The Mysterious Disappearance of Energy
When two lumps of soft clay collide head-on and stick together, coming to a complete stop with no visible flash, sound, or motion afterward, where did all the kinetic energy go, if energy is supposed to be conserved?
History and references
Settled in 1843 by James Prescott Joule (mechanical equivalent of heat) and the general first law of thermodynamics.
The recognition that mechanical energy 'lost' in friction, collisions, and deformation reappears as heat was established through the work of James Prescott Joule and others in the 1840s, cementing the first law of thermodynamics and the general principle of energy conservation across all its forms. Lange's Chapter 4 ('Conservation Laws in Mechanics') puzzle uses an everyday inelastic collision to force readers to extend their notion of 'energy' beyond visible motion, a foundational step in understanding thermodynamics.
It is the everyday collision that first forced physicists to admit that heat is a form of energy, not a mysterious separate substance.
Explanations that circulated
- Treating 'kinetic energy of the visible, macroscopic motion' as the only form of energy that counts, so that when it vanishes in a perfectly inelastic collision it looks like energy has simply disappeared from the universe.
- Not accounting for the small, often imperceptible rise in temperature and the permanent plastic deformation of the colliding bodies, which is exactly where the 'missing' macroscopic kinetic energy went — converted into microscopic (thermal) kinetic energy of atoms and into the energy stored in deforming the material.
References
- On the Mechanical Equivalent of Heat, James Prescott Joule, Philosophical Transactions of the Royal Society, 1850
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