# moment of inertia

### Where does an earthquake's energy come from?

"As seismologists gained more experience from earthquake records, it became obvious that the problem could not be reduced to a single peak acceleration. In fact, a full frequency of vibrations occurs." -Charles Francis Richter You've all been around long enough to be familiar with the severe damage that earthquakes can cause, rattling and cracking the ground, shaking down buildings, and creating catastrophic tsunamis tidal waves. In short, the largest ones that occur in the wrong places will cause billions of dollars worth of damage and will kill thousands of people. Image credit: AP / Press…

### 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…

### Could cars slow down the Earth?

Time 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…

### Moment of Inertia Demo

When I was talking about balancing a stick, I mentioned the moment of inertia. Moment of inertia is different than mass, but I like to call it the "rotational mass". What does mass do? Things with larger mass are more difficult to change their motion (translational motion). A similar thing is true for the "rotational mass". Things with larger rotational mass are more difficult to change rotational motion. Here is the demo. Demo for Moment of Inertia from Rhett Allain on Vimeo. Why do I like this demo? First, it uses ordinary things. I consider juice boxes to be pretty ordinary.…

### Balancing sticks - choose a longer stick

Maybe you have seen this trick. Basically, you hold by supporting it with two fingers from the bottom. You then move your hand around to keep it balanced while the stick is vertical. It is really not as hard as it looks. Also, there are two things that can make your job easier. Use a longer stick, or add an extra mass at the end of the stick. Here is a video of me demonstrating this. (I forgot, this also an event in the show Unbeatable Banzuke) Balancing a Stick Demo from Rhett Allain on Vimeo. So, how does this work? Let me start with a stick that is mostly vertical and supported by…

### CD Players in space and rotations of a rigid body

I 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…