or jog off into the distance.  There were no roads, and therefore no cars.  And, I realize now with a certain wistful bliss I had no knowledge of at the time, there were also very few people at hand that new what a baby…

### Lab Book 2014_07_10 More NaI Characterization

Summary: Much more plunking around with the NaI detector and sources today.  A Pb shield was built to eliminate cosmic ray muons as well as potassium 40 radiation from the concreted building.  The spectra are much cleaner, but still don't have the count rates or distinctive peaks that are expected.
New to the experiment?  Scroll to the bottom to see background and get caught up.
Lab Book Threshold for the QVT is currently set at -1.49 volts.  Remember to divide this by 100 to get the actual threshold voltage. A new spectrum recording the lines of all three sources, Cs 137, Co 60, and Sr 90, was started at approximately 10:55. Took data for about an hour.
Started the Cs 137 only spectrum at about 11:55 AM

Here’s the no-source background from yesterday
In comparison, here’s the 3 source spectrum from this morning.

The three source spectrum shows peak structure not exhibited by the background alone. I forgot to take scope pictures of the Cs137 run. I do however, have the printout, and…