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From: TomGee on 17 Oct 2005 15:41 Randy Poe wrote: > TomGee wrote: > > Randy Poe wrote: > > > TomGee wrote: > > > > Randy Poe wrote: > > > > > The situations are: > > > > > (a) No external forces. Velocity and KE are constant. > > > > > > > > > > > > > > Velocity can be constant but aren't you saying that KE does not change? > > > > > > If velocity is constant, KE is constant. > > > > > > > I asked you to support that claim before, to no avail. Reference > > > > please, again. > > > > > > What claim exactly? That when velocity is constant, KE > > > is constant? Or that KE and velocity are constant in the > > > absence of external forces? > > > > > > > > There > > > > > is no force to do any work, so discussions of "work" > > > > > are meaningless. > > > > > > > > > I repeat: But you said a force performs work on an object at CV, > > > > > > Situations (b) and (c) muddled. There are situations with force > > > (situation (a)), and situations without force (b and c). When > > > there is no force present, I didn't say anything about what a > > > (non-existant) force does, in terms of work or anything else. > > > > > > I also didn't say a force performs work on an object at constant > > > velocity. If a force is performing work on an object, the > > > velocity is NOT constant. > > > > > > > > Here's what you said, "work is done by the component of force in the > > direction of motion". > > And I said that after introducing circular motion into the > conversation. > > Prior to that point in the conversation, I was talking about > straight line motion in the absence of external forces. > > > Newton's body having no external net forces > > acting upon it moves in one direction at constant velocity, which > > supports your and everyone else's assertion that work is done by a > > "force in the direction of motion". > > Newton's body moving in the absence of external forces says > nothing whatsoever about what happens in the presence of > external forces. > > > > I brought up force in discussing situations in which force is > > > present. Constant velocity is not such a situation. > > > > > Yet in those same situations you insisted there was no right angle > > force present, > > There are two situations I have discussed where force is > present. One of them, situation (b), is the case where force > is at right angles to the motion. I have never insisted > "there was no right angle force present" in circular motion. > Your statement above is contrary to fact. > > > so which force were you talking about that was present? > > In situations (b) and (c), there is a force present, either > at right angles to the motion (b) or not at right angles > to the motion (c). > > Let's start from here. I don't recall your situations, but I will accept the above as being correct wrt (b) and (c). > > > > > Most recently, I brought the concept of force and work into > > > the discussion on the question of circular motion, which is > > > emphatically *NOT* either a forceless or a constant-velocity > > > situation. > > > > > No, not "most recently", but immediately. And above you state that the > > question of circular motion is "NOT...a forceless situation" (meaning > > that there is a force present), > > Right. There's a force present in circular motion. > > > so if there is no right angle force present, > > There is a "right angle force present". More correctly, there > is a force present and it is at right angles to the motion. > > > which force is present? I know the answer and you know the > > answer, and everyone reading this knows the answer - the question is, > > are you willing to let the correct answer come out of your mouth > > What comes out of my mouth and what you SAY comes out of my > mouth are two clearly different things. > > > > There is no "resistance to the right angle force". The force > > > is applied and the path changes. > > > > > Oh, of course there is, silly! Inertia is a body's resistance to > > change when acted upon by a directional force. > > Yes, but "inertia" is not a force. > > > The inertial force resists the gravitational force. > > There's not "inertial force". > Yes, there is, everyone knows that. > > > > Everyone knows that. > > Everyone does not "know that". > > I hold a rock over a hole in the ground. I let go. Please > tell me how the rocks inertial force is resisting the > gravitational force. Describe the forces present and how > they are acting. > > Sure. The Inertial Force inherent in the rock is resisting the acceleration imposed by the gravitational force. Since inertia is resistance to change, once the rock drops from your hands, inertial resists the change from being relatively unmoving to moving toward the ground. As gravitation increases the rate of fall, the rock resists such acceleration, but since it is already in motion, it is not resisting any longer the change from being stationary to moving, it is only resisting the change in the rate of its speed. > > > > But up above you said, "And I still don't think that I have to talk > > about "unnamed components of force" in situations where the force is > > zero.", yet here you are again doing that very same thing in saying, > > "When the force is zero, all components are zero."! What made you > > change your mind? > > I didn't change my mind. I do not have to consider any components > of force, since they are all zero, in situations where the > force is zero. You are seeing contradictions where they > don't exist. > > "The northward component of force is zero" does not contradict > "there is no northward component of force". > > "None" and "zero" are synonyms. I did not "change my mind" > when I talk about no components in one place and zero components > in another. > > - Randy
From: Randy Poe on 17 Oct 2005 17:00 TomGee wrote: > Randy Poe wrote: > > In situations (b) and (c), there is a force present, either > > at right angles to the motion (b) or not at right angles > > to the motion (c). > > > Let's start from here. I don't recall your situations, but I will > accept the above as being correct wrt (b) and (c). Goody. To refresh the conversation: (a) No external forces. Straight line, constant velocity motion. (b) Circular motion. External force at right angles to the motion at all time. Constant speed, non-constant velocity (because the direction is changing). (c) General motion. External force at arbitrary angle to the motion, so it is doing work. Both speed and direction in general are changing. > > There's not "inertial force". > > Yes, there is, everyone knows that. No, there isn't. Inertia is not a force. Can you pull a quote from Tipler where he refers to the force of inertia? He's a someone, isn't he? > > > Everyone knows that. > > > > Everyone does not "know that". > > > > I hold a rock over a hole in the ground. I let go. Please > > tell me how the rocks inertial force is resisting the > > gravitational force. Describe the forces present and how > > they are acting. > > > Sure. The Inertial Force inherent in the rock is resisting the > acceleration imposed by the gravitational force. Since inertia is > resistance to change, once the rock drops from your hands, inertial > resists the change from being relatively unmoving to moving toward the > ground. I see. And as we watch it change from nonmoving to moving, we can tell that something is "resisting" this change by...? And if that "inertial force" weren't resisting gravity, then we would see...? To me, it looks like as soon as we apply a force, e.g. gravity, the mass accelerates. Nothing prevents it from doing so. Nothing resists it. And the rate at which it accelerates can be determined precisely from F = dp/dt. - Randy
From: TomGee on 17 Oct 2005 18:08 Randy Poe wrote: > TomGee wrote: > > Randy Poe wrote: > > > In situations (b) and (c), there is a force present, either > > > at right angles to the motion (b) or not at right angles > > > to the motion (c). > > > > > Let's start from here. I don't recall your situations, but I will > > accept the above as being correct wrt (b) and (c). > > Goody. To refresh the conversation: > > (a) No external forces. Straight line, constant velocity motion. > > (b) Circular motion. External force at right angles to the motion > at all time. Constant speed, non-constant velocity (because the > direction is changing). > > (c) General motion. External force at arbitrary angle to the > motion, so it is doing work. Both speed and direction in general > are changing. > > > > There's not "inertial force". > > > > Yes, there is, everyone knows that. > > No, there isn't. Inertia is not a force. Can you pull a quote > from Tipler where he refers to the force of inertia? He's > a someone, isn't he? > > No, but if you're referring to the claim by some that some forces are psuedo-forces, or don't really exist, they can take that up with Newton, as psuedo-forces exist in certain situations. In mechanics, the gravitational mass of a body is the source of a force that exists between it and another body and it is effectively the same as its inertial mass, which is what determines the motional response of the body to any force exerted on it. > > > > > > Everyone knows that. > > > > > > Everyone does not "know that". > > > > Well, you're the only one who doesn't know that for a fact. > > > > > I hold a rock over a hole in the ground. I let go. Please > > > tell me how the rocks inertial force is resisting the > > > gravitational force. Describe the forces present and how > > > they are acting. > > > > > Sure. The Inertial Force inherent in the rock is resisting the > > acceleration imposed by the gravitational force. Since inertia is > > resistance to change, once the rock drops from your hands, inertial > > resists the change from being relatively unmoving to moving toward the > > ground. > > I see. And as we watch it change from nonmoving to moving, we > can tell that something is "resisting" this change by...? > > By measuring the amount of energy required to cause it to start moving. > > > And if that "inertial force" weren't resisting gravity, then > we would see...? > > Hmmmmmmm.... Faster rates of acceleration? > > > To me, it looks like as soon as we apply a force, e.g. gravity, > the mass accelerates. Nothing prevents it from doing so. Nothing > resists it. > > Yes, it seemed that way for centuries, but we know now that the bigger a body, the stronger the force required to change its velocity. If it were so that nothing resists a force set upon it, all sizes of objects, even the moon would be forced to change its velocity by a single feather and so it would no longer even be in our solar system since its been hit so often by objects much larger than a feather. > > > And the rate at which it accelerates can be determined precisely > from F = dp/dt. > > No, that will depend on the forces and the body mass sizes involved in the interaction..
From: Randy Poe on 18 Oct 2005 10:13 TomGee wrote: > Randy Poe wrote: > > TomGee wrote: > > > Randy Poe wrote: > > > > In situations (b) and (c), there is a force present, either > > > > at right angles to the motion (b) or not at right angles > > > > to the motion (c). > > > > > > > Let's start from here. I don't recall your situations, but I will > > > accept the above as being correct wrt (b) and (c). > > > > Goody. To refresh the conversation: > > > > (a) No external forces. Straight line, constant velocity motion. > > > > (b) Circular motion. External force at right angles to the motion > > at all time. Constant speed, non-constant velocity (because the > > direction is changing). > > > > (c) General motion. External force at arbitrary angle to the > > motion, so it is doing work. Both speed and direction in general > > are changing. > > > > > > There's not "inertial force". > > > > > > Yes, there is, everyone knows that. > > > > No, there isn't. Inertia is not a force. Can you pull a quote > > from Tipler where he refers to the force of inertia? He's > > a someone, isn't he? > > > No, Then you don't have any backing for your claim that "everybody knows" inertia is a force, do you? > but if you're referring to the claim by some that some forces are > psuedo-forces, I'm not. You stated something about inertia being a force, and that "everybody knows it". I asked you to back that up. You respond by changing the subject. > or don't really exist, they can take that up with > Newton, as psuedo-forces exist in certain situations. A pseudo-force, like any other force, is one that causes a change in velocity. They are perfectly well modeled by Newton's laws. What makes them "pseudo" is that they are artifacts caused by being observed in accelerating frames, such as the surface of earth. An observer in a non-accelerated frame of reference will see no changes in velocity and no force. A centrifuge works perfectly well, settling its contents as if there is a centrifugal force much stronger than gravity and pointing directly outward. Yet the situation is equally well described by a person standing in the lab who sees only an inward-directed, centripetal force. > In mechanics, > the gravitational mass of a body is the source of a force that exists > between it and another body and it is effectively the same as its > inertial mass, which is what determines the motional response of the > body to any force exerted on it. F = ma, or in general F = dp/dt Yes, mass determines the acceleration caused by a force. That does not make mass a force. Force is the thing on the left-hand side of those equations. You can't use the mass as a force in those equations. That's why it's given a different symbol. > > > > > Everyone knows that. > > > > > > > > Everyone does not "know that". > > > Well, you're the only one who doesn't know that for a fact. And Tipler, apparently. I asked you for an example of Tipler using inertia as a force. You say you can't provide one. So I'm not the "only one", am I? But fine, F = ma. Tell me how I use inertia for F to describe the motion of a body. > > I see. And as we watch it change from nonmoving to moving, we > > can tell that something is "resisting" this change by...? > > > By measuring the amount of energy required to cause it to start moving. The amount of energy required for it to have 1 Joule of KE is 1 Joule. The amount of energy required for it to have 10 J of KE is 10 J. The amount of energy required for it to have 3.485 J of KE is 3.485 J. All I see is work energy being turned into KE. Where is this additional thing, this "resistance", that requires energy to overcome? > > And the rate at which it accelerates can be determined precisely > > from F = dp/dt. > > > No, that will depend on the forces and the body mass sizes involved in > the interaction.. Yes. That's what it means to use F = dp/dt to determine acceleration. I plug in the correct expression for F and for dp/dt, using the correct value of mass, and I get acceleration. F = dp/dt is the equation I use to do this. - Randy
From: PD on 18 Oct 2005 11:25
Randy Poe wrote: > TomGee wrote: > > Randy Poe wrote: > > > TomGee wrote: > > > > Randy Poe wrote: > > > > > In situations (b) and (c), there is a force present, either > > > > > at right angles to the motion (b) or not at right angles > > > > > to the motion (c). > > > > > > > > > Let's start from here. I don't recall your situations, but I will > > > > accept the above as being correct wrt (b) and (c). > > > > > > Goody. To refresh the conversation: > > > > > > (a) No external forces. Straight line, constant velocity motion. > > > > > > (b) Circular motion. External force at right angles to the motion > > > at all time. Constant speed, non-constant velocity (because the > > > direction is changing). > > > > > > (c) General motion. External force at arbitrary angle to the > > > motion, so it is doing work. Both speed and direction in general > > > are changing. > > > > > > > > There's not "inertial force". > > > > > > > > Yes, there is, everyone knows that. > > > > > > No, there isn't. Inertia is not a force. Can you pull a quote > > > from Tipler where he refers to the force of inertia? He's > > > a someone, isn't he? > > > > > No, > > Then you don't have any backing for your claim that "everybody > knows" inertia is a force, do you? > > > but if you're referring to the claim by some that some forces are > > psuedo-forces, > > I'm not. You stated something about inertia being a force, and > that "everybody knows it". I asked you to back that up. You > respond by changing the subject. > > > or don't really exist, they can take that up with > > Newton, as psuedo-forces exist in certain situations. > > A pseudo-force, like any other force, is one that causes > a change in velocity. They are perfectly well modeled by > Newton's laws. > > What makes them "pseudo" is that they are artifacts caused > by being observed in accelerating frames, such as the > surface of earth. An observer in a non-accelerated frame > of reference will see no changes in velocity and no force. You will have to be careful to distinguish the motion and the forces of circular motion as seen in a non-accelerated frame, from the motion and forces as seen in the accelerated frame. TomGee is unlikely to see the distinction at first, as a result wandering into the weeds immediately. PD |