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Physics of Linerider Part II: Scale

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Scale 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).

![line rider](http://blog.dotphys.net/wp-content/uploads/2008/09/line-rider.jpg)

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 m/s2.

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The Physics of Linerider

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**Note: this is post from which IS my site, but I am putting here. – end note**
**Part I: Introduction**

[Linerider](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 example of a physics project that students could do as a project.

I also tried to write this in a method that would be instructive. The analysis may seem overly detailed, but I tried to give a good background to the physics needed. If you had physics in college or high school, this could be a good review.

**Disclaimer** There are likely to be some mistakes in here. I know some people may get all uptight about some of my equations. I tried to simplify things as much as possible (only using vector notation when absolutely necessary). This could lead to some equations that experts might call wrong (but they are not wrong).

**UPDATE:** I would just like to emphasize that the goal is this report is NOT to say that the Line Rider game is bad. It is very, very good and very entertaining. I have also talked to a famous flash programer (who pretends he has a master’s degree in physics, but he doesn’t). This programmer explained that with flash, it is nearly impossible to correctly calculate things in flash and that most (if not all) of the physics is faked. So there.

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Significant figures what are they for and what do they have to do with uncertainty?

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Suppose 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:
![Sigfig 1](http://blog.dotphys.net/wp-content/uploads/2008/09/sigfig-1.jpg)

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 principle that could be worse than the Large Hardron Collider coming online and destroying the world?

No, we can all calm down. This really isn’t a big deal. Unfortunately many people (*hint* like chemists) do get all freaky about significant figures. Now calm down chemists. I am not saying significant figures are entirely stupid. They do have a purpose. What I AM saying is that they are not some fundamental rule that can not be broken.

So what is the deal with significant figures?

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MythBusters pulling on a phone book: You are doing it wrong.

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The 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:

![tug1](http://blog.dotphys.net/wp-content/uploads/2008/09/tug1.jpg)

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 really goes back to the old question: Which would produce a greater tension, two horses pulling in opposite directions, or one horse pulling on a rope tied to a tree. The answer is that both tensions are the same. However, many say that the two horses create a greater tension. The likely thinking in this “two horses are more” answer is that TWO things are doing something must be greater than ONE thing doing something. This reasoning fails because if you tie a rope to a tree, it is doing exactly the same thing the other horse doing: not moving.

Why? A force explanation follows:

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Cat in the Hat sits on a throne of Lies!

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My 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:

![page](http://blog.dotphys.net/wp-content/uploads/2008/09/page.jpg)

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 not get absorbed. All colors of light interact with the water. Reds are mostly absorbed, the blue gets scattered.

[Here is a better explanation from the Library of Congress](http://www.loc.gov/rr/scitech/mysteries/oceanblue.html).

Would it be so hard to have someone look over the science in a book? I would do it if they asked.

Fake or Not: Physics analysis of “Extreme way to shop for groceries”

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Here 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 two tosses.

![grocery](http://blog.dotphys.net/wp-content/uploads/2008/09/grocery.jpg)

A few things to notice:
– The plots are mostly parabolic. This is what you would expect from a real toss (air resistance would be small)
– The two tosses have essentially the same acceleration.
– The acceleration of these two tosses is in the ball park of -9.8 m/s2. Yes, they are not right on, but I totally guessed on the scale of the video.

So, from this, I think that shot is real. It is still incredible. Maybe they did this for like two hours to get it right. Maybe they just got lucky. Maybe they used the force and the force is strong in their family.

I was going to add an analysis of how off they could be on their throws and still make the “basket”, but I got lazy. Sorry. Maybe I will add that later.

People in and out of a bar

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So, I am still in Alabama because of lack of electricity in Louisiana (although we got power back last night – YAY!). My wife convinced me to go out and hear this band since we are staying at my parents and they said they would watch the kids. In general, I am way too old and crotchety to go out – but it appears I had no choice. Overall, it was not too bad except for staying out too late. (the band was actually pretty good – I think they were called [Fly By Radio](http://www.fly-byradio.com/) and they played 80s musics) The one thing I thought about while in the bar was the long line of people waiting to go in. At some point, the number of people inside reaches its maximum capacity. People still come in because some people leave. Here is my rough sketch of this: (I call it a bar graph – get it?)

![bar graph](http://blog.dotphys.net/wp-content/uploads/2008/09/bar-graph.jpg)

My first idea was to think of the bar as a capacitor and maybe this works. Ok, now I made something positive out of my trip.

Basics: Projectile Motion

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**pre-reqs:** [kinematics](http://blog.dotphys.net/2008/09/basics-kinematics/)

My previous “basics” post was on kinematics (in one dimension). But what about two dimensions? In particular, what about projectile motion. My motivation here is that I was about to talk about analysis of a video that involved projectile motion and I don’t want to go over all the stuff again and again.

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The Sky is Falling (as always)

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One of my daughters was just reading Chicken Little to me. I don’t know if you are familiar with Chicken Little, but she is a chicken that runs around telling people “The sky is falling”. In my normal fashion, I started thinking about the plausibility of this. What would fall? What would you look for? Then I figured it out. The sky IS falling. It is ALWAYS falling and it has always been falling.

What is the sky? I am assuming the sky is the air. I will treat the air as a gas of single particles (which it isn’t, but that’s ok). So, why does this sky (air) do what it does? If you look at each individual sky particle, its motion is governed by two things.
1) Gravity. The gravitational force makes each particle fall. Without this, all the air would escape the planet. (this would be bad)
2) Collisions with other particles. This is what prevents the “sky” from collapsing.
I actually talked about this some before [when I talked about MythBusters and the Lead Balloon](http://www.dotphys.net/files/lead_balloon.html). The best way to see this interaction between gravity and collisions for a gas is with the excellent [PHET simulator](http://phet.colorado.edu/new/simulations/sims.php?sim=Balloons_and_Buoyancy)
![phet](http://blog.dotphys.net/wp-content/uploads/2008/09/phet.jpg)
If you adjust the “gravity” you can see that there are more particles down lower, but that a particle in between collisions looks like a projectile. Thus, I think it is ok to say that the sky IS falling.

Basics: Kinematics

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**pre reqs:** *none*

Often I will do some type of analysis that I think is quite cool. But there is a problem. I keep having to make a choice. Either go into all the little details, or skip over them. My goal for this blog is to make each post such that someone could learn some physics, but I also don’t want it to go too long. So, instead of continually describing different aspects of basic physics – I will just do it once. Then, when there is a future post using those ideas, I can just refer to this post. Get it?

Fine. On with the first idea – kinematics. Kinematics typically means a description of motion (not what causes that motion). In particular, kinematics looks at position, velocity, and acceleration. In this post, I will try to stay in one dimension. This will make things look simpler without really losing too much. Later, when I talk about vectors, I will make it all better.

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