rallain | September 23, 2008In my classes, I like to bring up the question:
*Why do astronauts float around in space?*
The most common response to this question is that they float around because there is no gravity in space. Some people take this a small step further and say that there is no gravity in space because there is no air in space. This is why they claim there is no gravity on the moon (even though there is - more on this later).
I like to start off with the concept of gravity. Gravity is an attractive force between any two objects with mass. Your pencil and your dog both have mass so there is a force pulling…
rallain | September 21, 2008**pre-reqs**: [kinematics](http://scienceblogs.com/dotphysics/2008/09/basics-kinematics.php) *I don't think you need [part I of this](http://scienceblogs.com/dotphysics/2008/09/basics-making-graphs-with-ki…) if you don't want*
So, you still want to make a graph with that kinematics data? You think that graphs on paper are too barbaric? Well, if you are ready, you can use a spreadsheet. But be careful. If you don't know what you are doing, you can cause some damage (much like flying a 747 after reading a blog about it). Spreadsheets allow you to do a couple of things.
make pretty graphs…
rallain | September 19, 2008Today I was talking about electric potential. My favorite analogy for electric potential energy is gravitational potential energy. But electric potential is something different. Electric potential (commonly called potential) can be defined as:
So, V is the electric potential in units of Joules per Coulomb or Volts.
What about gravitational potential? I am sure some astrophysicist use gravitational potential. Maybe they even have some units for it, but I have never seen it. My students asked me if there was such a thing as gravitational potential. I said, sure. Here it is:
I picked…
rallain | September 18, 2008I use equations quite a bit. What I have been doing is writing the equations in LaTeX and then taking a screen capture of the output. I think this makes nice looking equations, but it sure takes a while. Here I am testing [LaTeX for WordPress plugin](http://wordpress.org/extend/plugins/latex/). The following *should* be some equations:
$$\vec{F}_\text{net}=\frac{d\vec{p}}{dt}$$
$$a^2+b^2=c^2$$
End of test. This was only a test. Had this been a real post, it *might* have had something useful.
**update** not sure if I like the way the equations look.
rallain | September 18, 2008**pre reqs:** [kinematics](http://scienceblogs.com/dotphysics/2008/09/basics-kinematics.php)
Suppose there is some experiment in which you throw a ball up and collect position and time data (with video analysis). What do you do with this data? Your instructor told you to make a graph, but how do you do that?
Here is the fictional data you (or I) collected:
Here is the text file with the data if you want to reproduce the graphs I make here [kinematics data](http://scienceblogs.com/dotphysics/kinematics_data.…
rallain | September 17, 2008So here I was in thermal physics class. The students were talking about the assigned homework and then asked: "can't we get some homework credit for this? Why are we even doing this?" Immediately in my head popped "wax on, wax off". This was the same situation Mr. Miyagi (from [The Karate Kid](http://en.wikipedia.org/wiki/The_Karate_Kid)) was in with Daniel-san. Homework should not be done just for the points. Homework should help the students become more proficient at blocking blows from the test.
I really like the movie karate kid. Mr. Miyagi brings up some good points. How does…
rallain | September 16, 2008**pre-reqs:** *none*
I know who you are. I have seen you before and talked to you before. You are taking introductory physics and you are scared. Why does this have to be so difficult? It seems like there are a bazillion equations. Calm down, I will try to help. First, realize that algebra and trig are typically a pre requisite course for introductory physics. Your instructor probably expects that you have already mastered this material. Perhaps you did well in algebra (maybe you earned a B). But maybe you just worked hard and never really "got it". That is ok. There are many…
rallain | September 15, 2008I like computers, really I do. Computational physics is a good thing. However, there is a small problem. The problem is that there seems to be a large number of people out there that treat numerical methods and simulations as something different than theoretical calculations. You can tell who these people are because they refer to simulations as "experiments". But what do these simulations really do in science? What is science really all about?
**Science**
To me, science is all about models. Making models, testing models, upgrading models. Models. Some examples are the model of…
rallain | September 15, 2008There was this commercial on the radio about Trane heating and cooling units. The ad claimed that the units could use up to 50% less energy than your existing unit. This started me thinking (because before that I was in a complete state of non-thinking). Do you remember the Y2K problem? Basically, when people started writing programs back before Star Wars they had to be very conservative. The hardware of the time did not afford the programmers to have frivolous code. To conserve, they only used the last two digits to represent the year (1970 was represented as 70). Obviously this…
rallain | September 14, 2008**pre-reqs:** trig
Think of the following two things. Temperature and wind speed. These are two different things that you could measure, but there is one big difference. Wind speed has two parts to it - how fast and which direction. Temperature is just one thing (no direction). Temperature is an example of a scalar quantity (just one piece of information). Wind speed is an example of a vector quantity - multiple pieces of information. Here are some other examples:
**Scalar:** mass, money, density, volume, resistance
**Vector:** velocity (most physicist reserve the word "speed" to mean…
rallain | September 14, 2008Can you believe it? Have you seen this video?
Are you thinking what I am thinking? WOW. How could these people not follow my rules for cool internet video. Once again, here they are:
1Keep the camera stationary. This way I don't have to keep moving the origin in the movie.
2Don't Zoom. Same reason, this video followed that rule.
3Include a clear and obvious calibration object. A meter stick would work, or even a Kobe Bryant (I can look up his height). Maybe it could be a Ford F-150 that has a known length. Something!
4Include the mass and height of all people involved.
5Use high…
rallain | September 14, 2008I am sure I mentioned that I am moving to WordPress (from Rapidweaver). I would like to have all my important posts moved. To do this, I am just going to repost them in WordPress. Maybe there is a better way, but I am not sure. So, the point is that if you have been reading this for a while, you might see some repeats.
rallain | September 14, 2008Friction in Line Rider
Is there friction in Line Rider? Does it function as physics would expect? To test this, I set up a simple track:
Basically, a slope with a flat part to start with and to end with. Let me show you something simple before further analysis:
This is the x-position vs. time for the line rider on the first horizontal portion of the track (before he or she goes down the incline). This shows the rider…
rallain | September 13, 2008There is no air resistance in line rider. Sorry to spoil the suspense.
To test for the presence of an air resistance force, a track was created that let the rider fall.
(note the markers on the side. These are used to keep track of how the origin is moving).
Below is the y position of the rider as a function of time:
In this situation, the rider falls about 100 meters. A quadratic line is fit to the data…
rallain | September 13, 2008Scale of the Line Rider
First, we assume that the line rider is on Earth and for low speeds will have a free-falling acceleration of 9.8 m/s2. Next, an arbitrary distance is selected. In this case the length of the sled is chosen to be 1 LU (Linerider Unit).
The goal will be to put the linerider in a free fall (where air resistance should be able to be ignored) and determine his (it could be a she, it is difficult to tell) acceleration in LU/s2. Then we can determine the conversion factor from LU/s2 to…
rallain | September 13, 2008Part I: Introduction
http://linerider.com is a flash "game" in which the user can create tracks. A rider is then allowed to slide down these tracks. If you have not played this, I recommend you DON'T play it. It is very addicting and can consume many hours of your time (hours you would otherwise spend on Digg or surfing needlessly).
In this short report, I will analyze the physics involved in line rider. An obvious question is "why not just ask the line rider programmer?". Well, that would not be too much fun. Would it? So, there is the first reason - its fun. The second reason is to give an…
rallain | September 12, 2008Suppose 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:
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…
rallain | September 11, 2008The MythBusters aren't really doing it wrong, but they give me a chance to talk about some physics. In the latest show, they tested the myth that two phone books with their pages alternating were indestructible. To test this, they put the two phone books together and then pulled them apart in a sort of tug of war. Here is a diagram:
Looks great, what is wrong with this? The problem is that by pulling this way, the MythBusters produces 320 pounds of force on the book - but they could have done twice that. This…
rallain | September 8, 2008My kids like books. Especially when they are going to bed. I let my daughter pick a book and she picked "Clam-I-am. All About the BEACH" by Trish Rabe. It is nice, it rhymes. The pictures are pretty. Then I get to this page:
So, the ocean is blue because of the sky? How do you get green oceans? How about brown (I live in Louisiana, trust me - the gulf of Mexico can be brown)? What about when you are underwater, everything looks blue. The best answer to why the ocean is blue is that that is what color does…
rallain | September 6, 2008Here is the video in question:
Looks too incredible to be real for me. That is when I start to question things. Is this fake or not? To answer this, I took a clip that showed a person launching a grocery item over the isle. This was a good shot to look at because it was *mostly* perpendicular to the camera view. I then used [Tracker video analysis (free) tool](http://www.cabrillo.edu/~dbrown/tracker/) to get x-y-time data for the flying projectile grocery. The scale was difficult, so I just guessed that the guy on the left was 5 foot 10 inches. Here is the vertical position data for…