rallain | December 9, 2008VPython 5.0 is mostly out (in release candidate). To tell you the truth, I have been holding off on some vpython programs because I wanted to do them in the NEW vpython, not the OLD vpython. So, if you want to get the new vpython, go to http://vpython.org/index5.html. What is so great about VPython 5.0? Here a couple of things:
Runs in Mac OS X without X11. Maybe this isn't that big of a deal, but it makes me happy. If you don't use OS X, I guess this doesn't matter.
Textures. You can map textures and images onto objects. This won't really help with the calculations, but it will make…
rallain | December 6, 2008This comes from Buzz Out Loud Episode 865 which got the story from Slashdot regarding a possible new technology that would use piezoelectric devices to charge cell phones while you talk. The original article the slashdot story pointed to talked mostly about the advances in piezoelectric devices, but I want to look at the possibility that sound could charge a phone.
First for the basic physics. How do you make sound and what is it? Sound is a compression wave in the air. To make a sound you need something to push the air (yes, I simplified this quite a bit). When that something pushes the…
rallain | December 5, 2008Let me start off by saying I think I first read about this on HowStuffWorks.com (many years ago). So, why are christmas lights so cool? They are cool because they are a whole bunch of lights in series, but they still work if one of the bulbs gets burnt out. If you are not familiar with circuits, a series circuit is one in which all of the current goes through all of the items in that circuit (as compared to the case where the current gets split up). Here is an example of a series circuit with two bulbs.
In this case, the current comes out of the battery, goes through one filament and…
rallain | December 4, 2008Ok - I like Alabama football (sorry, but it's true). There is a clip on youtube of the end of the Alabama-Auburn game where the fans sing the traditional "Rammer Jammer". If you don't know what that is, don't worry. I am not sure I completely approve of the cheer, but it is a tradition. If you are curious, here it is:
So, what is cool about this? Notice that the entire crowd is yelling the same thing, but they are out of sync. Can this be used to estimate the size of the stadium?
I made an audio file from that youtube clip and looked at it with Audacity. Although I am completely…
rallain | December 2, 2008For many people, it is the time of the year to put the bikes away. I live in Louisiana, so now is the time to get the bikes out (too hot in the summer). Learning to ride a bike is a curious thing. Most parents use training wheels to get kids started. I do not think this is the best strategy. In this post, I will focus on teaching bike riding skillz. If you are looking for the physics of bike riding, there are some good links:
David Jones - "The stability of the bicycle" (pdf)This is a great article. The key aspect is that this guy modified a bike in unique ways to see if it was still "…
rallain | December 1, 2008This is something I have thought about for some time. It is also something I (as a father of 4) have a lot of experience with. The problem is these infant car carriers. In the car they are not a problem. The problem is out of the car. Not sure what I am talking about? Here is a picture:
I don't mean to attack the infant carrier industry. I am sure some people really like these things. For me, there is only two situations that are good for taking the seat out of the car.
In a restaurant. The infant seat works well with those toddler high chairs. Most of the current infant seat…
rallain | November 25, 2008You see this all the time in textbooks:
This is often described as "the acceleration due to gravity". Is this really the best thing to call this? No. A better name would be "the local gravitational field" and list it in units of:
Where does g come from? The usual source is from the weight, or gravitational force:
This is the best way to write it (in vector notation) but you will often see it written as:
which looses the vector nature. In the vector notion, the vector g is a vector pointing towards the center of the Earth.
Why not acceleration due to gravity?
Take the following case…
rallain | November 24, 2008This is an old video, but still interesting. In this video clip, Kobe Bryant jumps over a pool of snakes.
You are probably thinking what I thought when I see that - this doesn't look real. Really, it must be fake. Why would Kobe jump over a pool of snakes? Isn't that dangerous (even if there were no snakes in there he could hurt himself). Video analysis is needed. Is this fake or is it real?
Start with my favorite video analysis tool (tracker). Use this to get position time data.
Step 1: Get the movie. I used Mpeg Streamclip to get the movie in quicktime format from the flash video…
rallain | November 23, 2008One of my favorite podcasts to listen to while driving or mowing the lawn is [Buzz Out Loud](http://bol.cnet.com). Buzz Out Loud (BOL) is basically just a tech-based podcast, but very entertaining. Anyway, as part of my attention seeking disorder, I try to get mentioned on the show. I succeeded once when I posted my [Optimal Commuting Speed Calculator](http://www.dotphys.net/page1/page10/efficiency/calculator.html). When I heard Tom and Rafe talking about orbital speeds, I realized this was my chance. Find any tiny flaw in their reasoning and I could perhaps come up with a comment worthy…
rallain | November 22, 2008One 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.…
rallain | November 20, 2008Georgia Tech is playing Miami (not the real Miami, the other one Miami Florida). Right after a commercial, the camera shows this sign from a student (or just person) at the game:
(Sorry if the image isn't that great.) I had to pause the game and take a closer look at this sign. I am not sure I get it. In case you can't see, the sign shows two players and the trajectory of a ball (clearly labeled x and y axes). The equations are:
![Gtequ 1](http://scienceblogs.com/dotphysics/wp-content/uploads/2008/11/…
rallain | November 19, 2008Rarely am I up late enough to watch the Colbert Report, but by chance I saw it last night. I know Stephen Colbert is a big fan of Dot Physics, and so this is a message to him. Stevie, good job. Here is what he said during his show on the "The Word" segment. Just to give you the context, he was making some point about how if you say anything while talking like a hippie, you make what you said untrue:
Colbert: "Net force of the object equals mass times acceleration, MAN. There. I completely invalidated Newton's second law of motion."
First, the good. He said "net force". How many…
rallain | November 18, 2008Big trucks are not as popular as they used to be, but gas prices are going down so maybe this commercial for the ford F-150 will start showing up again:
They say its a real demonstration, but it has seemed odd to me. (they also say this is a professional driver on a closed course and don't try this at home - damn! I was SO going to do that this weekend). Here is my analysis:
Here is the important data I have gathered from the Internets.
The truck (a Ford F-150) stops a plane C-123 Provider
The curb weight of the F-150 is around 5,000 lbs - depending on options and stuff.
The empty weight of…
rallain | November 18, 2008This must have been mentioned in a previous post, but I am too lazy to search. Anyway, I think have enough stuff here (at blog.dotphys.net) to finish moving stuff from my previous version of the blog. What does this mean and why do you care? You care because if you are one of the two people that read these old posts (I am one of the two) then you will see them again.
rallain | November 17, 2008Atwood's machine is the name of a device that looks like this:
Also known as "two masses on a pulley". Surprisingly, this simple device comes up a lot in intro physics texts. It also brings up some interesting issues. I will go over the basic way to solve a problem like this (as an example) and then talk about the other interesting issues it brings up.
Problem: A small, low mass, pulley has a light string over it connected to two masses, m1 and m2. If released from rest, what is the acceleration of the two masses.
Where to start? This is actually a very difficult question for…
rallain | November 14, 2008Here is a video of a guy jumping 35 feet into a pool of water only 1 foot deep.
UPDATE: Apparently, that video went away. Here is another version.
How does this work?
I don't think I even need to do a video analysis of this motion, all the important info is given. I will assume that air resistance did not play a signficant role (and that is a good assumption - or good enough - see this for example: motion of a falling tennis ball). So, here is the situation.
Part 1: guy falls 35 feet 5 inches (10.8 meters).
 for my calculations on my posts. But there is a new game in town: [Easy Java Simulations](http://www.um.es/fem/Ejs/). What is the difference between these two? Which is better for students? Which is better for me? What are their strengths? What other questions are there? What other questions are there?
**VPython**
Let me start with [VPython](httP://vpython.org) since I am more familiar with that. VPython is…
rallain | November 11, 2008[In part I of this post](http://scienceblogs.com/dotphysics/2008/11/interaction-between-light-an…), I struggled to show that a particle in an infinite well can only exist at certain energies. If you try to put a particle with more than one energy, the probability oscillates at a frequency (E2 - E1)/h. So, what is next? Well I think I am ready to attack the photon.
According to the ultimate source of truthiness (wikipedia), [the photon is the elementary particle responsible for the electromagnetic interaction](http://en.wikipedia.org/wiki/Photon). In general, the photon is view as the…
rallain | November 11, 2008One of the fundamental aspects of physics is the study of light and how that interacts with matter. I have been putting off this post - mainly because I am not a quantum mechanic (I am a classical mechanic). There are lots of things that could be done in this post, but I am going to try and keep it limited (and maybe come back to the interesting points later). Also, most of my posts are aimed at the intro-college level or advanced high school level. This will be a little higher. If you are in high school, there is still a lot of stuff for you here.
Let me summarize where I am going to…
rallain | November 7, 2008On MythBusters this week, Adam and Jamie tested the bullet-proofness of various objects. The one that sticks in my mind is the ipod. The said there was a report of a solider being shot by an AK-47, but he was saved because the bullet hit his ipod. To test this, Adam shot an AK-47 at an ipod and it went through. Their conclusion was that he was also wearing body armor. I am not sure I like that conclusion. Why would someone report that the ipod saved him if he was also wearing body armor? Maybe they would, but not sure.
I was thinking, maybe the bullet went through the ipod because they…