The heat in a hot object is the mechanical energy of its atoms and molecules in motion. The sum of these is called mechanical energy. Kinetic energy is the energy of its motion when it starts rolling. Potential and kinetic energy are two of the most basic forms, familiar from high school physics class: Gravitational potential is the stored energy of a boulder pushed up a hill, poised to roll down. The forms that energy takes, however, are constantly changing. The universe itself is a closed system, so the total amount of energy in existence has always been the same. The law of conservation of energy, also known as the first law of thermodynamics, states that the energy of a closed system must remain constant-it can neither increase nor decrease without interference from outside. Even the seemingly paradoxical dark energy causing the universe’s expansion to accelerate, we will see, obeys this rule. Each of these situations, however, is simply a case of energy changing form. Sparks create a fire, which generates heat-manifest energy that wasn’t there before. The conservation of energy is an absolute law, and yet it seems to fly in the face of things we observe every day.
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