circular
rallain | August 17, 2010Let me start with the video. Here is a guy flying a plane in a barrel roll and pouring some tea at the same time. Talk about multitasking.
How can he pour upside down? Well, there are two ways to look at this. First, I can look at this in the frame of the plane. For this case, I can invoke the fake force - centrifugal force. Oh yes, I am going to do it. You probably remember all your physics instructors warning you to never ever do this. Well, they say that because they are afraid you will do something bad with it. Here, I will only use the centrifugal force for good.
What is the…
rallain | August 16, 2010I can't let it go. There is more here to explore. First, I can't believe that I looked at braking and then turning but not turning then braking. And what about braking while turning? I will model braking and then turning - but it won't stop here.
Consider a few motions. I could turn and then brake (which I am looking in this post). Another option would be to brake and then turn. I already showed that this takes a longer distance than just stopping. Braking and then turning in general won't work. Suppose I brake and slow down to a certain speed. Now I am a certain distance away from…
rallain | August 5, 2010This is a classic problem. You are in a car heading straight towards a wall. Should you try to stop or should you try to turn to avoid the wall? Bonus question: what if the wall is not really wide so you don't have to turn 90 degrees?
Assumption: Let me assume that I can use the normal model of friction - that the maximum static friction force is proportional to the normal force. Also, I will assume that the frictional coefficient for stopping is the same as for turning.
Stopping
I am going to start with the case of trying to stop. Suppose the car is moving towards the wall at a speed…
rallain | July 8, 2010In my last zombie post, I looked at a human moving in a circle to avoid a zombie (if they are stuck in a room). What if I build a zombie evading robot that always moves perpendicular to the path of the zombie? Would this work?
This shouldn't be too difficult to model. I can use my existing model for the zombie (where there is a force towards the human and a drag force). For the right-turning-robot, I will also have a drag force and a "driving" force. How do I find the direction of the driving force for the robot? Here is a diagram.
This Fdrive force will really be the frictional force…
rallain | July 1, 2010Title: Chased by zombies
When I heard word about the ScienceBlogs Zombie Day, I knew I wanted to participate with a post - but I had no idea what to do. My first thought was to somehow talk about living off the electric grid in the case of a zombipocolypse - you know, like how big of a solar panel would you need?
But you know what? Physics is difficult - but modeling is easy. How about I model something? How about a model for the motion of a zombie horde? This will be great.
Zombie motion model
What do I want in my model? What are the constraints? What real-life situations can I use to…
rallain | May 20, 2010And there's parliament. Ok - sorry, I had to make a "Tom (Swans on Tea)" title for this one. Tom, forgive me.
Here are two great circular motion videos. First, this one is from Dale Basler. He made himself a fine little floater-type accelerometer. Better than just make it, he made a video of the accelerometer in his car going around a round about. Check it out.
Bobber Meets Roundabout from Dale Basler on Vimeo.
So, if he is driving at a constant 10 mph, how big is the round about (traffic circle)?
Next video - more silly kids
First, I saw this one on ZapperZ's Physics and Physicists who…
rallain | April 27, 2010There are several free iPhone-iPod Touch apps that let you look at the acceleration of the device using the built in accelerometer. I was planning on reviewing some of these free apps, but I didn't. When I started playing around with them, it was clear that I needed some way to make a constant acceleration. There are two simple ways to do this - drop it, or spin it in a circle. I decided to go with the circular motion option because I like my iPod and because Steve Jobs told me to.
While playing with this, I realized that the acceleration depends on the distance of the sensor from the…
rallain | March 26, 2010Time for another Fermi problem. There was a recent story in Science News that talked about the effects of the Chilean quakes on the Earth's rotation. The basic idea is that some ginmourmous amounts of rock moved closer to the Earth's center. Since the angular momentum of the Earth is conserved, the angular rotation rate would increase. The estimated change of the day was by about 1.26 microseconds.
Could all the cars in the USA be used to change the rotation of the Earth?
Well, I shouldn't have phrased the question that way. Of course 1 car technically is all you need to change the…
rallain | February 17, 2010It is winter Olympics time and time for physics. The event that I always gets me thinking about physics is short track speed skating. It is quite interesting to see these skaters turn and lean at such high angles. All it needs is a little sprinkling of physics for flavor.
Check out this image of Apolo (apparently, it is not Apollo).
How about I start with a force diagram?
I know what you are thinking...Fcent....what force is that? Yes, I am going to use the centrifugal force in this case - but remember that sometimes fake forces are awesome. In short, if I want to pretend like Apolo…
rallain | January 1, 2010It's odd that I have talked about these forces so much. First, I talked about how centrifugal forces were not real and the difference between centrifugal and centripetal forces. Then I talked about how sometimes, fake forces are good. Finally, I talked about the origin of the words centrifugal and centripetal. (note: "talked about" means wrote a blog post)
In thinking about centripetal forces, I realized that I could come up with a situation in which the centrifugal force is the centripetal force. This is great. I can end all the confusion between centrifugal and centripetal by making a…
rallain | November 26, 2009This year's episode of Punkin Chunkin is coming up (I think tomorrow). Discovery just showed a teaser commercial with the specifications for one team's machine. If you are not familiar with Punkin Chunkin (World Championship Punkin Chunkin), the basic idea is to project some pumpkins. (note, if you waiting for the Discovery Channel show for the 2009 Punkin Chunkin, don't click on the previous link, it has the results already).
One of the categories for Punkin Chunkin is the centrifugal machine. These are machines that spin pumpkins around really fast in circles to shoot them. They are…
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 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 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 | August 31, 2009Maybe this is a little old (in internet age), but it is a great example. Here is the Loop-the-loop stunt from the show Fifth Gear.
I like this. First, it is a bold stunt. But also, there is some good physics here. Though, most importantly, the Fifth Gear producers were kind enough to include a shot that was very compatible with video analysis.
I went to the official site of this stunt - http://looptheloop.dunlop.eu. From here I found some useful info:
Loop is 40 feet tall
The car is a Toyota Aygo
Some physics-y guy calculated that the car must go 36 mph to do the loop (I think that is…
rallain | August 3, 2009Every introductory astronomy text and most intro physics texts talk about tides. The usual explanation is something along the lines of:
The moon exerts a gravitational force on the Earth and all the stuff on the Earth.
This force decreases with distance (1/r2).
Thus the moon pulls greater on one side of the Earth than the other
This doesn't matter except for oceans which can move.
BOOM. Two tides a day due to a bulge on the side close to the moon and the opposite side.
Oh, the Earth is slowing down.
Really, that is what almost all intro texts say. Go check for yourselves.
Yes, the tides…
rallain | July 28, 2009Thanks to reader Cleon for notifying me of this video on youtube. Check it out.
First, some notes.
I am sure you noticed that the aspect ratio is incorrect (at least that was the first thing I noticed). The boys must have made a 16:9 video, but then uploaded it to youtube as a 4:3. This doesn't really affect the analysis, but I had to fix it. I used the awesome firefox plugin NetVideoHunter to download the video and then used MPEG Streamclip to resize the video.
Other than that, they did follow my suggestions for making videos. The camera doesn't move and is mostly perpendicular to the…
rallain | July 18, 2009pre-reqs: vectors, kinematics
I haven't done a "basics" topic in quite some time. It's odd, I have used centripetal acceleration quite often, but I never derived the expression that I use. To get to the point, the magnitude of the acceleration of an object moving in a circle is:
Also, the direction of this acceleration vector is always towards the center of the circle the object is moving in. This is really not too difficult to derive (but it does use at least one "trick"). Let me start with an object moving in a circle at a constant speed. I am going to show to instances of the object…
rallain | June 19, 2009I have been wanting to look at this whole curved bullet thing, but I wasn't sure how to approach it. In case you are familiar with the myth, this is from the movie WANTED (which I did not see). Apparently, some people learn how to make bullets curve by moving their gun. Here is a shot of a bullet curving in front of someone.
Maybe the picture doesn't do the clip justice, but it is enough for you to get an idea. Before I do an analysis, this reminds me of a great educational activity. In the activity, you give groups of students a full sheet of paper with lines that look something like…