# centripetal

### Pouring tea in a plane - upside down

Let 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…

### More on turning and braking

I 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…

### Turn or go straight? Quick!

This 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…

### Mercedes Roll

Here is a commercial for some Mercedes car. The first part is quite boring, but check out the stunt at around 2:00 minutes into the video. I haven't bothered to check if this is officially fake or not. Instead, I will do what I do - see if this is even feasible. The common question people ask when they see something like this is: "how does the car defy gravity?" Well, it doesn't. Why doesn't it fall? In a sense it does. This is essentially the same as spinning a bucket of water over your head. Maybe a diagram of the car at the top of the tunnel will help. I tried to make the car stand…

### Hey, look kids! There's Big Ben.

And 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…

### Where is the accelerometer in an ipod touch?

There 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…

### RP 11: When the centrifugal force is the centripetal force

It'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…

### RP 10: Centripetal Force, Centrifugal Force - what's the deal?

A couple of commenters expressed concern over the use of centrifugal force after my rant on the use of the word force. So, what is the deal with these two terms? Are they ok to use? Are they real forces? First, are they real forces? It depends on what you mean by real. What is a force? Here is a quick overview of what a force is. I previously talked about real vs. non-real forces. For me, I say that if the force is essentially one of the 4 fundamental forces then it is "real". With this definition, centripetal force would be real and centrifugal not real. Centripetal Force Centripetal…

### More Punkin Chunkin Centrifugal Machine

This 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…

### Punkin Chunkin - why work to be wrong?

Here 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 in…

### Demo: How to not spill your drinks

I 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…

### Circular Motion Lab

Sometimes 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…

### A surprisingly fun demo

You 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…

### 5th Gear Loop the Loop

Maybe 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…

### Tides. Why are they so hard?

Every 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…

### Playing with a merry go round

Thanks 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…

### Basics: Centripetal Acceleration

pre-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…

### Mythbusters and WANTED - curved bullet

I 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…

### When the centrifugal force is the centripetal force

It'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…

### Centripetal vs. Centrifugal (word origins)

A student in my office temporarily confused the words centripetal and centrifugal. This started me thinking about these two words. They mean different things, but do sound and look similar. I have previously talked about the difference between fake and non-fake forces, but let me quickly define these two: Centripetal: This is the force needed to make something move in a circle. The force could actually be a number of things such as: friction, gravity, tension in a rope or any combination. Centripetal force is a name for a real force that has the role of making something move in a circle…