rallain | November 10, 2009Maybe this isn't that mysterious. However, quite some time ago my daughter found this rock in the backyard. First, you must understand that this is an odd occurrence. In this part of Louisiana, there really aren't any rocks, just dirt. When she showed it to me, I readily dismissed it as some lava rock from someone's garden. That was the end of the story until a year later when she found it again. This time, I held it and realized it was way too dense to be a a normal lava rock. Holding it up to a magnet, I found that there was some interaction.
Skip forward a little bit of time. I had…
rallain | November 10, 2009This is for commenter JimP. How do you take into account uncertainty when using video analysis? A great question. The first thing to think about is where does the uncertainty come from? My first guess would that it would be from the user. Where does the user click? Is it right on the object in each frame? Is the scale set correctly? I guess there could be other sources of error - maybe there are repeating frames that are a result of encoding. Maybe there is interlaced video frames.
Well, what to do? I will just look at one motion in particular and do the analysis several times. I…
rallain | November 5, 2009Very simply, parallax is an apparent motion of an object due to a change in observation position. Let me start with an example. Here are two photos. I took a picture of the cabinet in the background from two slightly different positions. In the foreground is a clone trooper that did not actually move.
I added the dotted line so you could see how the clone trooper appeared to change positions with respect to the background. Here is a diagram of the camera in the two positions along with the toy.
Since the camera changed positions, the object that is closer appears to have moved with…
rallain | November 4, 2009I already attacked the 2008 Punkin Chunkin Show. So, now I going to give the chunkers some tips. In case you aren't familiar, the Punkin Chunkin contest has teams create devices to launch a pumpkin. They have different categories, but I am going to focus on the air-powered devices. The basic idea is to make an over sized pneumatic potato gun. Here are the things I was inspired to think about.
It seems all the canons were aimed at about the same angle. Did they guess at the angle? Or is this trial and error? What would be the best angle for a pumpkin launch?
Does the optimal angle of…
rallain | November 2, 2009I am sad to say the following comes from a school that I attended. This was from an alumni newsletter regarding the activities of one of the physics faculty.
"Since the time of Archimedes, sciences advanced along two avenues, through new experiments and through theory...."
I have a problem with just that first part, but it goes on:
"For many centuries, theoretical physicists devised clever mathematical methods to describe many physical phenomena, yet some of the most important ones - like the properties of matter, of proteins and living things, or of weather patterns - are far too…
rallain | October 31, 2009Here is the deal. Why would you put a lot of effort into explaining something wrong? I just don't get it. Wouldn't it be easier and better to just not say anything? The particular show I am ranting about is the 2008 Punkin Chunkin on Discovery (I think). Fairly ok show, but they should have left the science out. The part that made me speak out was when they were talking about the different types of punkin chunkers. They have:
Compressed air
Catapult
Trebuchet
Centrifugal Force Machine
To explain these devices (the physics behind them), the show brought in the big guns. Let's bring…
rallain | October 30, 2009In the past couple of weeks, we have had two different publishers 'pitch' their online homework system. First, they are fine people and interesting products, but I just don't think this is going to work.
Online Books
I think the publisher's plan is to have a textbook, and offer extra stuff to go with it (including an ebook). However, for all the ones I have seen, there are problems with the ebooks.
You can't keep them forever. It depends on the terms of the service, and maybe this is long enough. The terms seem to vary from 1 year to 5 years. If you are in certain fields (physic or…
rallain | October 27, 2009I know I saw this demo somewhere. Maybe it was at an AAPT conference a few years ago. I have always wanted to build this, but never got around to it. Until now. Here is the demo (it is easy, you should make one too)
So, how does this work? I think the simplest explanation is that the drinks do not spill because the string can only pull in the direction of the standing glasses. A slightly better explanation is that the string lets the tray rotate so that the sum of the acceleration and the gravitational field is in the direction of the open ending of the cup. I am still not happy with…
rallain | October 24, 2009Sometimes it is difficult to come up with new labs. Ideally, a lab should show use some of the basic physics principles as well as have something the students can measure. What to do with circular motion? I don't know how I forgot this, but here is a lab I used to do as an undergraduate student. I also like it because it doesn't really need fancy stuff like PASCO probes or anything. The basic idea is that a small mass is swung around in a circle with the tension in the string controlled by hanging a mass on the other end. Here, let me show you.
Circular Motion Lab from Rhett Allain on…
rallain | October 21, 2009My 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,…
rallain | October 20, 2009I don't really know what that title actually means. So, I have been having problems with my PASCO projectile launcher devices. I will just call them launchers (they are really cannons). In my previous post, I looked at the launch speed from a launcher shot horizontally and vertically. The problem was that I was getting different launch speeds for the vertical and horizontal shot. So, here is my plan: shoot the ball and a variety of angles from 0 to 90 degrees and see how the launch speed changes. I will only use the data from video analysis (of course using Tracker Video Analysis)…
rallain | October 15, 2009The basic idea of the student response system is that each student gets some electronical (or not - see below) device that lets them answer multiple-choice questions. (Science Geek Girl has a good summary of clickers also) A computer then displays the distribution of responses for the class. Simple, no? They are becoming super popular, and I really like them. I used to just use them for large enrollment lecture classes (like 100 students). However, this semester I started to use them in my intro physics course for science majors with just about 30 students. I didn't realize the impact…
rallain | October 13, 2009The last time I looked at this projectile motion lab, I was confused. My different methods for measuring the launch speed of the ball were not even close to being consistent. So, I am bringing out the big guns - video. I made a video of the ball shot both horizontally off the table and vertically. No point posting the whole video (unless you really need it), but here is a screen shot of what the setup looked like.
These videos were made with my flip video camera, it doesn't have adjustable shutter speed so that there is some blur. Also, notice the carbon paper on the floor. This is so…
rallain | October 12, 2009I put together this short presentation on fake videos for a class. What the heck, I will also put it online so that maybe some other people can use it. So, here it is. I have it in many forms. First, a video of me going through the talk. Then I have the keynote and PowerPoint files with the movies. Feel free to use it as you see fit. You might want to modify some of the files, I have no problem with that.
How to spot a fake video from Rhett Allain on Vimeo.
And the other versions:
Keynote (fakevideos.key - 46.1 MB) - this has the movies embedded
PowerPoint (fakevideos.ppt - 2.6 MB) -…
rallain | October 11, 2009The other myth the MythBusters looked at last week was the phrase "knock your socks off" (along with the dropping and shooting a bullet myth). But before that, let me complain.
Maybe it is just me, but I totally cringe when these guys use the word 'force'. Force probably isn't the best term to use to describe a collision especially when you are talking about one of the objects. "oh, we will just give this object some more force to impact with that other object". Force is not a property of an object, but rather an interaction between two objects. When two things collide, you really need…
rallain | October 9, 2009If you didn't catch the latest MythBusters (yeah! new episodes), they did something straight from the physics textbooks. Just about every text has this example of shooting a bullet horizontally and dropping a bullet from the same height. The idea is that they should hit the ground at the same time. No one but the MythBusters could actually show this demo with a real gun.
The Physics
I am going to do some calculations, but I want to first write about the physics that accompanies this idea (and you can actually do it your self without the gun). What physics principle does this demo show?…
rallain | October 6, 2009Many textbooks are pumped up about Newton's 3 laws of motion. For me, not so much. First, (as many other's point out) these are really Newton's ideas about force. Second, the first law is pretty much a special case of the second law. Here are the first two laws (in my words):
Newton's First Law:
The natural state of an object is constant motion.
Yes. I know that is not how it is normally written.
Newton's Second Law:
The rate of change of an object's speed is proportional to the amount of net force on the object and inversely proportional to the mass of the object.
This could also be…
rallain | October 6, 2009You know I like demos, right? This one is quite fun to do even in a class. The basic idea is to take a bucket of water and swing it around in a circle over your head. Simple, but if you have never done this one, it can be a little intimidating. Here is an example.
Water Demo from Rhett Allain on Vimeo.
So, how does this work? What does it show? Really, the question is: why doesn't the water fall out of the bucket?
First, I like to talk about "fall" what does that mean? I guess that means that the object has a downward gravitational force, but no force upward to give it a zero…
rallain | October 4, 2009There were a couple of things that bothered me about the MythBusters' myth where they fired bullets in the air. The myth was that a bullet fired in the air could kill you. The first problem is that it is not a myth. There are several reported cases of people being killed from bullets that were fired in the air. The Mythbusters tested this by finding out how fast a bullet would be going if fired straight up. A couple of problems:
First, they measured the terminal velocity of a tumbling bullet, not a spinning one. I really don't know how long a bullet will stay spinning, but I guess this…
rallain | September 30, 2009This 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,…