Pendulum
For other uses, see Pendulum (disambiguation). For torsion pendulums, see torsion spring. For the mathematics involved with pendulums, see Pendulum (mathematics).
A simple gravity pendulum
or bob pendulum
' (plural pendulums
or pendula''), is a weight on the end of a rigid rod (or a string/rope), which, when given an initial push, will swing back and forth under the influence of gravity over its central (lowest) point.
The pendulum was discovered by
Ibn Yunus al-Masri during the 10th century, who was the first to study and document its oscillatory motion. Its value for use in clocks was introduced by physicists during the 17th century, following observations from
Galileo.
Pendulums (these may be a crystal suspended on a chain, or a metal weight) are often used for
divination and
dowsing. There exist many different techniques. One widely used form is the following. The user will first determine which direction (left-right, up-down) determines "yes" and which "no," before proceeding to ask the pendulum specific questions. In another form of divination, the pendulum is used with a pad or cloth that may have yes and no, but also other words written in a circle. The person holding the pendulum aims to hold it as steadily as possible over the center. An interviewer may pose questions to the person holding the pendulum, and it swings by minute unconscious bodily movement in the direction of the answer. In the practice of
radiesthesia a pendulum is used for medical diagnosis. However all these uses of pendulums are not scientifically tested or supported.
The most widespread application is for timekeeping. A pendulum whose time period is two seconds is called the second pendulum since most clock
escapements move the seconds hands on each swing. The period of the pendulum is the time taken for two swings (left to right and back again) of the pendulum. The formula for the period, T, is T² = 4π²l/g where
l is the length of the pendulum and
g is the acceleration due to gravity. The presence of
g in the equation means that the pendulum frequency is different at different places on Earth. So for example if you have an accurate pendulum clock in Glasgow (
g = 9.815 63 m/s
2) and you take it to Cairo (
g = 9.793 17 m/s
2), you must shorten the pendulum by 0.23%.The pendulum is therefore also used in surveying to measure the local gravity at any point on the surface of the Earth. A pendulum in which the rod is not vertical but almost horizontal was used in early
seismometers for measuring earth tremors. The bob of the pendulum does not move when its mounting does and the difference in the movements is recorded on a drum chart.As first explained by M. Schuler in his classic 1923 paper, apendulum whose period exactly equals the orbital period of ahypothetical satellite orbiting just above the surface of theearth (about 84 minutes) will tend to remain pointing at thecenter of the earth when its support is suddenly displaced.This is the basic principle of
Schuler tuning that mustbe included in the design of any
inertial guidance systemthat will be operated near the earth, such as in ships and aircraft.
Two coupled pendulums form a
double pendulum.
*
Pendulum clock*
Simple harmonic motion*
Foucault pendulum*
Spherical pendulum*
Double pendulum*
Kater's pendulum*
Harmonograph*
Metronome*
A more general explanation of pendulums
