Reactor

The reactor feature is very useful as it makes objects act how you want them to act, without having to animate a path for it. For example adding weight to a sphere so that it falls when the animation is played.  This can be done by right clicking on the blank space next to the render button, to bring up the reactor panel. First create a box, with several spheres above it and make all of the objects rigid, this will make them solid. Next add weight to the spheres which will make them fall onto the box. Example shown below:

Now create another sphere and convert it to an editable poly. Take the vertex on the top part of the sphere and bend it in to form a bowl. Now in the object properties editor box, in the reactor panel make the bowl a concave mesh, which will make the reaction more like a bowl. Place the bowl underneath the spheres and play the animation, the spheres will act differently when the fall into the bowl.

Curtains can also be made using the reactor feature. First create a curtain pole, using a cylinder and a couple of spheres, then make a couple of hooks and place them on the curtain pole. Next make all of these objects rigid, which will make them a solid shape. Open the object properties editor and check the box “unyielding” which will allow the objects to move.  Next create two planes and give both of them the cloth modifier; this will make the planes appear like material. Now we need to connect the planes to the curtain hooks, in the modify panel click on reactor cloth and go down to constraints. Click on attack to rigid body then select part of the vertex of the plane and click on the “none” button, then the object you want to attack it to, in this case one of the hooks. Repeat this step several times until all of the hooks hare linked to the planes. Now make sure that nothing is selected so that the animation part can begin. Using auto key move the hooks apart, which will move the curtains when the animation is run.