Explain why the transformation of mechanical energy into internal energy remained unnoticed for a long time?

The most fundamental law of nature – the law of conservation of energy was obtained from observations of the physical processes of the transformation of mechanical energy:

potential to kinetic;
kinetic to potential;
energy consumption when doing work.

Reasons for doubts about the conservation law
In some cases, it seemed that the energy was still disappearing. For example, in the classic experiment of dropping a lead ball onto a lead plate. The potential energy of the ball raised above the surface of the earth gradually decreases when it falls on the plate, turning into an increasing kinetic energy. This is followed by a hit on the plate and subsequent changes in the ball-plate system are not visually traced.

Only with the advent of high-precision instruments were they able to detect subtle deformation of the plate and ball and the heating of these objects to an insignificant temperature. This made it possible to understand that the mechanical energy of the ball did not disappear, but passed into the internal energy of the plate and the ball.

Internal energy
The development of the molecular kinetic theory made it possible to clarify the concept of the internal energy of a body. It includes the energy of interaction of molecules and atoms with each other and the kinetic energy of oscillating or chaotically moving particles of matter. The deformation of the plate and the ball indicates changes in the distances between their atoms, the emergence of elastic forces inside bodies – the potential energy of atoms. Heating the plate and the ball speaks of the intensification of the vibrational motions of atoms in the nodes of the crystal lattice of lead – the kinetic energy of atoms.

Therefore, the transformation of mechanical energy into internal energy remained unnoticed for a long time.