Energy: a currency that is conserved
At school it is often said that “energy is neither created nor destroyed”. That is a strong statement, but also a slightly mysterious one: what actually guarantees that the sum of the terms stays the same? Historically, no “first law” imposed this conservation: scientists built it, inventing new types of energy each time the balance sheet seemed not to close. It was Joule who recovered the energy lost to friction by calling it “thermal energy”; it was Mayer who guessed that the heat of the human body is transformed chemical energy; it was nineteenth-century engineers who invented the concept of elastic energy in springs. Each time, the idea was the same: if we want the balance sheet to close, it is convenient to admit that this new reservoir exists. Seen this way, conservation of energy is less an empirical discovery and more a successful research programme: nature, every time it has challenged us to find the missing coin, has been generous enough to hide it in an unexplored drawer. Feynman discusses this with great clarity in his Lectures (Feynman, Leighton and Sands 1963, vol. 1, ch. 4).
Try it — interactive simulation
The skater continually converts potential energy into kinetic and back: without friction total energy stays constant, with friction it dissipates as heat.
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Topics: Work and energy Concepts: Conservation of mechanical energy · Thermal energy from friction
Related exercises: Worked exercise — block on an incline with a spring and friction · Problem — Vertical projectile with resistance · Problem — Conservation of energy with friction