String (rod)It holds the bob. It can be a string or a rod.
Pivot (point of suspension)The fixed point the pendulum swings about.
What a drawing marks 6
LengthMeasured from the pivot to the center of the bob.
Equilibrium position (rest position)Where the bob hangs when it is not swinging.
AmplitudeHow far the swing goes from the rest position.
Arc (path of the bob)The path the bob follows as it swings.
Period (one full swing)The time for one swing over and back.
Gravity (weight)The pull that brings the bob back down.
How this list was compiled: Every name checked on 2026-09-30 and kept only where at least two of these use it. (1) Wikipedia, Pendulum, https://en.wikipedia.org/wiki/Pendulum : a weight (the bob) suspended from a pivot on a cord or rod; equilibrium position; period (the time for one complete cycle); amplitude (the width of the swing); length; gravity as the restoring pull. (2) Carhart and Chute, The Elements of Physics (1897, public domain), full text at https://archive.org/details/elementsofphysic00carhrich : the bob on a slender rod, centre of suspension, single and complete vibration, period, amplitude, length of the pendulum. (3) A. P. Gage, The Elements of Physics (1898, public domain), full text at https://archive.org/details/elementsphysics00gagegoog : pendulum bob, point of suspension, the arc through which a pendulum swings, period of vibration, amplitude, the length from the point of suspension to the center of the ball, the force of gravity acting on the bob, position of rest. Licence: own work
The drawing: Own drawing from plain geometry; nothing traced. Licence: own work
How it works
A pendulum is a weight hung from a pivot and free to swing. The drawing shows one at the end of its swing, with the rest position and the other end of the swing dashed.
Read the full guide
The three parts
The pivot is the fixed point at the top. The string, or a rod, hangs from it, and the bob is the weight on the end. The length is measured from the pivot to the center of the bob.
The swing
Pulled to one side and let go, the bob swings back through its equilibrium position and out the other side. The amplitude is how far it swings from that position. The period is the time for one full swing, over and back.
A longer pendulum has a longer period: it swings more slowly.
Questions and answers
What are the parts of a simple pendulum?
Three things: a pivot at the top, a string or rod that hangs from it, and a weight on the end called the bob. Drawings also mark its length, its amplitude and its equilibrium position.
What is the period of a pendulum?
The period is the time the bob takes for one complete swing, over and back to where it started. It depends on the length of the pendulum and, only slightly, on the amplitude.
What is the amplitude of a pendulum?
It is the width of the swing: the angle between the string at the end of a swing and the string hanging straight down.
What is the equilibrium position of a pendulum?
It is where the bob hangs at rest, straight below the pivot. Gravity pulls a swinging bob back toward it.