free body diagram
rallain | August 21, 2009Forgive me if I don't know the official parkour term for this move. This is where you have two walls that are close to each other and you vertically climb them. Here is a shot of Mark Witmer (from Ninja Warrior) doing the wall climb.
Doesn't look too hard, does it? Well, I think it depends on how far apart the two walls are. This is actually one parkour move that my kids like to do (Hey kids! Don't do that! Let me get my camera though because this will be perfect for my blog)
I am going to start with this second kind of wall climb. Simply because it is easier due to symmetry. So,…
rallain | August 12, 2009This Giant water slide video is extremely popular on the internets. Maybe you have not seen it (doubtful), then here it is:
This is such an incredible stunt that the very first question that comes up is - fake? or not fake? From my previous analysis, I can say:
Even though there is some slight perspective issues, the vertical acceleration seems to be constant and has a reasonable scale to give the acceleration of -9.8 m/s2
The horizontal motion is essentially constant (as a real jump would be)
The launch speed is about 19 m/s
The launch angle is 32.8 degrees
If the guy had no friction on…
rallain | August 5, 2009Parkour: the act of running and jumping like a crazy superhero. I can't do any of it. But I can analyze it. So, let me start with the wall-flip (or any kind of move that involves walking on vertical walls). This looks like a good example (there are a bajillion on youtube).
Pretty cool, huh? How do you run up a wall? Well, it has to do with friction. Remember, this is a fairly useful model for the magnitude of the frictional force on an object:
Where N is the force the surface exerts on the object (the normal force) and ?s is the coefficient of static friction. Note that this less…
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 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 | 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 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 12, 2009Jennifer over at Cocktail Party Physics has a nice post about her trip to Disneyland. The one ride that would be fun to play with (in terms of physics) would be the tower of terror. Think of the cool things you could do with a video camera during that ride. It would be like a mini-vomit-comet. Anyway, I want to talk about one part of Jennifer's post.
"As one would expect, this lifted us out of our seats slightly, as much as the straps would allow, and we got that one glorious moment of seeming weightlessness, before reaching a jerky stop and being raised back up for another drop."
It is…
rallain | June 9, 2009So, analysis of the movie Up is pretty popular in the blogosphere. Figure I might as well surf the popularity wave. So, I have a couple more questions.
The most important thing to estimate is the mass of the house. I am going to completely ignore the buoyancy of the house. I figure this will be insignificant next to the buoyancy needed. Anyway, let me go ahead and recap what has already been done on this in the blogosphere.
Wired Science - How Pixar's Up House Could Really Fly - from that post:
First, they calculated (seemingly correct) that the buoyancy of helium is 0.067 pounds per…
rallain | April 30, 2009ZapperZ already beat me to this, but if it is worth doing once it is worth doing again. Here is a video of an electric motorcycle that can apparently go from 0 to 60 mph in 1 second.
What acceleration would this have? If you want, you can look at my stuff on kinematics, but this case is pretty straight forward. Really, as straight forward as one could get. Let me start with the definition of acceleration: (in the x-direction since this is a 1-d problem)
I know both the change in speed and the time, so this is easy. I can make it even easier using google. The acceleration is 60 mph/s.…
rallain | March 31, 2009What is a fake force? A fake force is one of those forces that introductory texts tell you aren't real - like centrifugal force. They aren't real in the sense that they are due to one of the fundamental interactions. Basically, introductory texts (and even blogs like this one - not a bad summary of real vs. fake forces) attack the centrifugal force. This is because it is so common for students to want to use these faux forces in the wrong way. Better to just not use them at all.
Anyway, there are times when faux forces are awesome. Just to be clear, a faux force is needed to use normal…
rallain | February 21, 2009This is one of my favorite demos. Easy to do and doesn't really require any equipment. The basic idea is that you support a meter stick horizontally with two fingers. Slide your fingers in and they will both meet at the center of mass. Here is a video.
Meterstick friction demo from Rhett Allain on Vimeo
So, how does it work? To explain this, you need to understand friction and equilibrium. Well, nobody really understands friction - but anyway..
First, what is equilibrium? Equilibrium means that an object's motion does not change. For this case that means that both it's velocity and it…
rallain | January 30, 2009I said I would come back to this, and I am. I am a man of my word. Yesterday, I posted a link to a video of this really cool water rocket thingy.
How does this work? What is the physics going on here? I think this can be best explained with the momentum principle.
Let me start by pretending like I have some object that shoots out a piece of water (or really it could be anything). Also, let me pretend like this is in space ore something where there are no external forces.
Above is a before and after picture. Initially, there is something (the box) with water inside. Through some process…
rallain | December 29, 2008In part I of this post, I talked about the basics of projectile motion with no air resistance. Also in that post, I showed that (without air resistance) the angle to throw a ball for maximum range is 45 degrees. When throwing a football, there is some air resistance this means that 45 degree is not necessarily the angle for the greatest range. Well, can't I just do the same thing as before? It turns out that it is a significantly different problem when air resistance is added. Without air resistance, the acceleration was constant. Not so now, my friend.
The problem is that air…
rallain | December 18, 2008I saw this video on several places. It shows an astronaut playing with a CD player.
I wish I were an astronaut. I would probably not stop throwing up though. It would still be worth it. You can only throw up so much right? (I know the answer to this question). Anyway, this is a really cool demo. Look at the first CD player that is on. When the guy taps it, it doesn't rotate but rather it wobbles. This is a rather difficult concept, but I am going to try to give a reasonable explanation.
I will start with angular momentum. Angular momentum is sort of like momentum (linear momentum…
rallain | November 18, 2008Big trucks are not as popular as they used to be, but gas prices are going down so maybe this commercial for the ford F-150 will start showing up again:
They say its a real demonstration, but it has seemed odd to me. (they also say this is a professional driver on a closed course and don't try this at home - damn! I was SO going to do that this weekend). Here is my analysis:
Here is the important data I have gathered from the Internets.
The truck (a Ford F-150) stops a plane C-123 Provider
The curb weight of the F-150 is around 5,000 lbs - depending on options and stuff.
The empty weight of…
rallain | October 6, 2008**Pre reqs:** [Free body diagrams](http://scienceblogs.com/dotphysics/2008/09/basics-free-body-diagrams.php)
Friction is an interaction between two objects in contact that opposes relative motion of those two objects. It is not something fundamental (like gravity, or electromagnetic force), but it comes up enough that it will be worthwhile to talk about it. Let me start with a simple example. Suppose I have a book on a table. Here is the free body diagram for the book:
Simple enough - right? There…
rallain | September 30, 2008**Pre Reqs:** [Intro to Forces](http://scienceblogs.com/dotphysics/2008/09/basics-what-is-a-force.php), [Vectors](http://scienceblogs.com/dotphysics/2008/09/basics-vectors-and-vector-ad…)
Hopefully now you have an idea of what a force is and what it isn't. What do you do with them? The useful thing to do with forces is to determine the total force acting on an object. At the beginning of the introductory physics course, you will likely look at cases where the total force is the zero vector. This is called equilibrium. Even if you are looking at cases where the forces don't add up to the…
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 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…