# measurements

### Density of Wood

This isn't much - really it is part of another post I am working on. The point of this post is to calculate the density of this piece of wood. Really, there is a reason for this. I saw this little stick (sticklette?) and noticed that it was very cylindrically shaped. So, what if I just pretend it is a cylinder to calculate the volume? This way I won't have to get it wet or anything (because I might need this stick later). First the mass Yes, there is some uncertainty in the mass - but it is small. I put the stick on balance and I will use a value of m = 28.9 g or 0.0289 kg. Volume The…

### RP 7: Cool things the Greeks did in astronomy

One of things I like to think about in science is "how do we know that?" It is interesting how one thing builds on another. This is a story of how the Greeks estimated the distance from the Earth to the Sun (an important idea in the development of the model of the solar system). I like this story because it is not too complicated. In fact, one could easily reproduce these measurements themselves. So, here is what I will talk about: Measuring the size of the Earth. Determining the distance from the Earth to the moon and the size of the moon. Calculating the distance (and size) to the Sun.…

### Uncertainty and Video Analysis

This is for commenter JimP. How do you take into account uncertainty when using video analysis? A great question. The first thing to think about is where does the uncertainty come from? My first guess would that it would be from the user. Where does the user click? Is it right on the object in each frame? Is the scale set correctly? I guess there could be other sources of error - maybe there are repeating frames that are a result of encoding. Maybe there is interlaced video frames. Well, what to do? I will just look at one motion in particular and do the analysis several times. I…

### Parallax, what is it good for?

Very simply, parallax is an apparent motion of an object due to a change in observation position. Let me start with an example. Here are two photos. I took a picture of the cabinet in the background from two slightly different positions. In the foreground is a clone trooper that did not actually move. I added the dotted line so you could see how the clone trooper appeared to change positions with respect to the background. Here is a diagram of the camera in the two positions along with the toy. Since the camera changed positions, the object that is closer appears to have moved with…

### Cool things the Greeks did in astronomy

One of things I like to think about in science is "how do we know that?" It is interesting how one thing builds on another. This is a story of how the Greeks estimated the distance from the Earth to the Sun (an important idea in the development of the model of the solar system). I like this story because it is not too complicated. In fact, one could easily reproduce these measurements themselves. So, here is what I will talk about: Measuring the size of the Earth. Determining the distance from the Earth to the moon and the size of the moon. Calculating the distance (and size) to the Sun.…

### Significant figures what are they for and what do they have to do with uncertainty?

Suppose I am working on a problem and I wish to calculate the density of something. I measure the mass to be *m* = 24.5 grams and the volume is *V* = 10 cm3. In this case the density would be: ![Sigfig 1](http://scienceblogs.com/dotphysics/wp-content/uploads/2008/09/sigfig-1…) ALERT! ALERT! ALERT! This is not a test!!!! Something is drastically wrong! Clearly I messed up. How can I have the mass measured to **3** significant figures, the volume measured to **1** significant figure, but the density calculated to **3** significant figures? Isn't this a violation of some fundamental…