Ok its finished. Here it is. Thank you to all who helped.

PERFECT.skp

Alright i have built a working trebuchet. I have added 3 scenes which i would like to view the launch from. But i have no idea how to record it and turn it into a video. i have tried by using the record button but i just don't understand it. I found the Export Animation thing to turn it into a video but all that does is go from scene to scene with out launching. So if you know anything about this i would greatly appreciate some help.
I have attached the working trebuchet.

Working Trebuchet

Using Mr.K's weighted block idea, i got the arm to swing and its awesome. Now because this is a replica of my actually trebuchet. It has to have a string and sling to launch the item. Does anyone know how to make string?

I have made more things groups and have managed to get the base of the trebuchet to stop the arm after i manualy swing it. i still need to attach a weighted block so the arm will naturally swing itself. Having dificult adding the weight and attaching the block to the end of the arm ans still having it act natural like its attached using a string or something. If you could help would be great. Thank You

http://sketchup.google.com/3dwarehouse/details?mid=7cbbac7a3747ff363a6035083031a088&prevstart=0
New Link

i actually have tried searching other trebuchets for examples, but it seems that everytime i check GUI its set to default. Like it wont show me what they did to their groups.

I built a trebuchet in regualr sketchup for a school project. I need to animate the trebuchet, i installed sketchy physics but i dont understand how to use it. i tried adding weight to an object but it never seems to affect anything. even when i set the object to showcollision it doesnt collide. Here is a link to my trebuchet can someone help my add weight to the back of the arm so it will work on its own.
http://sketchup.google.com/3dwarehouse/details?mid=8c085389b45fa95f1cf22ac79f83b3b&prevstart=0

I change the density of my block and it will fall but when i connect it with the back of the arm nothing happends ofcourse i have a hinge for my pivot but idk what joint to use to connect the weighted box(which i cant seem to get weighted after changing density of box) connect weighted box to end of my arm. plz help.