For a school project we have to make a trebuchet (medieval weapon, like a catapult but with a sling)
I have got an arduino Uno, but I'm still a beginnen and I don't know that much. So the problem is that I want to launch the trebuchet with arduino when a button is pressed, but I don't know how to attach the sling to something and let it go when the button is pressend.
I hope you could help me.
Thanks in advance!
How about connecting the button to a piece of string which operates the release mechanism? Google will give you plenty of examples of trigger mechanisms used to release this sort of device, and they don't typically involve a microcontroller.
Now I have seen/ heard everything! 
I love trebuchets, people throw all sorts with them from watermelons to pianos.
I think your problem will be that there's a reasonable amount of stress on the sling so if you just use a pin or something to release it you will need a fairly powerful actuator.
Normally they have an nice trigger mechanism AFAIK, this allows a small force to be used. So really this is a mechanical problem first, and an Arduino problem second.
How would you release it manually? Have you searched for release mechanisms?
Rob
PeterH:
How about connecting the button to a piece of string which operates the release mechanism? Google will give you plenty of examples of trigger mechanisms used to release this sort of device, and they don't typically involve a microcontroller.
best answer.
Yes we often forget the simpler approaches and throw an Arduino at the problem. Like the other day when someone wanted a LED to light when a signal went high. The obvious response was something like
digitalWrite(ledPin, digitalread(signalPin));
But to my credit I did point out that a length of wire would do the same thing 
Still in this case I suppose it's designed to be a learning exercise more than a reasonable use of an embedded computer.
Rob
Poke around on "The Slingshot Channel" and get a look at his trigger mechanisms; you'll find pictures like these. The two cylinders grip the object on the front side and his trigger keeps them from moving apart (or allows them to move apart to launch).
It might not be appropriate for your particular projectile -- gripping the part in that way, but you can always connect a dowel (etc.) with a string to the back of the treb's pouch and grip onto that instead of the projectile itself.
For a electromechanical actuator just go with an RC servo. Very easy to use. I'm sure it would be easier to do this with a string, etc., but I'm guessing you have a requirement of using a microcontroller as part of the project.