# measurement

"?There are two types of nation" a recent Nature editorial begins; "...those that use the metric system and those that have put a man on the Moon."
Such a pro-American jingoistic statement must be deep British irony. Anyway, the editorial continues ...
The reliance of the United States on feet and pounds, along with its refusal to embrace metres and kilograms, baffles outsiders as much as it warms the hearts of some American patriots. But it is time for the country to give up on the curie, the roentgen, the rad and the rem.
This is about measuring radiation. There are several ways of…

It's been a long and brutally busy week here, so I really ought to just take a day off from blogging. But there's a new paper in Science on quantum physics that's just too good to pass up, so here's a ReasearchBlogging post to close out the week.
Aw, c'mon, dude, I'm tired. What's so cool about this paper that it can't wait until next week? Well, the title kind of says it all: they measured the average trajectories of single photons passing through a double-slit apparatus. By making lots of repeated weak measurements at different positions behind the slits, they could reconstruct the average…

I might as well make a new tag called "basketball throws" because I can't stop with the analysis of these crazy basketball shots. Watch - in the end someone is going to post a video about how all these were faked (and I have said there is no clear evidence they are fake). Oh, if you want to see some shots that I am talking about - just search for Dude Perfect on youtube.
Physically, these crazy shots are possible. Time of flight in the video is comparable to a numerical model. But, the question is: how difficult are these shots? Are these one in a million? Are they easy? Are they…

There is something I am working on that will need some type of angle sensor. I am going to use a potentiometer. First, who came up with that name? Isn't this name the same as a voltmeter? Something is wrong with that name. How about we just call it a variable resistor or something like that?
These things are pretty easy to find if you have some old electronic stuff. Here is one I found in our "junk room".
This one was used in a lab as a variable resistor with plugs built in. The normal potentiometer has three connections; one for each end and one for the variable position in the…

This week's big story in physics is this Science paper by a group out of Austria Canada (edited to fix my misreading of the author affiliations), on a triple-slit interference effect. This has drawn both the usual news stories and also some complaining about badly-worded news stories. So, what's the deal?
What did they do in this paper? The paper reports on an experiment in which they looked at the interference of light sent through a set of three small slits, and verified that the resulting pattern agrees with the predictions of the Born rule for quantum probabilities.
What does Matt Damon…

There are several free iPhone-iPod Touch apps that let you look at the acceleration of the device using the built in accelerometer. I was planning on reviewing some of these free apps, but I didn't. When I started playing around with them, it was clear that I needed some way to make a constant acceleration. There are two simple ways to do this - drop it, or spin it in a circle. I decided to go with the circular motion option because I like my iPod and because Steve Jobs told me to.
While playing with this, I realized that the acceleration depends on the distance of the sensor from the…

Note: The following is a repost from some time ago. Today is the officially day to celebrate Dr. Seuss, so here is my Seuss-related post. Enjoy.
In the second Cat in the Hat book (I think it is the second one), the Cat reveals that he has more smaller cats under his hat. They are labeled A - Z with Z being so small you can't even see. Question: What is the sequence of sizes for successive cats? How big would Cat Z be?
Here is the first picture that Cat reveals Cat A. It is not trivial to measure their relative sizes because they are in different positions. I drew two circles, one…

Some time ago, I wrote about the awesome things the Greeks did in astronomy. Basically they calculated the size of the Earth, distance and size of the moon and distance and size of the sun. The value obtained for the distance to the sun was a bit off, but still a bang up job if you ask me. (where bang-up is meant as a good thing) If the greeks were in my introductory physics lab, they would need to include uncertainties with their measurements. What would the uncertainty in the final value look like?
In my introductory physics lab course, I have students measure things and estimate the…

My friend Konrad showed me this awesome toy he made.
I know it doesn't make sense yet, so let me explain. Basically, you take this marble and roll it down the tray through the pegs. The pegs sort of randomize where the marble rolls through the hidden section. Inside the hidden section, it looks like this:
The object of this toy is to find the cross-sectional area of the rings. Konrad said he built this based off of a toy he was given in middle school. He wasn't told how to do it, just to do it. Maybe I shouldn't say anymore about that toy except that it is awesome. No instructions,…

This is really a lab that I have students do, but I am pretty sure they don't read this blog - so it is ok. If they are reading this, hi!
We have these projectile cannons that shoot small balls. In order to look at projectile motion, they need to first determine the launch speed of the ball. I have a great method for this. Basically, shoot the ball horizontally off the table and measure how far horizontally it goes. You can get the final location of the ball by having it hit a piece of carbon paper on top of normal paper. If you don't know what carbon paper is, you are young.
Anyway,…

I think the Mythbusters have a wonderful opportunity for educational outreach. Take this week's episode. One myth was to see if arrows fired from a moving horse penetrated more than arrows fired from a standing position. They first did this with real horses, but they said the data was not convincing.
I am pretty sure they had more than 10 trials recorded (there was a glimpse of the notebook). I would love to see this data and find (or let students find) the standard error of these measurements. This would be a great exercise to see how this whole uncertainty thing works.
As long as I am…

I previously talked about measurements (some) when I looked at the uncertainty in the distance to the Sun. One of the simple ways of determining the uncertainty of a calculated quantity is to use the uncertainty of the measured variables and find the max and min that calculated quantity can be. The example I used was in calculating the uncertainty for the area of a rectangle. So, the maximum and min areas would be:
And then the uncertainty in the area can be described as:
Yes, I know this is not as sophisticated as the normal procedure for error propagation, but it works. This is the…

Some time ago, I wrote about the awesome things the Greeks did in astronomy. Basically they calculated the size of the Earth, distance and size of the moon and distance and size of the sun. The value obtained for the distance to the sun was a bit off, but still a bang up job if you ask me. (where bang-up is meant as a good thing) If the greeks were in my introductory physics lab, they would need to include uncertainties with their measurements. What would the uncertainty in the final value look like?
In my introductory physics lab course, I have students measure things and estimate the…

Previously, I talked about science fairs. One of the problems is that students don't really have a good understanding of data analysis. For me, statistical analysis is just something to do with data. It isn't absolutely true. So, it doesn't really matter that students use sophisticated tests on their data. The important point is they use some type of test to compare data.
I just made up some arbitrary data analysis rules. Maybe if students and judges accept something like this, it could really improve science fair projects and judging.
To explain my analysis, I decided to have my own…

I'm a little cranky after a day of reviewing grant proposals, so it's possible that I'm overreacting. But commenter Neil B has been banging on about quantum measurement for weeks, including not one, not two, but three lengthy comments in Tuesday's dog post.
For that reason, I am declaring this post's comments section to be the Official Neil B. Quantum Measurement Thread. Until such time as I declare the subject open again, this is the only thread in which I want to see comments about quantum measurement. Attempts to bring the subject up in comments to other posts-- even other posts having to…

One of things I like to think about in science is "how do we know that?" It is interesting how one thing builds on another. This is a story of how the Greeks estimated the distance from the Earth to the Sun (an important idea in the development of the model of the solar system). I like this story because it is not too complicated. In fact, one could easily reproduce these measurements themselves. So, here is what I will talk about:
Measuring the size of the Earth.
Determining the distance from the Earth to the moon and the size of the moon.
Calculating the distance (and size) to the Sun.…

Suppose I am working on a problem and I wish to calculate the density of something. I measure the mass to be *m* = 24.5 grams and the volume is *V* = 10 cm3. In this case the density would be:
![Sigfig 1](http://scienceblogs.com/dotphysics/wp-content/uploads/2008/09/sigfig-1…)
ALERT! ALERT! ALERT! This is not a test!!!! Something is drastically wrong! Clearly I messed up. How can I have the mass measured to **3** significant figures, the volume measured to **1** significant figure, but the density calculated to **3** significant figures? Isn't this a violation of some fundamental…