RE: Name that forceI don't see this much differently than when we push on one box and that box is pushing another. There is a contact force between the first box (the train) and the second box (the rain). These are a N3L-pair.
In the free-body-diagram (FBD) of the first box (train), we include the force on the first box due to the second. In the FBD of the second box (rain) we include the force on the second box due to the first.
Therefore, as Ken Fox stated, we simply call this the rain-on-train force. Granted, it is more complicated than the two boxes, but I don't see why the extra complication should change the name.
I would like to make one disagreement with what Ken said, then I would like to say a bit more about the complicatedness of this situation.
* * * Subscript Quibble * * *
Ken said the rain-on-train force would be Fsub(rt). I would call it Fsub(tr). The subscript closest to the variable ought to specify the object the force is acting on.
The force on mass 1 because of mass 2 ought to be F12. The force on mass 2 because of mass 1 ought to be F21.
Similarly, delta-V21 ought to be the potential at point 2 relative to point 1; that is: V2-V1.
I have seen textbooks that do this backwards, textbooks that do it correctly, and textbooks that do it one way in some chapters and the other way in other chapters. This is very frustrating. I have also seen textbooks that insert a comma to imply the other way around. For example the force on mass 1 because of mass 2 ought to be written F12 but some authors write it as F2,1. It appears they know it should be F12, but rather than thinking "force on 1 because of 2" they would rather think "force of 2 on 1" and they write F2,1.
* * * Complication * * *
Of course it is only the new rain that causes a force. The old rain that has already accelerated to the velocity of the train is no longer exerting a force on the train, it is "part of the train." Also note that if the train car were powered such that it maintained constant velocity, then the force of the new rain on the train would be constant when the rainfall rate is constant. However, in the case of the coasting car, the force is not constant even if the rainfall rate is constant because the train is slowing down. Therefore, when we call this the force on the train because of the rain, Fsub(tr), we need to realize it is only the new rain that is currently accelerating that is causing the force, and we have to realize the force is not constant in this problem.
Perhaps it is the "new rain" aspect that bothers people in calling this "the force on the train because of the rain." That is, the "agent" causing the force keeps changing. Think of it like a bunch of people carrying something heavy. As they wall along, sometimes stumbling, sometimes getting a better grip, sometimes resting, the load distribution is always changing among the participants. But I don't have any problem using the collective word "people" and naming "this force" as "the force on the load because of the people."
It's an interesting problem. But I don't have a problem naming the force.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton College
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu
-----Original Message-----
From: SSHS KPHOX [mailto:kphox@CHERRYCREEKSCHOOLS.ORG]
Sent: Tuesday, October 28, 2003 9:13 AM
To: PHYS-L@lists.nau.edu
Subject: Re: Name that force
Forum for Physics Educators <PHYS-L@lists.nau.edu> writes:
What is a
good name for a force responsible for the
change in speed?
It is part of the action reaction pair that the train applies to the rain to start the water moving in a horizontal direction. Thus the rain pushes back. I call it the rain-on-train force; Fsub(rt)