A child sits in a swing at a playground. Her father pulls her back and releases her so that she swings back and forth. The motion of the swing changes over time, and eventually the swing is at rest.
What measurable quantity determines the potential energy in this situation? When is it a maximum? When is it a minimum?
What measurable quantity determines the kinetic energy in this situation? When is it a maximum? When is it a minimum?
When the swing is first released it has mechanical energy. When it comes to rest, it has none. If the law of Conversation of Energy says that the energy of a system stays constant, how can we explain this using the physics of energy?