After taking a break from the project, I'm now back! I will not put as much effort into it as I did before the Power Challenge deadline, but I want to make contious progress.

I had some interesting discussions with a friend of mine about measuring the energy trapped in the tree swing. My original idea was to mount a cylinder between the tree in question and another point in space (like another tree, a house or a pole). Inside the cylinder is a spring, such that the movement of the tree compresses the spring. Now I can measure the absolute position and therefore calculate the force needed to compress or stretch that spring. With that information I can then calculate the work (or energy) which is needed to compress the spring. But my friend objected, since the spring is springy, I can not do this continously (it is only valid for one swing), I have to dissipate the energy somewhere otherwise I can not differentiate if the system is swinging on its own or its swinging from a fresh blow of wind.

Currently I see two ways to approach the energy estimations:

1. Estimate the spring constant and the dampening by deflecting the tree with a known force and observe the movement. Mount a positional sensor onto the tree and calculate with the movment data then the energy trapped in there. See also the sketch below.
2. Use a more handson approach and build a generator which is more capable
   then my small stepper motor generator and use the energy harvested to charge up a battery or to heat up a power resistor. Monitor voltage and current and calculate the energy which has been harvested. Off course I need more data about the treeswing (eg amplitude) first, so basically I am back to approach 1.

I think building the position sensor is really the first thing I should do. What do you guys think, which approach is better (in terms of getting results and effort to build it)? Or would you go a completely different route with the research?