# tension

### Kids like demos

Sometimes I get asked to do some demos for kids. I like this, it is fun. This week, I was asked to do just such a demo. Kind of as a historical record, I am going to document what I did. The audience It is always important to think about who you are talking to. Is this a group of high school physics students? High school teachers? Middle school teachers? Kids? It could be anything. In this case, I was meeting with a group of kids from ages 8 to 12 as part of a science camp. The Objective Is there a reason for the demo show? In this case, there was no stated objective. That means I can…

### Experimental Rope Logarithms

Sue from Math Mama Writes... sent me an email about wrapping a rope around a pole. In that post, Sue thinks about rope looped around a post. When you wrap a rope around a post, the friction between the rope and the post can help you hold something (like a horse) that is much stronger than you. The first case she thinks about is using several posts. What if you wrap a rope around one post and pull? What if you use 2, or three posts? The idea is that if one posts 'multiplies' the force by 10, two posts would have an effect of multiplying by 100 and so forth. That seems reasonable. The…

### A small duct tape mistake

Yes, I am going to talk about the MythBusters latest duct tape episode. A couple of pre-post points: They call it "duct" tape. I call it duck tape. It doesn't work very well with ducts. Also, it is good for water proofing stuff - you know like a duck. For the rest of this post, I am calling it duck tape. (Wikipedia agrees there is some naming problem) You know I love the MythBusters - right? I hope you don't think I sit around waiting for them to make a mistake so I can pounce on them (I save that for ESPN Sport Science). In this case, they just made a small mistake. A mistake that…

### Oak Trees are awesome

In this part of the world, we have oak trees. Technically they are called live oaks - but I don't get it. Of course they are alive. I was at a soccer game and this is the tree I always look at. Look how far those limbs extend horizontally. That branch is about 12 meters long. Why is this amazing? Have you ever tried to hold an 8 foot 2 x 4 board horizontally by holding one end? Pretty tough. How about I calculate the forces needed to hold that branch in place? I will do a simple model and then maybe later I can make it more complicated. Suppose I replace that limb with one straight…

### 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…

### Video Analysis tutorial with Tracker Video

Previously, I showed how to analyze a cat video with Logger Pro. Logger Pro is nice, but so is Tracker Video. I also posted a comparison between Tracker and Logger Pro. Now, here is the same tutorial on the same video using Tracker Video. Record your screencast online I am not going to go through the analysis, since it would be the same as with Logger Pro.

### Video Analysis Tutorial and a cat

Cats can be entertaining - especially when they are someone else's cat and that someone made a video. Really, this post is about analyzing video with Logger Pro (in a tutorial type fashion). I just happens that I chose this cat video to analyze. Here is the video: I am going to look at the part where the cat gets on the fan. I will try to step through the analysis so you can do your own. Get the video Actually, the first step is to find a video. YouTube has tons of stuff. Also, you could make your own video with a camera. A couple of things to make your life easier: A non-zoom video.…

### An example of Atwood's machine (masses on a pulley)

Atwood's machine is the name of a device that looks like this: Also known as "two masses on a pulley". Surprisingly, this simple device comes up a lot in intro physics texts. It also brings up some interesting issues. I will go over the basic way to solve a problem like this (as an example) and then talk about the other interesting issues it brings up. Problem: A small, low mass, pulley has a light string over it connected to two masses, m1 and m2. If released from rest, what is the acceleration of the two masses. Where to start? This is actually a very difficult question for…

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

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://scienceblogs.com/dotphysics/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…