Illustration of colliding pendulums, changing colour to show the relationship between gravitational potential energy and kinetic energy. At the start, both pendulums are at rest and have no kinetic energy, but they have potential energy due to gravity (blue). When they are released, the potential energy is converted to kinetic energy as gravity accelerates them downwards (purple). At the bottom of their swing, the pendulums are travelling at their fastest and hence have greatest kinetic energy (red).