a frictionless pendulum
a mass on a spring in a vacuum
This is College Physics Answers with Shaun Dychko. A closed system is one in which there is no energy lost or gained. So the total energy in the system stays constant. So, now for earth, the system is not closed because it receives energy from the sun and then it re-radiates some of the energy and there's always this exchange of energy. Same with a car, there is energy taken away due to friction as it's traveling through the air and so it's not a closed system. For a pendulum, all the energy that it has you know, it stays in the system so if it has some kinetic energy, you know it's going to get turned into potential energy and back into kinetic and so on and so on and will continue going forever like that. It's the same kind of story for a mass on a spring in a vacuum.