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sungam
Oct 8, 2008, 11:20 PM
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robdotcalm wrote: jt512 wrote: robdotcalm wrote: jt512 wrote: mtnrock wrote: yea that's what i was thinking if it was moving like while letting him down to find the tenition it would be more like T=m(g-a) t is tention m is mass and g is accleration due to gravity and a is acceleration of the system A fundamental fact of physics is that motion at a constant velocity and motion of zero velocity are indistinguishable. Jay I must be mistaken, but I just saw two cars on the street. One was parked and one was moving at a constant velocity of 30 mi/hr. I was able to distinguish them based on the motion. On the other hand, both cars were moving at approximately 66,600 mi/hr around the sun, as were you. Still think you can distinguish constant velocity from zero velocity? Jay Yes, especially if I (first) stood in front of the parked car and then in front of the car going 30 mi/hr. I'm not sure, though, that I'm willing to do that experiment. Would you do it for me? r.c
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jt512
Oct 8, 2008, 11:54 PM
Post #52 of 123
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robdotcalm wrote: jt512 wrote: robdotcalm wrote: jt512 wrote: mtnrock wrote: yea that's what i was thinking if it was moving like while letting him down to find the tenition it would be more like T=m(g-a) t is tention m is mass and g is accleration due to gravity and a is acceleration of the system A fundamental fact of physics is that motion at a constant velocity and motion of zero velocity are indistinguishable. Jay I must be mistaken, but I just saw two cars on the street. One was parked and one was moving at a constant velocity of 30 mi/hr. I was able to distinguish them based on the motion. On the other hand, both cars were moving at approximately 66,600 mi/hr around the sun, as were you. Still think you can distinguish constant velocity from zero velocity? Jay Yes, especially if I (first) stood in front of the parked car and then in front of the car going 30 mi/hr. I'm not sure, though, that I'm willing to do that experiment. Would you do it for me? r.c
I think you're missing the point. There is no such thing as absolute velocity. Velocity is relative. The parked car appears to have zero velocity to you, but to an observer in another frame of reference, the parked car appears to be moving at 66,600 mi/hr.
You perceive one car to have a speed of zero and the other to have a speed of 30 mi/hr. But if the car you perceive to be traveling 30 mi/hr is traveling in the opposite direction of the earth's motion around the sun, then someone in the sun's frame of reference would observe the car that you see as the faster one to be moving 30 mi/hr slower than the car you perceive to be "parked." Neither of you would be right or wrong in the absolute sense, because there is no absolute sense in which to describe motion.
Jay
(This post was edited by jt512 on Oct 9, 2008, 4:25 AM)
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curt
Oct 9, 2008, 1:03 AM
Post #53 of 123
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jt512 wrote: mtnrock wrote: yea that's what i was thinking if it was moving like while letting him down to find the tenition it would be more like T=m(g-a) t is tention m is mass and g is accleration due to gravity and a is acceleration of the system A fundamental fact of physics is that motion at a constant velocity and motion of zero velocity are indistinguishable. Jay
Although most of us know what you meant, your statement is incorrect. What you meant to say is "motion at a constant velocity and motion of zero velocity both involve zero acceleration."
Obviously, if you plot position versus time for an object of constant velocity it will look quite different than a similar graph showing an object with zero velocity. The former will result in a straight line and the latter will result in a point.
Curt
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mtnrock
Oct 9, 2008, 1:14 AM
Post #54 of 123
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ok wait cause your guys are so set on how i word something. that's not what the point i was trying to get across. i was trying to show if there is no acceleration tension does not increase. and it really seems people are just on this site to piss people off
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jt512
Oct 9, 2008, 1:18 AM
Post #55 of 123
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mtnrock wrote: ok wait cause your guys are so set on how i word something. that's not what the point i was trying to get across. i was trying to show if there is no acceleration tension does not increase.
Oh, sorry. Next time we'll ignore what you write, and just read your mind instead.
Jay
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curt
Oct 9, 2008, 1:18 AM
Post #56 of 123
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mtnrock wrote: ok wait cause your guys are so set on how i word something. that's not what the point i was trying to get across. i was trying to show if there is no acceleration tension does not increase. and it really seems people are just on this site to piss people off
Some of us are only here to piss-off the retards. Why? Are you pissed?
Curt
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mtnrock
Oct 9, 2008, 1:24 AM
Post #57 of 123
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na just saying all you do is insult people
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curt
Oct 9, 2008, 1:31 AM
Post #58 of 123
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mtnrock wrote: na just saying all you do is insult people
Not true--I do that and a whole lot more. You'll see.
Curt
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sungam
Oct 9, 2008, 1:36 AM
Post #59 of 123
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curt wrote: mtnrock wrote: na just saying all you do is insult people Not true--I do that and a whole lot more. You'll see. Curt
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jt512
Oct 9, 2008, 1:39 AM
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curt wrote: jt512 wrote: mtnrock wrote: yea that's what i was thinking if it was moving like while letting him down to find the tenition it would be more like T=m(g-a) t is tention m is mass and g is accleration due to gravity and a is acceleration of the system A fundamental fact of physics is that motion at a constant velocity and motion of zero velocity are indistinguishable. Jay Although most of us know what you meant, your statement is incorrect. What you meant to say is "motion at a constant velocity and motion of zero velocity both involve zero acceleration." Obviously, if you plot position versus time for an object of constant velocity it will look quite different than a similar graph showing an object with zero velocity. The former will result in a straight line and the latter will result in a point.
I meant it more broadly. Since motion is relative, there is no Universal distinction between zero velocity and non-zero velocity. Zero velocity is just constant velocity observed from a special frame of reference.
Jay
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curt
Oct 9, 2008, 2:13 AM
Post #61 of 123
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jt512 wrote: curt wrote: jt512 wrote: mtnrock wrote: yea that's what i was thinking if it was moving like while letting him down to find the tenition it would be more like T=m(g-a) t is tention m is mass and g is accleration due to gravity and a is acceleration of the system A fundamental fact of physics is that motion at a constant velocity and motion of zero velocity are indistinguishable. Jay Although most of us know what you meant, your statement is incorrect. What you meant to say is "motion at a constant velocity and motion of zero velocity both involve zero acceleration." Obviously, if you plot position versus time for an object of constant velocity it will look quite different than a similar graph showing an object with zero velocity. The former will result in a straight line and the latter will result in a point. I meant it more broadly. Since motion is relative, there is no Universal distinction between zero velocity and non-zero velocity. Zero velocity is just constant velocity observed from a special frame of reference. Jay
Still, the two examples you cited are certainly not "indistinguishable" from a physics perspective. In one case, position versus time is changing at a constant rate--and in the other case position versus time is not changing.
Curt
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mtnrock
Oct 9, 2008, 2:38 AM
Post #62 of 123
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i see what jay is saying but yea the examples are different because to say one object has velocity you have to say that another doesn't as a frame of reference
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jt512
Oct 9, 2008, 3:01 AM
Post #63 of 123
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curt wrote: jt512 wrote: curt wrote: jt512 wrote: mtnrock wrote: yea that's what i was thinking if it was moving like while letting him down to find the tenition it would be more like T=m(g-a) t is tention m is mass and g is accleration due to gravity and a is acceleration of the system A fundamental fact of physics is that motion at a constant velocity and motion of zero velocity are indistinguishable. Jay Although most of us know what you meant, your statement is incorrect. What you meant to say is "motion at a constant velocity and motion of zero velocity both involve zero acceleration." Obviously, if you plot position versus time for an object of constant velocity it will look quite different than a similar graph showing an object with zero velocity. The former will result in a straight line and the latter will result in a point. I meant it more broadly. Since motion is relative, there is no Universal distinction between zero velocity and non-zero velocity. Zero velocity is just constant velocity observed from a special frame of reference. Jay Still, the two examples you cited are certainly not "indistinguishable" from a physics perspective. In one case, position versus time is changing at a constant rate--and in the other case position versus time is not changing. Curt
I disagree. In fact the example you give of why they are distinguishable — that they have different displacement–time graphs — is an artifact of the fact that the frame of reference you've chosen is unique; out of an infinite set of frames of reference, you've chosen the only one in which the one object appears stationary. Stated another way, for any two objects moving relative to each other, a frame of reference can be chosen in which one of the objects appears to be stationary. But physics is the same everywhere in the Universe; physics does not depend on frame of reference.
Jay
(This post was edited by jt512 on Oct 9, 2008, 3:04 AM)
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curt
Oct 9, 2008, 3:30 AM
Post #64 of 123
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jt512 wrote: curt wrote: jt512 wrote: curt wrote: jt512 wrote: mtnrock wrote: yea that's what i was thinking if it was moving like while letting him down to find the tenition it would be more like T=m(g-a) t is tention m is mass and g is accleration due to gravity and a is acceleration of the system A fundamental fact of physics is that motion at a constant velocity and motion of zero velocity are indistinguishable. Jay Although most of us know what you meant, your statement is incorrect. What you meant to say is "motion at a constant velocity and motion of zero velocity both involve zero acceleration." Obviously, if you plot position versus time for an object of constant velocity it will look quite different than a similar graph showing an object with zero velocity. The former will result in a straight line and the latter will result in a point. I meant it more broadly. Since motion is relative, there is no Universal distinction between zero velocity and non-zero velocity. Zero velocity is just constant velocity observed from a special frame of reference. Jay Still, the two examples you cited are certainly not "indistinguishable" from a physics perspective. In one case, position versus time is changing at a constant rate--and in the other case position versus time is not changing. Curt I disagree. In fact the example you give of why they are distinguishable — that they have different displacement–time graphs — is an artifact of the fact that the frame of reference you've chosen is unique; out of an infinite set of frames of reference, you've chosen the only one in which the one object appears stationary. Stated another way, for any two objects moving relative to each other, a frame of reference can be chosen in which one of the objects appears to be stationary. But physics is the same everywhere in the Universe; physics does not depend on frame of reference. Jay
Yet another way to differentiate a stationary object from an object moving at constant velocity (from a physics standpoint) is that any object in motion has momentum--and the one that is stationary does not. (i.e., p =mv) Looks like we need rgold to weigh in.
Curt
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jt512
Oct 9, 2008, 3:40 AM
Post #65 of 123
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curt wrote: jt512 wrote: curt wrote: jt512 wrote: curt wrote: jt512 wrote: mtnrock wrote: yea that's what i was thinking if it was moving like while letting him down to find the tenition it would be more like T=m(g-a) t is tention m is mass and g is accleration due to gravity and a is acceleration of the system A fundamental fact of physics is that motion at a constant velocity and motion of zero velocity are indistinguishable. Jay Although most of us know what you meant, your statement is incorrect. What you meant to say is "motion at a constant velocity and motion of zero velocity both involve zero acceleration." Obviously, if you plot position versus time for an object of constant velocity it will look quite different than a similar graph showing an object with zero velocity. The former will result in a straight line and the latter will result in a point. I meant it more broadly. Since motion is relative, there is no Universal distinction between zero velocity and non-zero velocity. Zero velocity is just constant velocity observed from a special frame of reference. Jay Still, the two examples you cited are certainly not "indistinguishable" from a physics perspective. In one case, position versus time is changing at a constant rate--and in the other case position versus time is not changing. Curt I disagree. In fact the example you give of why they are distinguishable — that they have different displacement–time graphs — is an artifact of the fact that the frame of reference you've chosen is unique; out of an infinite set of frames of reference, you've chosen the only one in which the one object appears stationary. Stated another way, for any two objects moving relative to each other, a frame of reference can be chosen in which one of the objects appears to be stationary. But physics is the same everywhere in the Universe; physics does not depend on frame of reference. Jay Yet another way to differentiate a stationary object from an object moving at constant velocity (from a physics standpoint) is that any object in motion has momentum--and the one that is stationary does not. (i.e., p =mv) Looks like we need rgold to weigh in. Curt
It would probably help to have a real physicist to weigh in, instead of two guys who sometimes play one on the internet; but all the real physicists I know want to get paid to answer questions like this. But, that said, I claim that since velocity is relative then so must be momentum, since they are proportional; so a momentum of 0 must also be an artifact of having chosen a special frame of reference.
Jay
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curt
Oct 9, 2008, 3:46 AM
Post #66 of 123
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jt512 wrote: curt wrote: jt512 wrote: curt wrote: jt512 wrote: curt wrote: jt512 wrote: mtnrock wrote: yea that's what i was thinking if it was moving like while letting him down to find the tenition it would be more like T=m(g-a) t is tention m is mass and g is accleration due to gravity and a is acceleration of the system A fundamental fact of physics is that motion at a constant velocity and motion of zero velocity are indistinguishable. Jay Although most of us know what you meant, your statement is incorrect. What you meant to say is "motion at a constant velocity and motion of zero velocity both involve zero acceleration." Obviously, if you plot position versus time for an object of constant velocity it will look quite different than a similar graph showing an object with zero velocity. The former will result in a straight line and the latter will result in a point. I meant it more broadly. Since motion is relative, there is no Universal distinction between zero velocity and non-zero velocity. Zero velocity is just constant velocity observed from a special frame of reference. Jay Still, the two examples you cited are certainly not "indistinguishable" from a physics perspective. In one case, position versus time is changing at a constant rate--and in the other case position versus time is not changing. Curt I disagree. In fact the example you give of why they are distinguishable — that they have different displacement–time graphs — is an artifact of the fact that the frame of reference you've chosen is unique; out of an infinite set of frames of reference, you've chosen the only one in which the one object appears stationary. Stated another way, for any two objects moving relative to each other, a frame of reference can be chosen in which one of the objects appears to be stationary. But physics is the same everywhere in the Universe; physics does not depend on frame of reference. Jay Yet another way to differentiate a stationary object from an object moving at constant velocity (from a physics standpoint) is that any object in motion has momentum--and the one that is stationary does not. (i.e., p =mv) Looks like we need rgold to weigh in. Curt It would probably help to have a real physicist to weigh in, instead of two guys who sometimes play one on the internet; but all the real physicists I know want to get paid to answer questions like this. But, that said, I claim that since velocity is relative then so must be momentum, since they are proportional; so a momentum of 0 must also be an artifact of having chosen a special frame of reference. Jay
What you're actually saying is that both velocity and momentum have no generally accepted meanings in newtonian physics--and hence, can not be used to differentiate between stationary and moving objects. Obviously, I think that's silly.
Curt
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jt512
Oct 9, 2008, 3:49 AM
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curt wrote: jt512 wrote: curt wrote: jt512 wrote: curt wrote: jt512 wrote: curt wrote: jt512 wrote: mtnrock wrote: yea that's what i was thinking if it was moving like while letting him down to find the tenition it would be more like T=m(g-a) t is tention m is mass and g is accleration due to gravity and a is acceleration of the system A fundamental fact of physics is that motion at a constant velocity and motion of zero velocity are indistinguishable. Jay Although most of us know what you meant, your statement is incorrect. What you meant to say is "motion at a constant velocity and motion of zero velocity both involve zero acceleration." Obviously, if you plot position versus time for an object of constant velocity it will look quite different than a similar graph showing an object with zero velocity. The former will result in a straight line and the latter will result in a point. I meant it more broadly. Since motion is relative, there is no Universal distinction between zero velocity and non-zero velocity. Zero velocity is just constant velocity observed from a special frame of reference. Jay Still, the two examples you cited are certainly not "indistinguishable" from a physics perspective. In one case, position versus time is changing at a constant rate--and in the other case position versus time is not changing. Curt I disagree. In fact the example you give of why they are distinguishable — that they have different displacement–time graphs — is an artifact of the fact that the frame of reference you've chosen is unique; out of an infinite set of frames of reference, you've chosen the only one in which the one object appears stationary. Stated another way, for any two objects moving relative to each other, a frame of reference can be chosen in which one of the objects appears to be stationary. But physics is the same everywhere in the Universe; physics does not depend on frame of reference. Jay Yet another way to differentiate a stationary object from an object moving at constant velocity (from a physics standpoint) is that any object in motion has momentum--and the one that is stationary does not. (i.e., p =mv) Looks like we need rgold to weigh in. Curt It would probably help to have a real physicist to weigh in, instead of two guys who sometimes play one on the internet; but all the real physicists I know want to get paid to answer questions like this. But, that said, I claim that since velocity is relative then so must be momentum, since they are proportional; so a momentum of 0 must also be an artifact of having chosen a special frame of reference. Jay What you're actually saying is that both velocity and momentum have no generally accepted meanings in newtonian physics--and hence, can not be used to differentiate between stationary and moving objects. Obviously, I think that's silly. Curt
So, are you saying that Newtonian physics does not recognize the relativity of motion?
Jay
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jt512
Oct 9, 2008, 4:00 AM
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Let me put it this way. There is nothing special about v=0. Velocity is relative. v=0 is exactly as distinguishable from v=30 as v=30 is from v=60. That is, independent of reference frame, the difference in velocities in each case is 30. So now we have something that is universal; that is, we can define velocity relatively. Since momentum is velocity times a constant, then the exact same argument applies to momentum.
Jay
(This post was edited by jt512 on Oct 9, 2008, 4:01 AM)
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curt
Oct 9, 2008, 4:08 AM
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jt512 wrote: curt wrote: jt512 wrote: curt wrote: jt512 wrote: curt wrote: jt512 wrote: curt wrote: jt512 wrote: mtnrock wrote: yea that's what i was thinking if it was moving like while letting him down to find the tenition it would be more like T=m(g-a) t is tention m is mass and g is accleration due to gravity and a is acceleration of the system A fundamental fact of physics is that motion at a constant velocity and motion of zero velocity are indistinguishable. Jay Although most of us know what you meant, your statement is incorrect. What you meant to say is "motion at a constant velocity and motion of zero velocity both involve zero acceleration." Obviously, if you plot position versus time for an object of constant velocity it will look quite different than a similar graph showing an object with zero velocity. The former will result in a straight line and the latter will result in a point. I meant it more broadly. Since motion is relative, there is no Universal distinction between zero velocity and non-zero velocity. Zero velocity is just constant velocity observed from a special frame of reference. Jay Still, the two examples you cited are certainly not "indistinguishable" from a physics perspective. In one case, position versus time is changing at a constant rate--and in the other case position versus time is not changing. Curt I disagree. In fact the example you give of why they are distinguishable — that they have different displacement–time graphs — is an artifact of the fact that the frame of reference you've chosen is unique; out of an infinite set of frames of reference, you've chosen the only one in which the one object appears stationary. Stated another way, for any two objects moving relative to each other, a frame of reference can be chosen in which one of the objects appears to be stationary. But physics is the same everywhere in the Universe; physics does not depend on frame of reference. Jay Yet another way to differentiate a stationary object from an object moving at constant velocity (from a physics standpoint) is that any object in motion has momentum--and the one that is stationary does not. (i.e., p =mv) Looks like we need rgold to weigh in. Curt It would probably help to have a real physicist to weigh in, instead of two guys who sometimes play one on the internet; but all the real physicists I know want to get paid to answer questions like this. But, that said, I claim that since velocity is relative then so must be momentum, since they are proportional; so a momentum of 0 must also be an artifact of having chosen a special frame of reference. Jay What you're actually saying is that both velocity and momentum have no generally accepted meanings in newtonian physics--and hence, can not be used to differentiate between stationary and moving objects. Obviously, I think that's silly. Curt So, are you saying that Newtonian physics does not recognize the relativity of motion? Jay
I am saying (in yet another way) that the first derivative of displacement, with respect to time, is velocity--and even mathematically it is clear that an object moving at a constant velocity and a stationary one can easily be differentiated. By the way, in the first post of yours that I replied to, you fixed the frame of reference by saying that one comparative case involved "zero velocity."
Curt
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jt512
Oct 9, 2008, 4:12 AM
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curt wrote: jt512 wrote: curt wrote: jt512 wrote: curt wrote: jt512 wrote: curt wrote: jt512 wrote: curt wrote: jt512 wrote: mtnrock wrote: yea that's what i was thinking if it was moving like while letting him down to find the tenition it would be more like T=m(g-a) t is tention m is mass and g is accleration due to gravity and a is acceleration of the system A fundamental fact of physics is that motion at a constant velocity and motion of zero velocity are indistinguishable. Jay Although most of us know what you meant, your statement is incorrect. What you meant to say is "motion at a constant velocity and motion of zero velocity both involve zero acceleration." Obviously, if you plot position versus time for an object of constant velocity it will look quite different than a similar graph showing an object with zero velocity. The former will result in a straight line and the latter will result in a point. I meant it more broadly. Since motion is relative, there is no Universal distinction between zero velocity and non-zero velocity. Zero velocity is just constant velocity observed from a special frame of reference. Jay Still, the two examples you cited are certainly not "indistinguishable" from a physics perspective. In one case, position versus time is changing at a constant rate--and in the other case position versus time is not changing. Curt I disagree. In fact the example you give of why they are distinguishable — that they have different displacement–time graphs — is an artifact of the fact that the frame of reference you've chosen is unique; out of an infinite set of frames of reference, you've chosen the only one in which the one object appears stationary. Stated another way, for any two objects moving relative to each other, a frame of reference can be chosen in which one of the objects appears to be stationary. But physics is the same everywhere in the Universe; physics does not depend on frame of reference. Jay Yet another way to differentiate a stationary object from an object moving at constant velocity (from a physics standpoint) is that any object in motion has momentum--and the one that is stationary does not. (i.e., p =mv) Looks like we need rgold to weigh in. Curt It would probably help to have a real physicist to weigh in, instead of two guys who sometimes play one on the internet; but all the real physicists I know want to get paid to answer questions like this. But, that said, I claim that since velocity is relative then so must be momentum, since they are proportional; so a momentum of 0 must also be an artifact of having chosen a special frame of reference. Jay What you're actually saying is that both velocity and momentum have no generally accepted meanings in newtonian physics--and hence, can not be used to differentiate between stationary and moving objects. Obviously, I think that's silly. Curt So, are you saying that Newtonian physics does not recognize the relativity of motion? Jay I am saying (in yet another way) that the first derivative of displacement, with respect to time, is velocity--and even mathematically it is clear that an object moving at a constant velocity and a stationary one can easily be differentiated. By the way, in the first post of yours that I replied to, you fixed the frame of reference by saying that one comparative case involved "zero velocity." Curt
Well, I don't know how to explain that there is no difference between zero velocity and non-zero velocity without using the phrase "zero velocity." I suppose that if I tried hard enough, I could do it, but it would have made the post even more abstract.
Jay
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curt
Oct 9, 2008, 4:24 AM
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jt512 wrote: curt wrote: jt512 wrote: curt wrote: jt512 wrote: curt wrote: jt512 wrote: curt wrote: jt512 wrote: curt wrote: jt512 wrote: mtnrock wrote: yea that's what i was thinking if it was moving like while letting him down to find the tenition it would be more like T=m(g-a) t is tention m is mass and g is accleration due to gravity and a is acceleration of the system A fundamental fact of physics is that motion at a constant velocity and motion of zero velocity are indistinguishable. Jay Although most of us know what you meant, your statement is incorrect. What you meant to say is "motion at a constant velocity and motion of zero velocity both involve zero acceleration." Obviously, if you plot position versus time for an object of constant velocity it will look quite different than a similar graph showing an object with zero velocity. The former will result in a straight line and the latter will result in a point. I meant it more broadly. Since motion is relative, there is no Universal distinction between zero velocity and non-zero velocity. Zero velocity is just constant velocity observed from a special frame of reference. Jay Still, the two examples you cited are certainly not "indistinguishable" from a physics perspective. In one case, position versus time is changing at a constant rate--and in the other case position versus time is not changing. Curt I disagree. In fact the example you give of why they are distinguishable — that they have different displacement–time graphs — is an artifact of the fact that the frame of reference you've chosen is unique; out of an infinite set of frames of reference, you've chosen the only one in which the one object appears stationary. Stated another way, for any two objects moving relative to each other, a frame of reference can be chosen in which one of the objects appears to be stationary. But physics is the same everywhere in the Universe; physics does not depend on frame of reference. Jay Yet another way to differentiate a stationary object from an object moving at constant velocity (from a physics standpoint) is that any object in motion has momentum--and the one that is stationary does not. (i.e., p =mv) Looks like we need rgold to weigh in. Curt It would probably help to have a real physicist to weigh in, instead of two guys who sometimes play one on the internet; but all the real physicists I know want to get paid to answer questions like this. But, that said, I claim that since velocity is relative then so must be momentum, since they are proportional; so a momentum of 0 must also be an artifact of having chosen a special frame of reference. Jay What you're actually saying is that both velocity and momentum have no generally accepted meanings in newtonian physics--and hence, can not be used to differentiate between stationary and moving objects. Obviously, I think that's silly. Curt So, are you saying that Newtonian physics does not recognize the relativity of motion? Jay I am saying (in yet another way) that the first derivative of displacement, with respect to time, is velocity--and even mathematically it is clear that an object moving at a constant velocity and a stationary one can easily be differentiated. By the way, in the first post of yours that I replied to, you fixed the frame of reference by saying that one comparative case involved "zero velocity." Curt Well, I don't know how to explain that there is no difference between zero velocity and non-zero velocity without using the phrase "zero velocity." I suppose that if I tried hard enough, I could do it, but it would have made the post even more abstract. Jay
I will now happily wait for rgold to arbitrate.
Curt
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colatownkid
Oct 9, 2008, 11:29 AM
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curt wrote: jt512 wrote: curt wrote: jt512 wrote: curt wrote: jt512 wrote: curt wrote: jt512 wrote: curt wrote: jt512 wrote: curt wrote: jt512 wrote: mtnrock wrote: yea that's what i was thinking if it was moving like while letting him down to find the tenition it would be more like T=m(g-a) t is tention m is mass and g is accleration due to gravity and a is acceleration of the system A fundamental fact of physics is that motion at a constant velocity and motion of zero velocity are indistinguishable. Jay Although most of us know what you meant, your statement is incorrect. What you meant to say is "motion at a constant velocity and motion of zero velocity both involve zero acceleration." Obviously, if you plot position versus time for an object of constant velocity it will look quite different than a similar graph showing an object with zero velocity. The former will result in a straight line and the latter will result in a point. I meant it more broadly. Since motion is relative, there is no Universal distinction between zero velocity and non-zero velocity. Zero velocity is just constant velocity observed from a special frame of reference. Jay Still, the two examples you cited are certainly not "indistinguishable" from a physics perspective. In one case, position versus time is changing at a constant rate--and in the other case position versus time is not changing. Curt I disagree. In fact the example you give of why they are distinguishable — that they have different displacement–time graphs — is an artifact of the fact that the frame of reference you've chosen is unique; out of an infinite set of frames of reference, you've chosen the only one in which the one object appears stationary. Stated another way, for any two objects moving relative to each other, a frame of reference can be chosen in which one of the objects appears to be stationary. But physics is the same everywhere in the Universe; physics does not depend on frame of reference. Jay Yet another way to differentiate a stationary object from an object moving at constant velocity (from a physics standpoint) is that any object in motion has momentum--and the one that is stationary does not. (i.e., p =mv) Looks like we need rgold to weigh in. Curt It would probably help to have a real physicist to weigh in, instead of two guys who sometimes play one on the internet; but all the real physicists I know want to get paid to answer questions like this. But, that said, I claim that since velocity is relative then so must be momentum, since they are proportional; so a momentum of 0 must also be an artifact of having chosen a special frame of reference. Jay What you're actually saying is that both velocity and momentum have no generally accepted meanings in newtonian physics--and hence, can not be used to differentiate between stationary and moving objects. Obviously, I think that's silly. Curt So, are you saying that Newtonian physics does not recognize the relativity of motion? Jay I am saying (in yet another way) that the first derivative of displacement, with respect to time, is velocity--and even mathematically it is clear that an object moving at a constant velocity and a stationary one can easily be differentiated. By the way, in the first post of yours that I replied to, you fixed the frame of reference by saying that one comparative case involved "zero velocity." Curt Well, I don't know how to explain that there is no difference between zero velocity and non-zero velocity without using the phrase "zero velocity." I suppose that if I tried hard enough, I could do it, but it would have made the post even more abstract. Jay I will now happily wait for rgold to arbitrate. Curt
Can I arbitrate?
I know it's only wikipedia, but seriously, as is said far too often on this site, do a search.
See http://en.wikipedia.org/wiki/Force:
"Newton's first law of motion states that objects continue to move in a state of constant velocity unless acted upon by an external net force or resultant force.[10] This law is an extension of Galileo's insight that constant velocity was associated with a lack of net force (see a more detailed description of this below). Newton proposed that every object with mass has an innate inertia that functions as the fundamental equilibrium "natural state" in place of the Aristotelian idea of the "natural state of rest". That is, the first law contradicts the intuitive Aristotelian belief that a net force is required to keep an object moving with constant velocity. By making rest physically indistinguishable from non-zero constant velocity, Newton's first law directly connects inertia with the concept of relative velocities. Specifically, in systems where objects are moving with different velocities, it is impossible to determine which object is "in motion" and which object is "at rest". In other words, to phrase matters more technically, the laws of physics are the same in every inertial frame of reference, that is, in all frames related by a Galilean transformation."
Further down the page it shows why Galileo was smarter than us:
"Simple experiments showed that Galileo's understanding of the equivalence of constant velocity and rest to be correct. For example, if a mariner dropped a cannonball from the crow's nest of a ship moving at a constant velocity, Aristotelian physics would have the cannonball fall straight down while the ship moved beneath it. Thus, in an Aristotelian universe, the falling cannonball would land behind the foot of the mast of a moving ship. However, when this experiment is actually conducted, the cannonball always falls at the foot of the mast, as if the cannonball knows to travel with the ship despite being separated from it. Since there is no forward horizontal force being applied on the cannonball as it falls, the only conclusion left is that the cannonball continues to move with the same velocity as the boat as it falls. Thus, no force is required to keep the cannonball moving at the constant forward velocity.[9]"
I'm gonna say Jay is right on this one. Jay's statement could be amended to say that "The force on an object in motion at a constant velocity and motion of zero velocity are indistinguishable." This might make it a lot easier to swallow and it directly answers the OPs question about tension while a climber is being lowered (that is, in the presence of constant velocity and no acceleration the tension does not change).
However, Galilean Relativity clearly states that Jay's original statement does in fact hold true. "Galilean invariance or Galilean relativity is a principle of relativity which states that the fundamental laws of physics are the same in all inertial frames." (see http://en.wikipedia.org/.../Galilean_relativity)
(Now, if we all really want to be completely honest about it, this is all technically wrong since time can not be considered absolute as the speed of light is approached. But let's not complicate things...)
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sungam
Oct 9, 2008, 2:06 PM
Post #73 of 123
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Registered: Jun 24, 2004
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colatownkid wrote: But let's not complicate things...
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jdefazio
Oct 9, 2008, 2:14 PM
Post #74 of 123
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Registered: Oct 29, 2007
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JT512 wrote: A fundamental fact of physics is that motion at a constant velocity and motion of zero velocity are indistinguishable. Jay curt wrote: Although most of us know what you meant, your statement is incorrect. JT512 wrote: ..."your rong"... curt wrote: ..."no yor rong"...
^^^repeat ad-infinitum.
Curt is correct here regarding the semantics of Jay's statement. The rest is like arguing over who is better looking, since the answer depends on who you ask (i.e. what reference frame they are in).
In classical physics, when comparing measurements obtained in different reference frames, the forces/accelerations are identical if the relative motion betwen these frames is of constant velocity. This is the Galilean relativity CI mentions, essentially saying the classical laws of physics (i.e. Newton's F=ma) are invariant between inertial (non-accelerating) reference frames.
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curt
Oct 9, 2008, 3:18 PM
Post #75 of 123
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Registered: Aug 27, 2002
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jdefazio wrote: JT512 wrote: A fundamental fact of physics is that motion at a constant velocity and motion of zero velocity are indistinguishable. Jay curt wrote: Although most of us know what you meant, your statement is incorrect. JT512 wrote: ..."your rong"... curt wrote: ..."no yor rong"... ^^^repeat ad-infinitum. Curt is correct here regarding the semantics of Jay's statement. The rest is like arguing over who is better looking, since the answer depends on who you ask (i.e. what reference frame they are in)...
Fine. Then I'm better looking too. 
Curt
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