A simple pendulum, is a concentration of mass on the end of a string or rod

In an ideal world, a pendulum when set in motion would continue to oscillate indefinately. However, due to energy loss through air resistance, the amplitude of the oscillation will reduce and eventually the pendulum will come to rest. See free vibration.

Another type of pendulum is made up of a stiff body (with its own mass) vibrating about an axis. For these pendulums their orientation will not matter, they will oscillate with a constant frequency whether they have a mass attached to the top or the bottom of the stiff body. (see the example on video below.) In these cases, the frequency of the pendulum is dependant upon the stiffness of the material and this is a property that buildings have in common. They are stiff bodies with concentrations of mass up their length. ie.at the floor levels where beams are attached to the columns. They columns in buildings can be looked at as being very stiff pendulums. This idea is expanded on in the video clips, the free and forced vibration page and in the design and analysis of buildings in earthquakes.