# Can there be energy with no force or energy with no power?

I think that both force (number of newtons) and power (p=ui(?)) implies that there is energy so we can't have force without energy and we can't have power without energy(?)

But can there be energy that is energy and no force and no power?

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I'm having trouble understanding your question. Force is defined as the rate of change of momentum and power is the rate of change of energy. If force/power are zero that implies that the momentum/energy resp. are constant, not absent... And energy is energy tautologically. What do you have in mind for energy that is not energy? –  Michael Brown Mar 16 '13 at 13:44
Exactly what I had in mind was whether I'm correct in presuming the existence of energy based on the knowledge that there is some force or some power. I'm trying to know what I can know if neither force nor power are mentioned while energy is the topic. –  Niklas in stockholm Mar 16 '13 at 13:56
If I throw a ball in outer space, it has energy (1/2 mv^2), and if it never interacts with anything else, it will have no force and no power. @Nick, I think if you provide some physical examples to illustrate what you are asking, it may help us understand. –  Mew Mar 16 '13 at 15:07
I wonder how e.g. radiation can be a power or a newtonian force. We can get radiation from the creation of the universe but we can't really call that radiation a newtonian force or a power from the big bang or can I say that the big bang is causing newtonian movements today(?) e.g. is radiation a newtonian movement(?) –  Niklas in stockholm Mar 16 '13 at 15:48
A single photon of radiation has a constant energy E = hf, and there will be no force or power if this photon does not interact with anything. However most radiating sources, are releasing many photons per second, and therefore one attributes "Power" to the source, which is related to the number of photons emitted per second. In the case of the Sun, the sun is losing its internal energy, and transferring the energy to light over time, and therefore has "Power". Each individual photon of light however does not have force or power until it interacts with something else. –  Mew Mar 17 '13 at 0:17

## 4 Answers

You ask:

I wonder how e.g. radiation can be a power or a newtonian force. We can get radiation from the creation of the universe but we can't really call that radiation a newtonian force or a power from the big bang or can I say that the big bang is causing newtonian movements today(?) e.g. is radiation a newtonian movement(?)

Radiation classically carries energy and if the radiation interacts with a body, then power is transmitted , where power is the rate of change of energy. For example there is radiation pressure in vacuum , in this link. Radiation does transfer energy in a newtonian way, where the force can be estimated from dp/dt, the instantaneous change of momentum .

The big bang is not a simple story to be expounded here and used as an example. Have you read the wiki article?

But can there be energy that is energy and no force and no power

Yes, as said in the comments to your question only interactions need the concept of force and power.

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Are you saying that the universe itself is energy with no force or power (providing there's nothing else in a multiverse for it to interact with)? Otherwise, radiation pressure in a vacuum would exert forces on its container, and anything with mass would exert graviational forces on everything else. –  Ehryk Jan 1 at 20:10
@Ehryk I am not discussing the big bang in this answer , just pointing out that radiation does carry momentum and thus in interacting can give a dp/dt . –  anna v Jan 1 at 20:29

A body when is in equilibrium and is still in a state of motion may have energy but has 0 net force. And Power is work done per unit time and in many cases work is done changing type of energy. If a body is in equilibrium, a body is most possibly in uniform motion, so energy is not getting transformed into anything else, thus there is no change in amount of energy resulting no work done i.e. no power. So , there can be energy but no force and power.

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You can have energy without force or power.

Rest energy ($c^2m$) is an energy with no force. Kinetic energy is another energy without a force (unless you think of inertia as a force, but I wouldn't recommend it). Changes in energy generally are due to forces.

But change in energy is power. But you have to be careful, you can have an energy density, and then the time rate of change of that is a power density. But you can also deliver work in a region, and there is a time rate of that, power delivered in a region. They aren't exactly the same conceptually.

For instance ... imagine a spatially uniform amount of energy density, moving at a uniform direction and rate, since both are uniform the power density doesn't change, so there is no power density, but power is being delivered, it's just that every region has a no net power delivered but it gets an amount equal to what it gives.

So, I'd still say break it into nouns and verbs, and don't confuse which is which.

Forces are changes of momentum, a secondary effect of a change in momentum might be to change the kinetic energy. So a force may (or may not) change an energy. If the energy changes then its change would be measured by power. So power and force change energy and momentum respectively. If you start asking about which did what by how much, then you might talk about momentum or energy imparted by a particular agent, which could be different that the total change in momentum or energy.

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Rest energy necessitates mass, would exert gravitational forces on everything around it. –  Ehryk Jan 1 at 19:48
The forces of external objects are not required for the rest mass energy to exist, you can make those forces as small as you want by placing the body very far from other masses. Once a body has the rest mass energy it has, no external forces are required for it to persist in having that energy. You can also use General Relativity where gravity is not a force, and there is still rest mass energy. –  Timaeus Jan 1 at 19:59
I made no claims on persistence; but unless it was a single particle (thus not exerting gravitation on itself), which is not really accurate since it would still consist of quarks exerting internal forces on each other, it would have to be the only mass in existence in the entire universe to exert no force. Even a single particle billions of lightyears away would exchange some force. –  Ehryk Jan 1 at 20:13
The OP was about having it (i.e. persistence). And you can have a lepton if you don't want to worry about quarks. And they you can use GR to not have a gravitational force, or you can consider a universe with just that lepton. Or you can note that since those forces can be made arbitrarily small they clearly aren't required for the persistence of the energy. Energy is a thing that can persist on its own without some minimum forces. A fixed amount of rest energy can exist in a universe where there is arbitrarily small amounts (or no) force. –  Timaeus Jan 1 at 20:24
I read the question differently, but sure if theoretical alternative universes are allowed then sure perhaps a forceless energy would be possible. But not in the one we currently live in. –  Ehryk Jan 1 at 20:30

From my interpretation of your question, are you asking if there is a form of energy which exerts no forces? If so, I'd say absolutely not.

Basically, to not exert any forces it would have to be: motionless, massless (or otherwise not interact with gravity), nonvisible, at absolute zero (if it had mass), uncharged, and noninteractive with matter.

If you're comfortable with only gravitational forces, then Neutrinos and Neutrons at rest and close to absolute zero are pretty close to forms of energy that do not exert much force.

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