[Garrett.d.w]

The easiest (albeit crudest) way that I can think of doing this involves some 2*4 lumber, a few half inch bolts, a drill, and duct tape.

First, build two triangles out of some 2*4 lumber (these triangles can be any size that you want, as long as they are the same).

Next, mount them so that they are parallel to each other. The mounts must be sturdy if you don't want this thing to discombobulate in the middle of the demonstration.

Finally drill a hole that is a half in diameter at the same height on both triangles (this will be the pivot point for your lever.

That will be your base.

Now for the arm.

Simply take a large piece of lumber, drill a hole 3/4 of the way down the length (1/2 to match the holes that are located on the frame).

Using tape, or any other method, secure your hammer to the end of the longer side of the arm.

Attach this to the frame using a half inch bolt through the mounting holes we drilled earlier.
With any luck, it should now look like a medieval trebuchet.

Now we need to power this contraption. To do this, we will attach a short length of 2*4 lumber to a variable speed drill (this will be our cam). You will have to play around with the mounting and size of the cam. Your goal is to have the cam push the short end of the lever down (raising the other end), and then drop it so that the hammer can descend.

If you want, I can draw this all for you. It will probably be easier to understand.