List of joints

=Use of the joints=
 * 1) You can create joints trought the SketchyPhysicsJoint toolbar. If you can't see it, go to View/Toolbars and click on the corresponding button.
 * 2) To select a joint, right-click on the corresponding button.
 * 3) Click one time in your model to set the joint center. Click a second time to select the joint axe.
 * 4) To link a joint to an object (group) select the "Joint connector" tool, click on the joint, then Ctrl+click the object. To check if the two objects are linked, click any of them. You will see a yellow cage around the joint and a green one around the object.
 * 5) Object can be linked to more than one joint, and vice versa. There is a bug with connecting muliple objects to a "controlled" joint.
 * 6) Joint can be grouped to objects in order to make them movable. Joints grouped with objects became new objects and can be linked to another joint.
 * 7) You can add values to the joint's variables, like maximum and minimum. This is another subject, but remember that angular values are expressed in degrees, and linear values in inches.

=Types of joints=
 * 1) Hinge: Object linked to this joint can spin around his center.
 * 2) Servo: Basically a controlled hinge, suitable to control arms, doors and similar things.
 * 3) Motor: Another controlled hinge, build to control wheels- only maximum speed is available.
 * 4) Slider: Object linked to this joint can move linearly between the maximum and minimum.
 * 5) Piston: The controlled version of the Slider joint.
 * 6) Gyro (for gyroscope):Object linked to this joint will have the corrispondent axe locked.
 * 7) Fixed: This joint was made to fix an object to another in order to remedy some problems that comes with grouping special objects(emitters, thrusters) to other objects. It is also useful for his "breakingForce" variable- if something hits the object with enough force, the link breaks up.
 * 8) Corkscrew: Object linked to this joint can spin around his axe and move between maximum and minimum.
 * 9) Spring: Object linked to this joint can move linearily along his axe. The spring joint will try to bring back them to the first position.
 * 10) Ball: Object linked to this joint can rotate freely around the joint. The object axe points to the joint.
 * 11) Universal: Object linked to this joint can rotate almost freely around the joint. Unlike the Ball one, the Universal joint won't allow the object to rotate around his axe.
 * See those images (I found them in an old post of CPhillips) to understand the concept:
 * http://www10.atwiki.jp/bambooflow/?plugin=ref&serial=19
 * http://www.impulse-based.de/manual_images/balljoint.jpg