When I was younger, I loved swinging on the playground swings. I always tried to go as high as possible because it made me go faster when at my lowest point, which was an exhilarating feeling. What I didn't understand then I understand now when considering the physics behind it. If you set your reference level at the lowest point of the swing, then when you first get on the swing, you have no mechanical energy. Then, when someone applies a force to you, they do work on you, causing you to accelerate and travel to a higher level. The work that they do in pushing you increases the mechanical energy of the system. The more work that they do to you, the more mechanical energy they add to the system. Since mechanical energy is conserved in a closed system, the potential energy at your highest point equals your kinetic energy at your lowest point. Therefore, the higher you are able to go, the more kinetic energy you will have at your lowest point, which means that you will travel at a higher velocity at the bottom of your swing.