Introduction
This works the same way the Bottle balance works. It's a triangle, the center of mass is at the end of the hammer where it meets the edge of the table, and if the end of the hammer goes past the edge of the table the whole system becomes unbalanced; causing the hammer, ruler, and piece of string to come crashing down in three pieces.
Principle
this is yet another great example of how torque works. The formula for torque is, Torque equals force in Newtons times the distance form the center of mass.
Materials
1 Hammer
1 10 centimeter string
1 12 inch ruler