rallain | July 25, 2009This is not really physics stuff. Well, it could be, but I am not going to take it that way. This is just a post that I consider to possibly add to the usefulness of the internet. When ever I want to do something, I always search online first. Suppose I want to make one of my dive lights into an led - if someone has done it that will help.
So, I am super pumped up. I am getting ready to dive the Oriskany (off the coast of Pensacola) soon with some of my old dive buddies. Will I need a light? You bet. Do I think led lights are awesome? Oh, yeah. Do I want to spend tons of money on a…
rallain | July 23, 2009I am pretty sure this came up on some email discussion listserv. Someone mentioned that students could just look up the answers to homework on cramster.com. Maybe you are more with it than I am, but I had never heard of this. Of course I had to check it out.
At the basic level, cramster gives solutions to homework problems. I was surprised how many introductory physics texts were available. They even had Classical Electrodynamics by Jackson. They did not have the text I use for calc-based intro physics, Matter and Interactions.
So, I tried it out. The site is pretty nice. They have…
rallain | July 21, 2009The Anacapa Society is a professional organization promoting research in all areas of theoretical and computational physics at primarily undergraduate institutions.
If you are at such an institution, I would recommend taking a look and perhaps joining. In my views, undergraduate research is one of the best things about an undergraduate institution. What better way to learn physics than by doing physics. My institution is primarily undergraduate and I can say that although working with undergraduates has it's challenges, it has benefits also. I think it is the thing that smaller schools…
rallain | July 21, 2009Chad is posting some very useful stuff at Uncertain Principles. Basically, he has quite a few scientists that are not professors or such answering some questions. This is a great way for students to get a feel for what scientists actually do. If you don't subscribe to Chad's blog, you totally should. A great resource for physics stuff.
Here are the interviews he has. I am not sure what the P stands for in PNAS, but the NAS is probably Non-Academic Scientist.
If you are a high school or middle school student looking to do some type of report or presentation, this would be a great place to…
rallain | July 21, 2009So, I built a new accelerometer. Why? The jelly-jar one was just not doing it for me. Plus, the cork was starting to make the water all yellowy. It was a good start, but I can do better. What was wrong with the jelly-jar one? First, it didn't let the cork move very far before hitting the wall. Second, it was kind of hard to see exactly where the cork was. Lastly, there was no way to get a reading of the acceleration from the jelly-jar. Now, I am going to fix that.
My new design uses a sphericalish glass flask. The floating bob is anchored in (near) the center of this sphere. Here…
rallain | July 21, 2009This comes up everytime I teach physics for elementary education majors. The curriculum I use (Physics for Everyday Thinking - which is awesome) says that the colors in white light are ROYGBV (Red-Orange-Yellow-Green-Blue-Violet). Typically, I will get a student that says "Hey! What about indigo? Shouldn't it be ROYGBIV?" My first reaction to this was "huh?" Really, does it matter? Here is the spectrum you would see looking at a white light source.
You could break this into as many or as few colors as you like. So, it doesn't really matter. But this leads to a great question: Who…
rallain | July 18, 2009pre-reqs: vectors, kinematics
I haven't done a "basics" topic in quite some time. It's odd, I have used centripetal acceleration quite often, but I never derived the expression that I use. To get to the point, the magnitude of the acceleration of an object moving in a circle is:
Also, the direction of this acceleration vector is always towards the center of the circle the object is moving in. This is really not too difficult to derive (but it does use at least one "trick"). Let me start with an object moving in a circle at a constant speed. I am going to show to instances of the object…
rallain | July 18, 2009Note to self: don't do the mechanical equivalent of heat lab again. It doesn't really work that well and there are better labs to do.
So, what is the mechanical equivalent of heat lab? It is actually a pretty cool idea. Take and object and drop it. What happens to the kinetic energy the object had right before it hit the ground? Most of it goes into thermal energy of the object and surroundings. In this lab, the students measure the change in gravitational potential energy for a falling object (where object is really lead shot or something) and then measure the change in temperature in…
rallain | July 16, 2009Yesterday, I uploaded a post from my old site about slowing down time to make videos look like they were on the moon. If you haven't read that, the short story is that it doesn't look right when speed Apollo videos up so that the acceleration is -9.8 m/s2.
What started that whole thing was an even earlier post. Here is an example of using video analysis (Tracker Video Analysis) to measure the acceleration of a jumping astronaut on the moon. In this case, it is the famous "jump salute" from Apollo 16 astronaut John Young.
First, thanks John (or whoever videoed it). This is a good video for…
rallain | July 15, 2009Note: This is a repost from my old site. Time to move it over.
First, what is different about motion on the moon and on Earth? Since the moon has a smaller mass in spite of its smaller size*, the moon has a smaller gravitational field. The gravitational field on the surface of the moon is 1/6th the field on the surface of the Earth. This means that the acceleration of a free falling on the object would be 1/6th the acceleration on Earth. So the question is: how would you change the time scale of a movie so that it looks like its acceleration is 1/6th of 9.8 m/s2?
*If you make the size of…
rallain | July 14, 2009This has been on my list for quite some time. Really, it must be since i posted about measuring acceleration in free fall with an iphone. So, this post will be all about accelerometers.
How does an accelerometer work? Really, an accelerometer measures force some way on a known mass. Let me show an extremely simple accelerometer - a mass on a spring.
(image from Science Buddies where they have instructions on building such a device)
Suppose I put this accelerometer in a stationary and non-accelerating elevator. Let me draw a free body diagram for the mass on the end.
No magic here, right…
rallain | July 11, 2009My friend J sent me a link to this gyrobike (http://www.thegyrobike.com/index.php). From what I can tell, it's a flywheel that you put in the front wheel of a bike. The site claims that this will help kids learn how to ride a bike.
So, what do I think? Clearly, this is a real product, but I am not so sure how effective it would be. As I mentioned in a previous post, it's not really about angular momentum that prevents you from falling over. Well, actually I didn't really say that. I referred to an excellent article on the physics of bicycles, David E. H. Jones and published in Physics…
rallain | July 8, 2009What do you see when you are in a completely dark room with no lights?
That is a great question to ask. It can bring out some interesting ideas. Anyway, here is an easy demo to show the color black. The basic idea is to build a box that has a small opening. Here is what it looks like from the outside:
As you can see, just a basic box. I have a door on the top, and I put a paper towel tube for a window. To make it look pretty, I covered it with black paper (so you couldn't tell where I stole the box from).
To demo this to students, I first go around and let everyone look inside. I ask…
rallain | July 6, 2009July 4th can be fun. One activity my family enjoys is playing in the lake at my parents house. Along with this comes the jumping off the dock. Great fun, and great physics. Here is a short clip.
Work Energy Example from Rhett Allain on Vimeo.
Notice that I violated my own rules for making videos. In particular, the camera was not perpendicular to the motion. Also, I can handle panning cameras, but not when there is nothing but sky in the background. This video is therefore not appropriate for a video analysis. That is ok. I don't need it to talk about physics. So, here is the…
rallain | July 4, 2009I created another screen cast showing how to get started with a numerical calculation for one of my classes. In this case, the students are attempting to model the motion of an object falling with air resistance. The following example doesn't show the steps including the air resistance, but maybe it is enough to get someone started.
Record your screencast online
rallain | June 29, 2009Slate's Explainer has an answer and question post about moonwalking. Here is one of the very good questions answered there.
Would it be easier to moonwalk on the moon?
The Explainer says "absolutely not" and attributes this to the awkwardness of walking on the moon. The article gives an example of Earthly legs being too powerful as the "astronaut's hop". The explainer also says it is awkward because of the pressurized space suits.
I think the problem is almost entirely the pressurized suits. I believe that the astronauts do their moon-hop because it is difficult to bend their legs in the…
rallain | June 29, 2009I finally saw the movie Hancock. Yes, I know it has been out for a long time but I don't get out much. You know me, I can't leave something like this well enough alone. It's not my fault, I was born this way. It shouldn't spoil the movie too much if I tell you this one scene (you have probably already seen it anyway).
Basically, Hancock gets upset with this boy and throws him in the air to scare him or something. In case you did not time it, the kid was in the air for 23 seconds. I claim that in order for Hancock to throw a person in the air for this long, the acceleration during…
rallain | June 27, 2009I have already talked about science and why everyone should take some science courses. The short answer (in case you don't want to read the previous post) is that everyone should take science because science (along with art and other stuff) is what humans do.
So, tomorrow I will be off to attend a workshop on a new physical science curriculum. The goal is to bring more active-learning styles courses to large enrollment classes. This is essentially a similar idea to the Physics for Elementary Teachers course that I teach with one major difference - size. I would love to teach the course…
rallain | June 26, 2009Was the moonwalk fake? No, not the Apollo landings. I am talking about Michael Jackson's moonwalk. You got to admit, he had a big impact on a lot of stuff and this is my way to give him respect - physics.
I am sure you know about the moonwalk. Maybe you can even do the dance move yourself, but how does it work? First, here is a clip of MJ doing his stuff.
As a side note, I can't remember where I saw it but there was a great discussion of the history of the moonwalk. If I recall correctly, some were saying Michael didn't create this move. One thing is for sure, he made it popular. Now…
rallain | June 24, 2009I made this screen cast for my algebra-based lab. Maybe you will find it useful also. This is a tutorial using Tracker Video Analysis (an awesome free program as I have said many times). In this tutorial I analyze a moving cart that shoots a ball up and lands back in the moving cart (called a Howitzer cart). The video is available at the LivePhoto Physics site.
Screencasts and videos online