The nature of force

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by tvanflandern</i>
<br /><blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Thomas</i>
<br />For a collision of two particles you need a force acting between them because otherwise their momentum could not change (F=dp/dt) and you would thus by definition not have a collision.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">By definition? What definition of "collision" are you using?<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
The definition that a particle changes momentum on encounter with another particle . Now since dp/dt=F , a momentum change must be associated with a force field acting between the particles. Without a force field the particles would not 'know' of each other and would be carrying straight on on their original path and you would thus not have a collision and hence no resulting macroscopic force either.

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www.physicsmyths.org.uk
www.plasmaphysics.org.uk

Thomas, EBTX,

We are still waiting for a *physical* justification for this force field you believe in. Until one is provided, my inclination is to read "magic field" when you write "force field".

LB

NOTE - "physical justification" means that, in general, no math is needed.

BTW, if any one else has an idea about how to turn one of these magic fields into something real, now would be a good time to speak up ...

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Thomas</i>
<br />For a collision of two particles you need a force acting between them because otherwise their momentum could not change (F=dp/dt) and you would thus by definition not have a collision.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">This seems to confuse a property of something with the thing itself, and to define an action by its effect rather than as a cause. If I paint something and it changes color, it would be weird to define painting as the act of changing color.

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">The definition [is] that a particle changes momentum on encounter with another particle.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">This implies that the momentum change causes the collision. But what about neutrons, photons, and in general entities that have no field? Or what if we have two particles in isolated space that collide inelastically, but we have no means to measure anything except their relative momentum, which does not change? Does that mean they did not really collide?

I find your definition to be inadequate for discussing physics, and contrived to lead to a predetermined outcome. I grant that one way bodies can collide is for their fields to interact. But not all physical entities have fields. If two swarms of bees fly through one another, I see three possible outcomes:
(1) No bees interact or change momentum. [By your definition, this means no collision occurred.]
(2) Some bees see the other swarm approaching and choose to randomly change their own flight path. [By your definition, this means a collision did occur because the momentum changed. The "force" acting is will power.]
(3) Some bees run into one another. [Does the fact that no force acts between the bees prior to contact mean that contact is not possible?]

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">Now since dp/dt=F, a momentum change must be associated with a force field acting between the particles.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">This is not the case in some of the examples I gave. Again, what about the action of photons on matter, or of neutron collisions, or of swarm encounters?

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">Without a force field the particles would not 'know' of each other and would be carrying straight on on their original path and you would thus not have a collision and hence no resulting macroscopic force either.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">So as I understand it, you maintain that dense particles of comparable size but lacking fields can occupy the same place at the same time without effect? -|Tom|-

The two view points here seem to me to basically revolve around the way force is transfered by contact or at a distance. There are so many details it is hard to know where to begin. Maybe the electron as modeled currently by all models should be first since it was the first quantifed detail and was measured as 1.6X10E-19J and is a universal constant. The current view of the electron is different. A second detail here is density-how is it that density decreases with scale since everything is made of smaller things? The field is not yet known very well and so what is?

[Jim] " ...how is it that density decreases with scale ... "

Your meaning is not clear. Do you mean " ... with (an increase of) scale ... ", or do you mean " ... with (a decrease of) scale ... "?

It is an observational fact that smaller things are more dense, on average, than larger things. For the range of scale that we are able to observe, of course. Our expectation is that as we expand that range with new microscopes and new telescopes, the observed trend will continue.

[Jim] " ... since everything is made of smaller things?"

I guess you still don't have a very good grasp of infinite divisibility?