### Sun SPOTs and Trebuchets

My son is enrolled in a summer science camp that he is enjoying quite a lot.

Last night he brought home a small trebuchet that he made in class. It was pretty nice and once we determined that he could throw an AA battery the quite a way in the back yard. He asked me the question, "I wonder how many Gs that battery gets when its thrown?" Well, dad's live for these kind of questions, especially when they have a set of Sun SPOTs in the house. So we got out the Sun SPOTs started throwing them around the yard. The telemetry application that comes with the Sun SPOT kit is ideal for this project. It provides a live feed of the accelerometer data from the Sun SPOT. That gave us live data as the SPOTs flew around the yard. Its a pretty small trebuchet, so it couldn't throw anything as heavy as a Sun SPOT very far, but it was interesting to see that the acceleration as the Sun SPOT was dragged along the platform was only about 3Gs, while once it started its real swing it was accelerated right up beyond 6 Gs. Then in the throw graphed below (displaying only y and total acceleration), you can see that it spun in the air 5 complete revolutions before landing unceremoniously with a >6G shock and bouncing in the grass several times then finally coming to rest. It was pretty fun.

We just had a short time before dinner, so we didn't try to calculate how high it went in the air or anything tricky like that, but I see no reason not to since you can tell when the trebuchet let go of the Sun SPOT and when it landed. You can calculate the acceleration involved and determine, to some approximation, how high it went.. I think this type of project could make for a fun grade school science project.

I've had ideas for other Sun SPOT related grade school projects like the SPOT-drop. Its like an egg drop contest where kids design enclosures for the eggs and then drop them from a reasonable height onto a hard surface. If the egg breaks they lose, if it doesn't, they win. Well, if you substitute a Sun SPOT for the egg, you can give the students actual data accelerometer data about how well their enclosure did rather than a binary break/no break indicator. If you have real data you can get into a lot more subtleties of what designs are best at protecting their cargo. Anybody want to give it a try and tell me how it goes?

Very cool. I want one of those Sun SPOTs (and a son that is interested in more than hacking Linux).

Posted by Thommy M. on July 14, 2008 at 11:31 PM PDT #

That's interesting. Once the SunSPOT leaves the arm and is in the air, it should be in free fall until it hits the ground, yet it clearly shows the tumbling. What do you supposed causes that effect, spinning? It doesn't look like the tumble could be fast enough. Hmmm.

Posted by Brian Utterback on July 15, 2008 at 01:16 AM PDT #

@Brian - Yes, you are absolutely correct. What appears to be happening is that accelerometer is not at the center of mass of the Sun SPOT, so you are seeing the centrifugal force of the spinning SPOT in the air. Because the eDemoBoard is near the top of the Sun SPOT and the accelerometer is on the top of that board, it is nearly impossible for it to be at the center of mass. Since the Sun SPOT will rotate around the center of mass the accelerometer will register the superfluous centrifugal forces.
Of course, the Sun SPOT was spinning because it was actually a little large for the "pocket" of the trebuchet. It was designed to throw very small objects like marbles. The Sun SPOT is a bit off balance as it gets thrown... in fact we had a tough time keeping it in the pocket long enough to get a good throw.

Posted by guest on July 15, 2008 at 03:48 AM PDT #

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Roger Meike, Senior Director of Area 51 and Director of Operations Sun Labs

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