How to feel energy so I can imagine it?

Question

Please tell me what is energy in reality and how can I imagine it . Please don't rearrange the question . Thanks

Answer 1

The simplest definition of energy is that it is a capacity to do work. Energy occurs in various forms, some of which are:

• Kinetic energy - energy that an object has due to its speed. If two objects have the same speed, the one with more mass has more energy. If two objects have the same mass, the one with the greater speed has more energy.
• Gravitational potential energy - energy that an object has due to its position in a gravitational field. Moving an object against the force of gravity - such as carrying it up a hill - gives it gravitational potential energy.
• Chemical potential energy - energy that an object has due to the chemical bonds between its atoms. This can be released by various chemical processes, such as burning an object.
• Heat energy - energy that an object has due to its temperature, which is in turn a measure of the thermal vibrations of its atoms and molecules.
• Radiant energy - energy carried by electromagnetic radiation. Each photon carries energy, and a shorter wavelength photon carries more energy than a longer wavelength photon.

In general relativity, mass and energy are equivalent and are related by Einstein's equation

$$E=mc^2$$

For more information read this Wikipedia article.

Answer 2

I like to think of energy as "stored" motion. Or more technically, "stored" momentum.

• Speed and position is is what defines motion.
• Acceleration is a change in motion.
• Forces cause acceleration.
• Interactions and collisions with their momentum transfers cause forces.
• What then allows for interactions to take place, for momentum to be transfered?

That would be energy. The fact that momentum is carried and stored is what we describe as kinetic energy. The fact that momentum can be initiated when something is released is due to potential energies. Etc.

Energy seems to explain - which none of the other factors above do - this fact that future motion can be "stored" over time. When released, motion can appear (when a compressed string is let go, when a book is pushed off the shelf etc.).

Answer 3

Energy is not the ability to do work. There is no relationship, in general, between energy of a body and work done by a body. For example, the kinetic energy of a body $K$ represents the work that the forces applied to $K$ have to do to slow it down to zero speed. And what is the relationship between the work of the forces applied $\textit{on}$ $K$ and the work of the forces applied $\textit{by}$ $K$? In general, there is no relationship whatsoever.

Concrete example. An object $A$, with positive electric charge, is launched along a horizontal plane, without friction, towards a fixed object $B$ (nailed down, it can not move) which is also positively charged. Since each of the body generates a repulsive force on the other one, $A$ moves towards $B$ losing speed. If the initial speed is not that large, it will stop before touching $B$ and it will restart to move in the opposite direction. So, after slowing down, $A$ loses all of its kinetic energy. How much work has it done? Zero, since $B$, on which the force coming from $A$ is applied, has not moved. On the other hand, even though $B$ has not kinetic energy, it has done a work equal to the kinetic energy lost by $A$. The conclusion is that a body without kinetic energy can do a lot of work. Hence, the kinetic energy of a body does not represent its ability of doing work.

One can extend this argument to potential energy. The potential energy of a body is not the work that it can due to its position. This is not okay, because defining the potential energy of a body $K$ as a function of its position means defining the potential energy of $K$ in terms of displacements from its actual position and the reference position. However, for conservative forces, what we can associate to the displacements of $K$ is the work of the forces applied to $K$, not the work of $K$ on other bodies (recall the definition: a conservative force is a force with the property that the total work done in moving a particle between two points is independent of the path taken).

Moreover, if really the potential energy $U$ of $K$ were the work that $K$ can do, the variations of $U$ were due not to the work done by the forces acting on $K$, but to the work done by $K$. And this would be catastrophic, because even if all the forces applied on $K$ were conservative, the energy of $K$ (sum of kinetic and potential energy) would not be conserved.

Instead, one could define the potential energy as the work that the conservative forces applied to $K$ would do, whenever $K$, for whatever reason, changes its position and reaches the position of reference. è il lavoro che le forze conservative applicate a K compirebbero qualora, peruna qualsivoglia ragione, K cambiasse posizione, portandosi nella posizione di riferimento

Hence, energy is not, in general, the ability to do work.

References

Prof. Giovanni Tonzig, "Cento errori di fisica pronti per l'uso" (in Italian language).