diy
rallain | December 20, 2009This is one of my favorite demos. I like it because anyone can do it at home and people usually find the results surprising. Here is the situation:
How much of yourself could you see in a small mirror? What if you move farther away from the mirror, can you see more of yourself? If you want to do the demo yourself, now would be a good time. All you need is a small mirror (I used a platter from a 10 GB hard drive - they make awesome mirrors). It will help out a lot if you can mount the mirror on a surface that is very near to vertical.
Here is my version of this demo (in case you can't…
rallain | December 14, 2009The solar system is difficult to show correctly. Why? It is difficult because the size of things are vastly different. Let me use units common in solar-system astronomy, the Astronomical Unit (or AU). One AU is the distance from the Sun to the Earth. If I want to look at all the planets, I would need to go out to about 30 AU (to Neptune - remember that Pluto is not a planet). That is not a problem but then if I want to look at the size of even the Sun, it is just 0.001 AU across. And the Earth is even smaller, at 0.0001 AU. So that is the problem. The distance from the Sun to Neptune…
rallain | December 1, 2009One of the roads near my house was just redone. They added some awesome retroreflectors in the middle. Here is a shot. No wait, I don't have a picture of that. I tried to take one, but it just didn't turn out very well. Oh, you know what it is supposed to look like. It looks like little tiny lights in the middle of the road.
What makes these things so cool? Why do they look like they are battery powered or something? Maybe it would be helpful to compare retroreflectors to some other materials. I can group stuff in the following manner:
Shiny stuff
Non-shiny stuff
Retroreflectors…
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 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 | August 18, 2009I got this question. How does this game work? Really, this is one of those silly things that gets forwarded a lot. It is called Regifting Robin. The basic idea is:
pick a 2 digit number, like 37
subtract both the number in the tens place and the number in the ones place from the original number
find the number you have left on a grid of "gifts" and robin will guess what your gift was
You may have seen stuff like this before and figured it out. It really isn't too bad as long as you know two things (one is a trick)
In a two digit number, like 37, the number is really 3*10 + 7*1. I know…
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 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 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,…
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 | May 29, 2009My brother found this, but he is too afraid of the internet to post something in the comments on my previous post about acceleration and the iphone. It turns out that this too awesome to just be in a comment. It deserves its own note.
Anyway, I was looking for acceleration data from the iPhone and this guy put one in his model rocket.
iPhone Rocket Launch and Interview (from MobileOrchard.com) from Mobile Orchard on Vimeo.
Here is some more detail (with graphs!) about the iPhone model rocket.
rallain | May 25, 2009When I was talking about balancing a stick, I mentioned the moment of inertia. Moment of inertia is different than mass, but I like to call it the "rotational mass". What does mass do? Things with larger mass are more difficult to change their motion (translational motion). A similar thing is true for the "rotational mass". Things with larger rotational mass are more difficult to change rotational motion. Here is the demo.
Demo for Moment of Inertia from Rhett Allain on Vimeo.
Why do I like this demo? First, it uses ordinary things. I consider juice boxes to be pretty ordinary.…
rallain | May 12, 2009Maybe you have seen this trick. Basically, you hold by supporting it with two fingers from the bottom. You then move your hand around to keep it balanced while the stick is vertical. It is really not as hard as it looks. Also, there are two things that can make your job easier. Use a longer stick, or add an extra mass at the end of the stick. Here is a video of me demonstrating this. (I forgot, this also an event in the show Unbeatable Banzuke)
Balancing a Stick Demo from Rhett Allain on Vimeo.
So, how does this work? Let me start with a stick that is mostly vertical and supported by…
bioephemera | April 26, 2009
One of the sucky things about viral interest in offbeat art from small vendors is that it always immediately depletes the supply. Like this fabulous minimalist plywood octopus from inthewoods' etsy shop. Sure, you can still get a plywood squirrel with a plywood acorn, but where's the sinister, feverish Cthulhulian majesty in that? Frustrated now!
Via monoscope
rallain | April 22, 2009Some time ago I was looking for materials to possibly build a foucault pendulum. Of course the first step is google. There was a site that suggested two old issues of Scientific American, and it happens that we have tons of old Scientific Americans in the store room. I found the two that I needed. I will talk about foucault pendulum in second, but let me show this picture.
This is a device to prevent elliptical motion of the pendulum. Part of it has a ring of nickel. Now for the quote:
"The nickel ring, perhaps appropriately, should have the dimensions of a U.S. five-cent piece, with a…
rallain | March 16, 2009Better technology sometimes causes problems. In this case, technology has made video cameras really small. Small? That's a problem? Well, the problem (as I discussed previously) is that small cameras are not stable and make "shaky" videos. Well, then increase the mass of the camera - simple. I wanted to test this idea and make a test video with and without extra mass on the camera. I was thinking - what could I attach to it? A 2 lb or 5 lb scuba weight came to my mind. Then it hit me: a water bottle.
When do people use these little video cameras? To film their kids or something? If…
sporte | November 27, 2008If you're not cooking today, why not experiment? Here's something fun you can do with Mentos and Diet Coke - and for those of you who think these experiments are too messy, you can still watch the movie.
Enjoy the music in the video, then go outside, and enjoy the show. Later, go to EepyBird.com and learn about the science behind the fountain effect.
technorati tags: Mentos and Diet Coke, chemistry, science you can do at home