A pendulum is an object that consists of a mass on the end of a string, wire, or thin rod.
The string, wire, or rod is attached to a surface so that the mass hangs below it.
When the pendulum hangs straight down, it is in its equilibrium position. The weight of the
mass is balanced by the tension in the string. There is no net force acting on the mass at the
end of the pendulum, so it does not move.
When the mass on the end of the pendulum is raised to one side by hand, the weight
of the mass still acts straight downward, but the tension in the string acts diagonally
upward. The tension only partly counterbalances the weight of the mass. This results in a net
force that is perpendicular to the string.
When the pendulum is released, the net force accelerates the mass, and the mass moves closer
to its equilibrium position. However, when the mass reaches its equilibrium position, it has a
sideways velocity, and it overshoots its equilibrium position. The mass moves past the
equilibrium position, and since the string keeps the mass at a fixed distance away from where
the pendulum is attached to the surface, the mass moves upward as well as to the side.