Talk:Body force

message: I had always thought "body-force" was the ratio of force to mass, not volume i.e. for gravitational force it is "g" or "g cos(angle)" if there is an angle between the direction of flow and the direction the gravity force acts.
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There are some major mistakes on this page, or lack of information maybe.  I changed the equation F=rho*a to F=rho*velocity squared*area as is shown in the fluids books.
I came here looking for a better understanding of body forces for my fluids class and,... no dice. eximo (talk) 01:13, 7 February 2008 (UTC)

message: Hey I linked this page from fluid statics and upon checking it out would like to help improve it.  I'm just starting to look at the article though.  Could someone explain need to include the following line in the intro paragraph:
The fictitious forces associated with a non-inertial reference frame may be viewed as body forces. 
I understand you are referencing Coriolis and centripetal affects, but the term fictitious force is not widespread and in my opinion adds confusion to what should be a straightforward opening paragraph for a relatively simple topic.  I'm not going to get into a debate about whether the term fictitious force is relevant or not, as I'm sure many physicists and possibly engineers as well utilize it.  I'm making the statement that even though I am a graduate student in mechanical engineering and fully versed in dynamics, this term confused me as I had never heard of it.  If you want to maintain it as an example, just place it in a paragraph in the article.  But since this should be a very fundamental topic, why risk alienating readers who may not have heard of this, since even on the fictitious forces talk page there is ongoing debate about the term's validity.  
Iron_Engineer (talk) 07:18, 1 July 2009 (UTC)
thread_title:  Editing 

message: There is a paragraph in the qualitative definition section which says,
"A body force is distinct from a contact force in that the force does not require contact for transmission. Thus, common forces associated with pressure gradients and conductive and convective heat transmission are not body forces as they require contact between systems to exist. Radiation heat transfer, on the other hand, is a perfect example of a body force."
I have bolded the last statement because it looks like it's probably incorrect, which seems plausible given that it is uncited and others seem to think this article is generally rather bad right now. It would make sense to say that radiation is a body force if not requiring contact between the objects was all that was necessary to make something a body force. Thus, it seems plausible that someone added this after seeing the preceding sentence (and/or a similar one elsewhere) and noted that radiation does not require contact between objects made of "matter" to transfer heat, unlike conduction and convection, and so decided to note it.
I have two main problems with this statement:
Firstly, radiation heat transfer is not a force at all on a macroscopic scale, although I suppose it does involve a whole bunch of random "forces" pushing on particles in the two objects to slow down particles in the cooling object and speed them up in the heating object (on average relative to the center-of-momentum frame of reference of each object). The concept of body forces versus surface forces seems very much to be a macroscopic concept likely to occur in continuum mechanics or other classical theories but not in statistical mechanics or quantum physics, so I think it's fair to say that heat transfer, in and of itself, is not a force. If this reasoning holds, then the wording of this statement, at the very least, is wrong.
Secondly, for objects that are opaque to the type of radiation being considered, the macroscopic forces resulting from radiation pressure (which acts on objects emitting, absorbing, or reflecting the radiation) behaves like a surface force, not like a body force: For opaque objects, radiation is effectively only emitted at the surface of the object, because any internally emitted radiation (of the type considered) is immediately absorbed, producing no net change in momentum (or temperature). Similarly, for opaque objects, all absorption and reflection of radiation also happens at the surface, because no radiation (of the kind under consideration) can penetrate any deeper than that.
The only situation in which it makes sense to call radiation pressure a body force is when objects that are semitransparent to the relevant types of radiation are considered. In this situation, emission, absorption, and reflection, and therefore momentum transfer, i.e. force, is distributed throughout the body and not just at the surface. Even this situation is a somewhat unusual type of body force, though, since the radiation attenuates exponentially as it passes through the semitransparent material. Thus, the force exerted on each part of the semitransparent body is dependent on the length of the path(s) the radiation took to get there (as well as the specific opacity of the parts it passed through if the object is not homogeneous with regard to opacity). I don't think this prevents it from being a body force, and I even think that other, perhaps more clear-cut, types of body force, like forces from static electromagnetic fields, may sometimes have properties somewhat like this (e.g., due to differences in permittivity and permeability); however, when we combine this with the fact that many if not most objects are effectively (though perhaps not totally) opaque to most of the radiation involved in the radiation heat transfer situations usually considered and the fact that radiation pressure is usually negligible anyway, I think it's quite dubious to call this "a perfect example of a body force", even if we were to replace "radiation heat transfer" with "radiation pressure" or "forces associated with radiation pressure".
Also, I personally don't know what forces related to conductive heat transfer there are, but I don't doubt that there are some.DubleH (talk) 22:02, 23 January 2021 (UTC)
thread_title:  Radiation heat transfer