Feynman Lectures Vol.1 10.5 Relativistic momentum: Why does heat energy can be easily "hidden" in random motions of the atoms of a body? In section 10.5, Feynman says that

In some of these cases, heat energy for example, the energy might be said to be “hidden.”

He then further explains why the heat energy can be "hidden."

The random motions of the atoms of a body furnish a measure of heat energy, if the squares of the velocities are summed. This sum will be a positive result, having no directional character. The heat is there, whether or not the body moves as a whole, and conservation of energy in the form of heat is not very obvious.

The problem is that I did not fully understand this explanation. It seems very vague to me.
What I've understood is that Feynman was trying to say it is hard to find how much heat energy is in the atoms of the body just by measuring the squared velocities of the atoms because the squared velocities are involved with both movement of the whole body and the random motion of the atoms. Hence, we cannot accurately calculate how much heat energy is in the atoms  and Feynman called this as heat energy being "hidden".
Am I on the right track? If not, how should I understand the "hidden" heat energy as?

Just in case, I have put whole 2 paragraphs that contain above content related to my question including the 2 quotes above.

In Chapter 4 we saw that the law of conservation of energy is not valid unless we recognize that energy appears in different forms, electrical energy, mechanical energy, radiant energy, heat energy, and so on. In some of these cases, heat energy for example, the energy might be said to be “hidden.” This example might suggest the question, “Are there also hidden forms of momentum—perhaps heat momentum?” The answer is that it is very hard to hide momentum for the following reasons.


The random motions of the atoms of a body furnish a measure of heat energy, if the squares of the velocities are summed. This sum will be a positive result, having no directional character. The heat is there, whether or not the body moves as a whole, and conservation of energy in the form of heat is not very obvious. On the other hand, if one sums the velocities, which have direction, and finds a result that is not zero, that means that there is a drift of the entire body in some particular direction, and such a gross momentum is readily observed. Thus there is no random internal lost momentum, because the body has net momentum only when it moves as a whole. Therefore momentum, as a mechanical quantity, is difficult to hide. Nevertheless, momentum can be hidden—in the electromagnetic field, for example. This case is another effect of relativity.

 A: Feynman isn't talking about the difficulties of calculation -- he's referring to the question of whether, just "by looking"* we can see where energy/momentum is stored. Since energy sums up between atoms, motion that is in all different directions can add up to give net energy even if the object is not moving and so does not "look" high-energy. On the other hand, momentum cancels out with momentum in the opposite direction. So if we "look" at an object and it isn't actually moving, we know it doesn't have a store of momentum hidden in the atomic motion, since that atomic momentum adds up to the net momentum of the bulk object.
*("look" here being a loose word that could include basic experimental measurements)
A: Adding to what Zeldredge said, atoms are too small to easily measure. You see a continuous body instead of individual atoms. And even if you did get out your electron microscope, there are way too many atoms to measure all of them. And even if you could measure all of them, the energy of each changes every nanosecond or so.
Think of a crowd in a stadium. It is easier to measure the total noise than to measure how loudly each person is cheering.
A: It is rather easy to disentangle the movement of the body as a whole and the relative movements of the atoms in respect to the body - oen simply has to subtract the velocity of the center of mass of the body from those of the atoms. One can similarly exclude the rotation of the body as a whole. In fact, this is usually assumed to be done by most texts on statistical mechanics/thermodynamics, even though not all books focus attention on this point.
What could be more difficult is account for various forms of energy with a body, particularly when these are treated on quantum mechanical level. E.g., even in a gas consisting of molecules, one needs to take into account not only the velocities of these molecules, but the energy of their rotation and the energy of their atoms vibrating in respect to each other. One could also take into account the energy stored in their chemical bonds. Descending to lower scales, one would have to consider the energy of interaction between the electrons and the nuclei, and then the energy of interaction between nucleons in the nuclei. One could gow down further on quark level. All this becomes even more complex in a liquid or solid body, where the particles may be involved in various collective motions - e.g., the energy of aligned spins in ferromagnet, the energy of spin exitations (magnons), the exciton energy in photoexcited semiconductors, etc.
A: Feynman is essentially distinguishing between the macroscopic and microscopic view of kinetic energy and momentum.
The macroscopic view is that of the motion of an object as a whole, that is, the motion of an object we can actually see. This is the purview of classical mechanics and deals with motion of the object with respect to an external (to the object) frame of reference.
The microscopic view is that of the motion of the molecules that comprise the object, the motion that we can't see (that is "hidden"). This is the purview of thermodynamics and statistical mechanics and deals with the motion of the molecules with respect to the reference frame of the object.
While we can't observe the motions of molecules directly, we can indirectly measure the average of the squares of the velocities of the molecules, which is proportional to their average kinetic energy, in terms of the temperature of the object.
If the object as a whole is not moving, it tells us there is no average direction of the motion of the molecules, i.e., the motions are random and there is no net momentum involved. If the object as a whole is moving, we can say it has momentum and that the total kinetic energy of the object is the sum of the internal (microscopic) random motion kinetic energy of its molecules, plus the kinetic energy of the object as a whole, its macroscopic kinetic energy, which is independent of the internal kinetic energy. One is hidden, the other is not.
When Feynman says "The random motions of the atoms of a body furnish a measure of heat energy" what he is really referring to is a measure of the internal microscopic kinetic energy. That's because, in thermodynamics, the term "heat" strictly refers to the transfer of energy due solely to temperature difference. A body does not "contain" heat.
Hope this helps.
