I'm currently learning about potential energy, kinetic energy, and mechanical energy, and I have a few questions:
Does everything have potential energy? If yes, how so? For example, does a book laying flat on the floor have potential energy?
Can an object have only kinetic energy and no potential energy? And vice versa. And if there are instances like this, can the energy be considered mechanical energy?
No. Only systems of objects which interact with forces which do path-independent work have potential energy. If your system is simply a book it doesn't have gravitational potential energy (taking a guess at what you're thinking.) If your system is the book and the earth, that system has gravitational potential energy.
Individual objects don't have potential energy. If you consider a massive object to be composed of smaller objects, and those smaller objects exert forces on each other (e.g., electromagnetic interactions) they can have potential energy. For objects which do not change shape, these potential energies will not change and can be ignored for purposes of mechanical energy calculations.
As with most things in physics, the answers to your questions depend on your considerations in the system.
Generally, potential energy is related to the forces that an object is under influence by. If the object has no forces acting on it, you can't really say it possesses any potential energy. The potential energy indicates the ability for the force to do work on the object, or in other words, to increase its kinetic energy if it is unimpeded. The book laying on the floor is exposed to the gravitational force, so it has gravitational potential energy. The normal force from the floor acting on the book impedes the gravitational force, so it cannot increase the book's kinetic energy. If the normal force did not exist, such as if the floor broke, the book would fall, and its potential energy would decrease.
Not all forces, however, have corresponding potential energy, because not all forces have the ability to do work. The book does not possess a "normal potential energy" because the normal force can't increase the book's kinetic energy.
Of course, you could go further and say, the particles of the book are influenced by electric and magnetic forces! These will give potential energy to the system. The nucleons are tied together by the strong force, which also gives more potential energy. There are many sources of potential energy, but most of the time we do not account for them because the forces are in balance. In other words, the forces are impeded, so they can't change the kinetic energies of stuff. The most important potential energy to consider for the book would be for the gravitational force, because it may not always be in balance with other forces, like the normal force from the floor.
As said above, if an object has no forces acting on it, you can consider it to have zero potential energy. A block moving in space at constant velocity would have zero potential energy. Vice versa, the book in the above example has gravitational potential energy with no kinetic energy. Mechanical energy is the sum of both kinetic and potential energies, so yes, both cases would have mechanical energy.
What you're referring to is "gravitational" potential energy.. which is one of many forms of potential energy (gravitational, electrical, chemical, nuclear, etc.)
The idea behind potential energy is that it is energy that hasn't been yet released in an "obvious" form like movement, heat or radiation.
Think of it like money in the bank.. you don't say that it will potentially be money.. IT IS money! It is just money in a less liquefied form.
Same thing applies to energy. Potential energy - in all its forms - is not potentially energy - IT IS energy - the word "potential" comes from the fact that it will "potentially" be released as WORK (movement, heat, radiation, etc.) - or some form of "visible" energy - if you'd like.
Again with the bank analogy - potential energy is like "energy credit" that is stored within a particle due to its position in space. A book lying flat on the floor has zero potential energy when your "y = 0" is set to be the floor. If, by any chance, your book moves and ends up below the floor - it will have negative energy - which is not "negative energy" per se.. it is just an indication that your "zero" or "ground" wasn't deep enough.. which is not necessarily a bad thing.. When applying physics to real life situations.. it is often helpful to choose your "zero" of energy to be in such a place that is convenient to measure with respect to - this all points to concepts in Galilean relativity and how "frames of reference" relate to each other.
Now I am going to answer your question. From what I understand - you're referring to the "ABSOLUTE" energy an object can possess. In the sense that it is measured with respect to the "lowest zero" that can ever be.. and while that's a valid question.. It is very difficult and perhaps not even possible or even useful to quantify that "absolute zero".. There are just too many boxes energy can be hidden in.. Even if you drive that book to the center of the earth.. The book will still have gravitational potential energy with respect to the sun, moon, other planets and distant galaxies!
And that's not even talking about the electrical potential energy between the protons and electrons inside each atom of the book. And the nuclear potential energy that will be released if that book was to undergo nuclear fission, or the chemical potential energy released as heat and light if you were to set that book on fire, etc etc etc.
Potential energy is related to the arrangement of things in space - the arrangement of bodies within gravitational and electrical (and nuclear) fields giving rise to all the forms of "potential energy" or "energy credit" I've talked about. So in order to "find" the "absolute energy" you need to take into consideration all the spatial arrangements of your book with respect to other objects. In the sub-case of gravitational energy due to earth (your system is composed of the book and the earth only).. you'd need to measure your book's energy from the center of mass of the earth - that's ignoring other gravitational effects from the sun and moon, etc.
And that's calculable by integrating the gravitational force in spherical coordinates from the r = 0 (the center of the earth) to r = h (the height of the book from the center of the earth).
But even if you find that quantity - it will be quite practically useless unless you have a system that involves having your book dive deep into the center of the earth and you need to calculate the "potential energy released".
So that's why the idea of "frames of reference" is important in physics to "abstract away" the information that is relevant to us.
Hope this answers your question.
