Pendulum
A simple pendulum oscillates in the vertical plane due to gravity.
A simple  or mathematical  pendulum consists of a rigid body suspended from a fixed horizontal axis about which the body may oscillate in a vertical plane due to the action of gravity.
The oscillating period for a simple pendulum can be calculated as
T = 2 π (l / a_{g})^{1/2} (1)
where
T = oscillating period for one cycle (s)
l = length of the pendulum (m, ft)
a_{g} = acceleration of gravity (9.81 m/s^{2}, 32.17 ft/s^{2})
Example  Calculate the length of a pendulum with an oscillating period of 1 sec, 10 sec and 100 sec
The length of a pendulum with a given oscillating period can be calculated by transforming (1) to:
l = (T / (2 π))^{2} a_{g}
Length for 1 sec  typically for an old fashion pendulum clock:
l = ((1 s) / (2 π))^{2}(9.81 m/s^{2})
= 0.249 (m)
Pendulum clocks were the world standard for accurate timekeeping for 270 years until the quartz clock was invented in 1927.
Length for 10 sec:
l = ((10 s) / (2 π))^{2}(9.81 m/s^{2})
= 24.9 (m)
Length for 100 sec:
l = ((100 s) / (2 π))^{2}(9.81 m/s^{2})
= 2487 (m)
Four Types Of Pendulums
It is common to differentiate between four types of pendulums:
 Compound or Physical Pendulum  A rigid body suspended from a fixed horizontal axis. The body may oscillate in a vertical plane due to the action of gravity.
 Simple or Mathematical Pendulum  Similar to the compound or physical pendulum but the mass is concentrated in a single point and oscillates back and forth in the vertical plane. The mass point is connected to a horizontal axis with a weightless chord.
 Conical Pendulum  Similar to the simple pendulum except that the weight suspended by the chord moves in uniform speed around the circumference of a circle in the horizontal plane.
 Torsional Pendulum  A disk fixed to a slender rod. The rod is fastened to a fixed frame. If the disk is twisted it will oscillate back and forth about the axis of the rod.
Related Topics

Dynamics
Motion of bodies and the action of forces in producing or changing their motion  velocity and acceleration, forces and torque.
Related Documents

Harmonic Oscillator
A simple harmonic oscillator.