Consider a tire swinging like a pendulum back and forth under the influence of gravitational force. The pendulum-Earth system is assumed to be an isolated system. As the pendulum swings, the system's energy varies back and forth between gravitational potential energy and kinetic energy, while the system's mechanical energy—the sum of the two—remains constant. When the pendulum is at its lowest position, it's at its maximum speed, and all the energy is kinetic energy. At its highest position, where the pendulum momentarily stops, the speed is zero, and all the energy is potential energy. At the intermediate positions, half the energy is kinetic energy, and half is potential energy. In any isolated system, the law of conservation of mechanical energy states that the total mechanical energy of the system at any instant remains constant. The subscripts refer to different time instants during an energy transfer process.