Where does the lost kinetic energy go in a perfectly inelastic collision?

Question

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Answer 1

Transferred to the surroundings as other forms of energy:

Imagine 2 trains crashing:

Deformation - due to energy
Heating of the surroundings - due to energy
Sound of crash - due to energy
Flying shrapnel - due to energy

All these other energy processes are transferred from the original kinetic energy, although the bulk of it is transferred as heat.

Answer 2

The kinetic energy is converted to heat energy if there is no velocity after the collision. Otherwise the kinetic energy is the momentum of the combined masses.

Answer 3

For large slow objects, it generally and mostly goes into heat energy. The plastic deformation entails internal friction which increases the objects' temperature. A popular demonstration of this is to kneed a clay ball for a period of time. It becomes noticeably warmer.

For atomic or subatomic collisions, it goes into internal excitation and/or particle ejection and/or particle creation.