Why a pendulum loses height
At the end of a swing, a pendulum briefly stops before reversing direction. Much of its energy is then stored as gravitational potential energy: the bob is raised above its lowest point. As it falls, that stored energy becomes motion, carrying the bob through the bottom and up the other side.
In an ideal system, the energy would transfer back and forth without loss, so the bob would rise to the same height each time. A real pendulum also has to move air aside, and its pivot resists motion. These forces divert a little energy from the swing into the surroundings, mainly as heat and sound. The bob therefore has less energy available to lift it on the next pass.
The slowing is gradual
This energy loss is called damping. It reduces the size of the swings over time rather than changing the basic back-and-forth motion. The missing energy has not vanished; it has spread into tiny motions and warming in the air, pivot, and pendulum. Because the losses are usually small on any one swing, the bob may continue moving for many passes before the motion becomes hard to see.
That is why a pendulum clock needs a mechanism to give the pendulum small, regular pushes. The mechanism replaces energy lost to damping, keeping the swing going—and its timing steady—instead of letting it fade away.
