It also has a measure of entropy and so it has a temperature.
treis, you’re confusing the definition of a unit with the measurement of some quantity. If you follow Trisk’s link, you’ll see that the meter is actually defined in terms of a velocity: that of light. It’s the distance light travels in a certain amount of time. (1/299,792,458 of a second)
If a particular object is not moving, that doesn’t change the definition of the unit “meter”.
I have two globs of clay 1 kg each traveling at 1 m/s towards each other and are currently 1 meter apart. Time is flowing becuase they have a velocity and are covering a distance. They collide inelastically and are now stick together motionless. Since there is no longer any velocity time is undefined physically this doesn’t make sense. If you are telling me that I cannot define my universe to be just those two globs of clay then you are telling me that the way we do physics is wrong.
No there is a difference between undefined and not being able to measured.
You said we don’t use velocity becuase its easier to measure distance and time. I am saying it is IMPOSSIBLE to measure velocity independently.
Tell me CandidGamera what is velocity? If you can give me a definition that doesn’t include change in distance/change in time I will concede this argument. Since you can’t you should realize that any unit you give me for velocity will be able to be broken down into some unit of distance/some unit of time. Since it can be written as a combination of two units it is not a fundamental unit.
Look at Desmostylus cite it does a better job explaining than I am doing.
Rob, friend, currently the second is quantified by cesium radiation it does not mean that the quantity we know as time is actually cesium radiating. If all the cesium in the universe went away it doesn’t mean that the quantity we call time no longer exists. It is just a way to quantify the unit we call a second in terms of a physical constant.
They still have motion on a molecular level. On the other hand, if your single-glob universe is at absolute zero - by which I mean, nothing’s happening at all - IS time passing in a meaningful sense?
All in your frame of reference, buddy.
If we take your assertions as true, then you’ve just argued that the argument that mass is a fundamental unit breaks down, because, according to you, it cannot be directly measured. That’s point one.
Point two, I had discussed generally that the ease of measurement of a quantity might influence its status as a “fundamental” measure. If you’re correct in that there’s no way to measure velocity directly, and because of that, it is not a fundamental unit, that substantially supports the position I’ve been spouting all along, one which you seemed to be attempting to refute.
Point three, are you familiar with Red-Shift and Blue-Shift of light wavelengths? If I recall correctly, it allows us to measure the velocity of a distant star without knowing how far it has travelled in what amount of time, directly. Or hey, the Uncertainty Principle. We can either know the velocity or the position of an electron with reasonable certainty, but not both. How do we get the velocity, if we don’t know the distance it’s travelled in a time interval? Because to know that, we’d have to know it’s position all along the way (since it’s not travelling in a straight line.)
Tell ME, treis. What’s mass? What’s time? I can define mass as an observable property of an object, one which is based on the strength of the gravitic attraction between it and another object. Does that mean that Force, and not Mass, is the fundamental unit? That the equation should be m = F/a?
We in fact in the English system we do use Force as a fundamental unit (pound) and when we need the mass divide by 32.1 ft/s^2. I am afraid I would be in over my head on a discussion of what mass actually is and whether or not it is a fundamental unit. What I do know however is that velocity is not.
I was under the impression that you were contending that we use distance and time becuase it is convienent. I am arguing that we use them becuase they are the only things that are possible for us to measure. It goes beyond the measure of convience and into the realm of possibility.
We can measure its velocity in the radial direction i.e. coming towards us or going away from us by measuring the change in the wavelength (change in distance) but not its lateral distance. So you aren’t calculating its velocity rather its velocity in one direction. Plus what you are actually measuring isn’t the velocity rather a change in wavelength (distance).
I fail to see your point here.
Time is fairly easy in a non-relativistic sense I define an axis I call time and treat it like another spatial demension.
Mass on the other hand I freely admit I have no idea what it is and I get the feeling that I am not the only one. What I do know is that every object has some quality that we define as mass but as of yet are unable to directly measure. But it is the mass that causes the force not the force that causes the mass. Perhaps we should use force instead of mass as the fundamental unit I don’t know. I know if you do calculations in the English system you use a force measurement as the standard and divide by 32.1 to get your mass.
I see a problem with defining mass as the magnitude of the gravitational force between two objects. If I define a universe with only object there is no gravitational force but the object still has a mass.
You may know that velocity isn’t a fundamental unit in the English or SI systems. My overriding point is that it just as easily could be; the reason we don’t is because it is easier to calculate velocity than measure velocity.
And yet, you assert it’s impossible to measure mass, but that’s a fundamental unit in the SI system.
It is a component velocity, but that’s okay. The point here is that if I want to know how long it takes Star A to move one-light year away from us… in other words, I’m choosing the distance, and want to know the time, we can calculate that time because we can find the velocity before we know the time. In other words, we can get velocity without dividing distance by time.
Just that velocity, contrary to your assertions, doesn’t need to have a known distance and time before it can be determined.
And yet the SI system uses Mass as a Fundamental unit.
You can’t ever observe such a universe. You could never measure anything about that object.
My overriding point is that it is impossible to measure velocity without measuring a change in distance and change in time. Therefore any unit of velocity you give me can be written as a distance/time.
I never asserted that it was impossible to measure mass. I said that we currently have no way to measure it. My point is that if I had you a snapshot it is impossible for you to determine a velocity. You need to measure some change in time and then from that using the distance an object traveled calculate its velocity. (I know we can get ‘snapshots’ of velocity by using the derivitive but thats another discussion)
You are only choosing its distance in one dimension though. Assuming its a constant velocity you will know its displacement in that one direction however you don’t have information about the other two dimensions.
I still don’t follow. The uncertainty principle just says we can’t know an objects velocity and its position. It doesn’t tell us how to calculate velocity without knowing distance and time.
I can measure its spatial dimensions. I can measure a change in time. From these I can calculate any number of values including velocity.
I think we have disconnected the fundamental units from the fundamental quantities. The fundamental quantities being intrinsitc properties of either an object or the universe. If you a
Hit reply on accident…
I think we have disconnected the fundamental units from the fundamental quantities. The fundamental quantities being intrinsitc properties of either an object or the universe. If you ask yourself is velocity a property of the universe or is it something we calculate from measuring intrinsic properties?
Answering this one first. Check the OP. He wanted to know why his professor said temperature (in degrees Kelvin, in this case) was a fundamental quantity.
The English system says that Force is a fundamental quantity. Why? Well, ease of measurement. And it having been constructed under a somewhat more naive world-view.
Note that I have never said, nor am I saying now, that velocity is, in any current system of measurement, a fundamental quantity. Further points to be addressed when I get to your previous post.
I mentioned two ways that velocity can be determined without change in distance and change in time.
Again, just the same as velocity - why are you so certain that velocity can never be measured directly, but mass might be, maybe?
In a snapshot it’s impossible to measure time, too.
It’s just difficult for us to do that with electromagnetic waves without a second point of reference, for triangulation. But it doesn’t matter. We could use a closer object, one for which we can set up the three necessary points of reference to determine a complete velocity.
If that were true, the uncertainty principle is meaningless. Since we can’t know both, and we can’t ever know velocity without distance (in your assertion) we can never know velocity for an electron, only position. So it’d be much easier to say that. The way the principle is phrased leads me to believe that scientists can determine the velocity of an electron.
No, you can’t, because if you were able to observe it, you’d be a second object (or a compact collection of millions of objects, strictly speaking) in that universe.
You need a fixed number of fundamentals (five, by my count), but there are a number of different fundamentals one can choose. One could choose as fundamentals mass, length, time, charge, and temperature, which is what SI does. But you could also choose fundamentals with dimension of (length*time[sup]-1[/sup]); (length[sup]3[/sup]*mass[sup]-1[/sup]*time[sup]-2[/sup]); (length[sup]2[/sup]masstime[sup]-1[/sup]); (length[sup]3[/sup]masstime[sup]-2[/sup]*charge[sup]-2[/sup]); and (length[sup]2[/sup]masstime[sup]-2[/sup]*temperature[sup]-1[/sup]). In fact, it’s common in some areas of physics to choose fundamentals with exactly those dimensions. The choice isn’t completely arbitrary: One cannot, for instance, set length, time, and speed as three of the fundamentals. But any independent set of fundamentals will work.
It’s also quite possible to measure the mass of something without any recourse to gravity at all. Take a spring, and use it to pull on a standard mass (like, say, that platinum-irridium cylinder they keep in Paris). Pull in such a way that the spring stays at a constant length, and measure the acceleration of the mass. Now attach the same spring to something you want to measure, and pull on it in such a way that the length of the spring is the same as it was before. a[sub]2[/sub]/a[sub]1[/sub] = m[sub]1[/sub]/m[sub]2[/sub]. Or as one of my professors put it, “Mass is how hard something hurts when you kick it”.
Bingo. Thank you, Chronos, for putting things better than I ever could.
Any enlightment as to how they measure the velocity of an electron?
Eh? Up to now I’ve been labouring under the impression that the pound is a unit of mass the way a kilogram is, and that in both cases we sloppily use the name of the mass unit when we actually mean the weight unit (pound force and kilogram force), each of which is defined as the amount of force required to accelerate the unit mass by one standard gravity (or the amount of force exerted to hold said mass up against one standard gravity).
The Imperial unit of force analogous to SI’s newton is the poundal - a force that will accelerate a one-pound mass at a rate of one foot per second per second.
Now back to the argument…
Nope. The Imperial mass unit is the slug. A slug of mass weighs a bit over 32 lbs in a 1G field.
Let d=5. Then we have 5/0 = t, which is undefined. Translating this equation into a word problem, we’d say, “An object moves at a velocity of zero for a certain period of time. At the end of this period, it is 5 meters from where it started. How long was that period of time?” You will find that there is no number of seconds which can answer the question.
I’m not a physicist, but I’m with Chronos and CandidGamera on this. Velocity always CAN be expressed as (distance) / (time), but distance also CAN be expressed (velocity)*(time). Having distance, time, charge, etc. be fundamental makes the most sense, (at least in situations I’m familiar with) but they don’t HAVE to be.
To address the OP, I assume temperature would be energy / particles, like Joule/mole (which in turn is newton*meter/mole, which in turn… etc.) This probably runs into some problems when you take E=mc[sup]2[/sup] into account, because you end up with mass/mole or something. I don’t know how to resolve that.
Okay. On reading the Wiki entry, it looks like “slug” is to “pound force” as “pound mass” is to “poundal”. Great! Another useful fact learned 