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EducationMass vs Weight
26th Oct 2017 20:27 UTCOwen Lewis
> Donald B Peck October 23, 2017 05:07PM
..... Yes a scale measures weight . . . but a balance measures mass ;<})
and
> Donald B Peck October 24, 2017 01:06AM
> Hi Thomas,
> True that a balance will not work without gravity, but both pans are essentially at the same point in the same gravitational field . . . thus independent of > the strength of the field. BUT . . . let's not hijack Krisha's thread.
> Don ;<})
And Doug agreed with him (!)...
Horsefeathers, gentlemen! And certain to throw sand in the eyes of newbies or those with shaky science education.
The truth is, balance or scale, single pan or two-pan, all of these instruments *weigh* and do not directly determine mass. Balances and scales actually measure the gravitational attraction of one object (very large - say planet Earth) on a smaller object under test. Weigh a given mass here on Earth and on the Moon and on Mars, and you will get three very different results. Trying to use either scale or balance under conditions of nil gravity (i..e. weightlessness ) and neither will function at all.
Mass is a fundamental property of matter and is unaffected by gravity. Weight is the pull of gravity on some given mass. The founding fathers of our systems of quantising the properties of matter made the convenient with a quite arbitrary decision to equate units of mass to the units of gravimetric pull here on Earth (e.g. weight).
It is quite possible to explain - as I am sure that Don and Doug could - how to measure mass anywhere and with a constant result even where gravity is zero. But it is definitely *NEVER* by using a two-pan balance! ;-) I leave it with them (for the moment) to give here a method for doing so and thus showing the difference between mass and weight..
Oh, gentlemen, gentlemen.... dear me....
26th Oct 2017 23:36 UTCRob Woodside 🌟 Manager
The remarkable thing is Einstein's Principle of Equivalence. Suppose you are abducted by aleins. You wake up in a windowless box with a meter stick, clock and a flash light. If you let go of these things they fall to the floor. Remembering your first year physics you quickly use these tools to see how fast things accelerate to the floor. With pride you say that is the acceleration due to gravity. However Einstein points out that maybe you are lost in space and the aliens are accelerating the box upward with the acceleration you have just measured. If you let go of an object it stays where it is obeying Newton's First Law about remaining at rest with NO external forces. The object remains there and the floor accelerates up into the object. So which is it? Does the body accelerate down or the floor accelerate up? Einstein says you can't tell. That is the principle of equivalence. So by that principle gravitational charge and resistance to acceleration are the same!!! Still weight is the force of gravitational attraction measured in Newtons or Pounds and mass is the resistance to acceleration measured in Kilograms or Slugs. The idea that they should be different things stems from bad philosophy that thinks concepts are defined by the measurements that determine them. So in this confusion weight and mass are totally different and should have NO expected connection because you measure them differently. The principle of equivalence says they are the same!
27th Oct 2017 02:40 UTCDoug Daniels
My agreeing with Don was more that we shouldn't be hijacking Krisha's thread. The comments were going to an argument of weight versus mass, which wasn't the point of the thread. I agree that weight is a result of the pull of gravity on a mass, and one would get a different weight for a given mass when measured on the Earth, Moon, or Mars (or any other body you may choose). And yes, on the International Space Station, one cannot measure weight. As far as Rob's mention of using a spring to determine mass in the lack of gravity, that method could work. But, try to use it with small masses (say a half carat diamond, for grins), and you may not have a spring that can measure it. Generally, when we measure the mass of an object, we will be on a planet with a gravitational pull. As such, things such as the good-old chem-lab three beam balance is useful; even though gravity is involved in the measurement, the value of the acceleration (F = mg, where g is the force of gravity) is cancelled on both sides of the balance (such a balance will give accurate results on Earth, Moon, Mars, and any other body). The measure of mass by such a balance comes more from the idea of torque (and, I have to go back and check my basic physics; it's been a year or two since I've used it).
27th Oct 2017 12:09 UTCRob Woodside 🌟 Manager
The real problem with Newton's gravity law comes from Newton's third law which says forces occur in equal and opposite action and reaction pairs. Two bodies in contact exert equal and opposite forces on each other. However with Newton's gravity the bodies are separated by a distance, R, in the gravity law. So how can two bodies separated by empty space possibly exert forces on each other??? For this reason Galileo knew that the moon could NOT cause the tides as it was pure astrology or magic to think that distant bodies could influence one another. This is the famous action at a distance problem for which the continental scientists criticized Newton's gravity law. Action at a distance was so ridiculous that if there was any truth to the gravity law it had to be coincidence. Boyle, the discoverer of Oxygen, complained to Newton about this and Newton replied, "No adept at Natural Philosophy could be so foolish as to believe in such an absurdity as action at a distance." As a result the Gravity law, though it foretold Coulomb's electrical force law in 1784, was not taken seriously until William Herschel found Mizar in the Big Dipper in 1803 to be a double star apparently obeying the gravity law. It wasn't until Faraday's electrical researches in the 1830's that the action at a distance problem suffered by both gravity and electric force laws was solved. The price paid was the invention of a new kind of thing, the electric and magnetic field for Faraday and by analogy the gravitational field. For many this was too high a price, but the field concept eventually won out. So now a body would be acted on locally by the field in which the body was immersed. It took Einstein to realize that space time curvature, or at least the traceless part of it, was the gravitational field and that density as well as other components of the stress energy tensor was the rest of the space time curvature and the source of the gravitational field.
With this understanding Newton's laws take new meanings. The first law actually defines a straight line in curved space. (With no external forces a body at rest remains at rest and a body in motion travels a straight line at a constant speed.). Such straight lines can be used locally to set up a special coordinate system called an inertial frame. These inertial coordinates are where Newton's laws obtain. So Newton's second law gives you the acceleration relative to a set of inertial coordinates that a body suffers from an external force. (F = ma). It's a bit of a swizz but you can take earth fixed coordinates as inertial as the accelerations of our daily rotation and orbit about the sun etc. are small. So standing by the roadside and watching a car accelerate past you, all is well because you are essentially in an inertial frame. However in the accelerating frame inside the car things are very different. As the car accelerates forward, there's a magical force that pushes you backwards into the seat. In this accelerating frame the second law becomes F -ma = 0. The -ma is the force pushing you back into the seat and the 0 is there because you are at rest in the accelerating frame. This -ma force is the "inertial force" or the "fictitious force". It is "fictitious" because unlike a real force it has no action reaction pair. This is exactly the fake gravity force in the Equivalence principle. So Newton's gravity force is a fictitious force with no action reaction pair and is the force a body exerts trying to return to the straight line motion that it would have in free fall.
27th Oct 2017 13:25 UTCJohn Collins
Physics is difficult for most people because we view the physical world from inside our bodies rather than from outside it.This is especially problematic when our body is being accelerated.
When you are sitting in a forward accelerating car, Your body (due to its inertia) pushes backward on the seat.The seat pushes forward on you causing you to accelerate with the car. No "fictitious" force here.
The action force is the seat pushing forward on you and the reaction force is the your body pushing back on the seat (deforming it slightly).
Now, back to weight vs mass....
Fun discussion.
Regards,
John
27th Oct 2017 14:09 UTCKevin Hean
But. A Meteor trundling through Space does not have Weight but it does have Mass..
Kevin.
27th Oct 2017 14:15 UTCRob Woodside 🌟 Manager
John, I'm afraid you are confusing the contact forces, which are real forces in an action reaction pair, with the inertial body force that is not in any such pair and therefore by definition 'fictitious'.
Remember that the second law uses ONLY external forces on the body to determine its motion. An external force is what acts on the body and any force the body exerts is irrelevant to the body's acceleration. Consider a block sitting on a table at rest in a gravitational field. What are the external forces on the block? Newton would say there's the body force mg due to the weight acting at the mass centre of the block. For the block to remain at rest, this weight must be balanced by the support force the table exerts on the block to hold it up. That support force acts at the surface of the block on the table and is a real force in an action reaction pair. It's opposite is the downward force the block exerts on the table at the table's surface. But that is a force exerted on the table, trying to accelerate the table, not the block.. So only the support force and the weight are the external forces determining the body's motion. If this did not happen on the surface of a planet, but lost in space in an Einstein cage accelerating upward, the weight acting at the body's centre of mass would be replaced by the inertial force acting at the body's centre of mass by the equivalence principle. This proves that the inertial force is a body force and not a contact force as John supposed.
27th Oct 2017 14:16 UTCRob Woodside 🌟 Manager
27th Oct 2017 14:35 UTCJohn Collins
J
27th Oct 2017 14:38 UTCJohn Collins
You are assuming that this interstellar rock has no gravitational force acting on it (from any source). Weight is a measure of the force of gravity on an object.
J
27th Oct 2017 14:47 UTCBob Harman
Just prior to an astronaut's trip to the Moon he/she is put on a scale with his full space suit. He is weighed.
Just after he lands on the Moon and emerges from the landing pod, he/she is put back on the same scale and again weighed.
Of course he/she weighs less, but his Moon mass is identical to his mass at the earthly weighing. Or is it??
Their weight differs, of course, because the mass of the Moon is much less than that of the Earth and the mass of the Moon and Earth (and all space objects) each directly affects the weight, but not the mass (?), of all objects on the surface of that object.
Or maybe I am wrong: if man would go to Jupiter, of course their weight would be much greater, but would the tremendous gravity and pressure also affect the mass???
Or, put another way, the mass of an object affects, thru its gravity, the weight of all objects on its surface, but does it also effect the mass of objects on its surface?
Am I correct???? Also, as our moon is called Luna, can I still capitalize the word moon when I refer to it as '"The Moon"?
CHEERS.......BOB
27th Oct 2017 15:10 UTCJohn Collins
For your questions, the answers are:
1) Yes
2) No The larger gravitational attraction is the cause of the extra weight. [Remember weight of an object is a measure of the force of gravity on it.]
3) No [Mass of a object is a measure of the amount of matter in that object]
4) Re the moon vs the Moon, I don't know.
Regards,
John
27th Oct 2017 20:28 UTCRob Woodside 🌟 Manager
John, when Newton "justfied" the third law in the Principia he talked of pushing down on a surface with your thumb or fingers and that the colour change in the skin as you push down proved that the surface was pushing back. Apparently at the time people thought you had to have a conscious being to perform the action and it was Newton's contribution to realize that there in fact was also a reaction, opposite and equal to the action. Your mention of the car seat deforming as you sit in the car accelerating forward is exactly Newton's argument, but for the action instead of the reaction. That pair of contact forces between your back and the accelerating seat are the classic example of Newton's third law as you said.
27th Oct 2017 21:14 UTCRonald J. Pellar Expert
Don't confuse mass with density. An object on Jupiter could have a higher density than on earth if it is not incompressible. However, the mass is the same regardless of gravitational strength.
Owen:
Summarizing the many of the comments, a scale measures weight as it measures the force directly acting upon the mass. A balance beam measures mass as it is a comparison between an unknown mass with a know mass (gram mass(es)) and will yield the same result on earth, the moon, of Jupiter. However, a balance bean would not work in zero gravity because the gravitational force is used to make the comparison between the know mass and the unknown mass.
Rob:
The mass measurement using the spring frequency of oscillation is used to measure very, very small masses by means of very small cantilevered flexible wires. They measure the frequency of oscillation of the bare wire and then again with a very small mass attached. the difference in frequency yields the mass of the object.
27th Oct 2017 21:40 UTCBob Harman
27th Oct 2017 22:55 UTCRob Woodside 🌟 Manager
28th Oct 2017 00:09 UTCRonald J. Pellar Expert
I think I read somewhere that masses in the nanogram and maybe even pictogram renges are measurable. Amazing indeed!! Also the orientation of the cantilever spring is not important as long as it doesn't change between measurements and it works in freefall!
28th Oct 2017 13:33 UTCJohn Collins
Re the block sitting on a horizontal table here on Earth, there are two forces acting on the block:
1 - the downward force of gravity ON the block due to the the Earth's gravity.
2 - the upward balancing force of the table ON the block.
These two forces are equal, opposite in direction and both act on the block. They are balanced forces and so no acceleration of the block results.
At the table surface where the block sits, the block exerts a downward force of X grams force on the table due to its weight. The table exerts an upward force of X grams force on the block.
These two forces are equal and opposite but NOT balanced as they act on different objects.
BTW, when I taught this stuff in the old days, I scotch taped a thumb tack in a sponge glued to a wall and invited students to use a finger to push on the point. Action and reaction forces were immediately observable.
John
28th Oct 2017 14:55 UTCAlfred L. Ostrander
Then you should be able to figure out why Don Peck is right!!!
28th Oct 2017 15:11 UTCDoug Schonewald
Would there not be a third force on the block? Atmospheric pressure also exerts force on any object (if indeed they reside within an atmosphere). In addition there could other atmospheric forces at work; wind, falling precipitation, humidity, and temperature can all play a small part in exertion of force. In this discussion it is likely not relevant, but exists nonetheless.
28th Oct 2017 21:09 UTCRob Woodside 🌟 Manager
Doug, Students like physics problems when they can do do them as they are very crisp and clean. They ignore all the complications of reality, which might be small as advertised or defeat any hope of experimental confirmation. Things usually happen on frictionless surfaces and mostly in a complete vacuum. I think the idea is to get the student to focus on the important physically significant effects. That makes for good theoreticians, but leaves experimentalists wondering.
Ron, Measuring nanograms in free fall- cute!!! I think you and others are right about Don's point that pan balance will measure mass when the unknown is compered to a known mass, provided you have one. Then for the balance to work you need either downward gravity or upward acceleration in an Einstein cage.
I'm always amazed at the 40 Hertz rumble in deep space and the synthetic gravity in the spaceships in the movies. At least we can synthesize gravity with acceleration. As for the deep rumble, Einstein was once asked about quantised space-time and he replied, "Having a discrete space-time without continuity is rather like trying to breath in a vacuum."
28th Oct 2017 23:50 UTCThomas Lühr Expert
I think there is no doubt to ANY ONE that a pan scale is actually able to determine the mass and does NOT gives any value of an object's wight.
As far as i understand, Owen's point is rather that the pan scale uses the whight as a kind of "secret interim value" and does not work at zero gravity.
Would you agree to say, "The pan scale determines the mass of the test object by measuring its wight and compensating this wight with the wight of a reference mass"?
I have a little problem to use the word "measure" here and used "determine", because one could do exactly the same with a fisherman's scale, that only is able to measure the wight, as we all know. One only would have to replace the test object with a calibrated mass to arrive the same value (of wight) on the scale - whatever the exact number may be.
Rob, as you said, if you don't have the calibrated referenz mass both scales are useless. So is it measuring or determining ?
Of course, it's a bit hair splitting, but that was said by Owen in the other thread exicitely ;)
Thomas
29th Oct 2017 00:36 UTCRob Woodside 🌟 Manager
29th Oct 2017 01:42 UTCHoward Heitner
However, nobody uses either of them is used anymore, just electronic balances or for large vessels, load cells.
29th Oct 2017 03:08 UTCDoug Daniels
29th Oct 2017 05:01 UTCRob Woodside 🌟 Manager
29th Oct 2017 22:53 UTCDoug Daniels
29th Oct 2017 23:48 UTCThomas Lühr Expert
Thomas
30th Oct 2017 01:19 UTCJohn Collins
A double pan balance compares the weight of the sample plus that of its pan with the weight of a predetermined mass on the other pan plus the weight of the other pan. If both total weights are equal, the instrument will show a balance and that means that the masses on the pans are equal. (Since weight of an object is proportional to its mass.)
A triple beam balance operates by a comparing the torque created by the gravitational force on the object being measured plus the weight of the pan with the torque caused by the weight of the beam assembly plus that of the sliding weights on the beams.
Both of these instruments will work anywhere there is a uniform downward gravitational force. Using the law of the lever is essential to the understanding of the operation of this device.
For a simple spring balance however, the weight of an object being measured will vary depending on the strength of the gravitational force at the location of the device.
Regards,
John
30th Oct 2017 22:22 UTCRonald J. Pellar Expert
Scales of the non-balance type are weight measurement devices that are simply calibrated in mass units under a given and constant gravitational field. These scales will show difference in so-called mass when used at a mountain top and compared to the same measurement in a valley. This is also true between the north pole and the equator on earth. On the moon the calibration between weight and mass will not be correct for scales brought from earth and would require calibration whereas balance scales would not have to be recalibrated at all.
Also to clarify, a gram is a unit of mass NOT weight. The corresponding unit of weight is the newton which is a force unit. Speaking of balanced forces ash regards to the block and table, if there is no relative motion between the block and the table then by definition the forces are balanced. Otherwise the unbalance between the forces would result in relative motion between them.
1st Nov 2017 02:02 UTCOwen Lewis
-------------------------------------------------------
> Bob:
> Don't confuse mass with density. An object on
> Jupiter could have a higher density than on earth
> if it is not incompressible. However, the mass is
> the same regardless of gravitational strength.
It's worth adding that, for most everyday purposes, matter in either of the solid or liquid states can be considered incompressible. Matter that is compressible is in the gaseous state.
> Owen:
> Summarizing the many of the comments, a scale
> measures weight as it measures the force directly
> acting upon the mass. A balance beam measures mass
> as it is a comparison between an unknown mass with
> a know mass (gram mass(es)) and will yield the
> same result on earth, the moon, of Jupiter.
> However, a balance bean would not work in zero
> gravity because the gravitational force is used to
> make the comparison between the know mass and the
> unknown mass.
You neatly destroy your own point, I think. Take another run at it.
No balance measures mass. If it did, it would work in weightless conditions and we all know that it will not. Balances measure weight. Weight is a measure of some constant force acting on a mass; for all everyday purposes this force is the gravitational attraction exerted by a hugely larger mass (think planet-sized) on a relatively small one (usually not more than a few tonnes).
Weight = Mass x Gravitational Force. The gravitational force (G) exerted by the planet Earth at its surface is arbitrarily assigned the value 1 and is thus given as 1G. It follows that, on Earth, a mass of 1 gram has a weight of one gram. Other gravitational fields may be greater, lesser or equal to that of Earth. Another way of quantising gravitational force is as the acceleration in free fall of a relatively small mass to a very large one. 1G = 9.807 metres per second per second.
There are several designs of balances. Beam balances all use the principle of an equalising of turning moments, this permitting much better accuracy that a direct comparison of weights. In general, the longer the beam and all else being equal, the more accurate the balance.
>
> Rob:
>
> The mass measurement using the spring frequency of
> oscillation is used to measure very, very small
> masses by means of very small cantilevered
> flexible wires. They measure the frequency of
> oscillation of the bare wire and then again with a
> very small mass attached. the difference in
> frequency yields the mass of the object.
But the essential point (for this thread) is that one can determine mass correctly in any gravitational field including 0G. Another method and quite useful with larger masses many tonnes (in space) is to substitute gravitational force with the centrifugal force generated by moving a mass around a circular path at a constant velocity. The angular velocity (speed of rotation) of masses rotating around a fixed central point causes a centrifugal force to act on the hull and loose items within it. Space stations are rotated about an axis at a fixed angular velocity sufficient to create a centrifugal force approximately equal to 9.807 m/s2 and thus creating a 'pull' on objects inside the skin of the station, towards the skin of the hull, of about 1G, so allowing people to walk rather float about, drinks to stay in mugs etc..
Coming down to Earth and the start of this thread. As designed, both scales and balances only work correctly and without modification in a 1G gravitational field strength. Neither will function in 0G condition unless gravity has been replaced by such as a centrifugal force of 9.807 m/s2 (=1G). In a gravitational field >< 1G, scales and balances can work, subject to the application of some constant correction to equalise the gravitational force in the local environment to 1G.
I'm sure this stuff is more fun now that it was when I was 14, and first learning it :-)
1st Nov 2017 19:00 UTCRonald J. Pellar Expert
Balances:
Balances use some form of force differential to determine equivalency of mass.
Δw = m1a – m2a = 0
Where w is weight, m is mass, and a is acceleration. When Δw is equal to zero then m1 is equal to m2 as long as a > 0. Thus balances will work in any accelerating environment, earth, moon, mars, asteroid, accelerating space ship. Etc.
When a balance condition is reached we have;
m1a - m2a = 0
and dividing out a (provided that it does not equal zero) we get
m1 - m2 = 0 or m1 = m2.
When a = 0 then, of course, a balance will not work. Beam balances also use a known mass that is positioned relative to the pivot point using the lever principle to provide equivalent mass for comparison. The lever principle is also independent of gravitation (it works in any orientation even on earth). The acceleration need not be gravitational at all, it can be electro-magnetic forces, oscillating spring forces, etc. By Newton’s second law we get m = f/a where a is any kind of acceleration.
Scales:
Scales work on the principle of calibration:
m = c(a)w
where m is the mass being measured and c(a) is a constant the depends on a the accelerating field. Any change in the accelerating field will require a new calibration. Thus using a scale calibrated at the equator will not give an accurate reading at the north pole since the earth is an ovoid shape. In fact, on your example of a simulated gravity using centrifugal force on a space craft, the calibration would have to be done for any change in height of the table that the scale rest upon, since the acceleration varies with the radius from the center of roation. The balance, however, is will work at any table height on the space craft. The balance only requires that the comparison masses that are used are of known mass.
In fact, from Newton's second law the only way that you can measure a mass by using a force (or weight) and dividing the resulting acceleration into the force, i.e., m = f/a. Therefor some sort of force must be used to establish the an unknown mass. The standard for mass is a hunk of platinum with a particular weight at a particular spot on the earth's surface. Since measuring a mass requires an accelerating force the real question is can it be done independent of the magnitude of the accelerating force? The answer is yes for a balance but it is no for a scele.
When force is zero there is no acceleration and mass is meaningless, i.e., any mass between zero and infinity will satisfy Newton's second law which is the equation that defines mass. In free fall, the only context is forces that are applied to a mass by other means then gravity. Thus an astronaut pushing on an object could be used to define the objects mass. Again, in the absence of any force mass has NO meaning. To actually measure a mass a force applied to the mass is required to measure it.
So Don was right, balances measure mass and scales measure weight.
1st Nov 2017 21:25 UTCOwen Lewis
In 0G, all balances return a reading of 0 whatever the mass being weighed. In other words, weghing does not measure mass but something else (the G force). But mass is as 'meaningful' in 0G at it is anywhere else is basic in some way to most engineering calculations, ranging from the maximum mass of a bullet or shell that can be safely discharged in a given gun and with a given powder charge, to the power that an engine must develop to accelerate a vehicle to a given speed in a given distance.
It is *weight* that becomes 'meaningless' in an environment of 0G..
Our modern system of standard units of measurement (MKS) has its roots in post-revolutionary France. It was popularised across Europe by Napoleon as part of his re-organisation of the administration of most of continental Europe In this French-invented system, a kilogram was defined as the mass of a cubic decimeter of pure water. No weighing involved. The modern standard kg is a lump of platinum/indium of very different volume but (more or less) the same mass as the earlier MKS standard.This more stable material was adopted by international agreement in 1889 around 100 years ago and this standard remains unchanged to this day
Read this and weep. .https://physics.nist.gov/cuu/Units/kilogram.html I think you're in a deep hole, Ron, and should stop digging. Balances are instruments for making weighings.
P.S. For those in the English-speaking world who are old enough to have used the old Imperial system of measurements which antedates even the original the MKS system, a 1 gallon volume of pure water has a mass of 10lb or 1 pt has a mass of 1.25lb.
1st Nov 2017 22:17 UTCRob Woodside 🌟 Manager
So in the space station we can use springs to "mass" something. Another more crude method might be to tie a known mass on a string of appropriate length and tie the other end to the unknown mass and set it rotating. The system will rotate about its centre of mass which can be found by superposing two frames of a video of the rotating system. The string weighing far less than the masses will from each frame intersect at the mass centre. Knowing its position, then the unknown mass can be found.
The MKS system has the remarkable feature of having the fundamental unit of mass as a thousand times what ought to be the fundamental unit, the gram. It may be nit picking but the Kilogram ought to be renamed the Einstein, in honour of his work and E = mc2. If force gets named after Newton then mass should be named after Einstein!.
Another way to determine mass is to somehow find the total energy E and divide by c2. That works for electromagnetic fields and things that don't easily sit on a pan.
2nd Nov 2017 00:18 UTCOwen Lewis
-------------------------------------------------------
> ..... When
> there's no force as in free fall, like in an
> orbiting space ship, using a balance is a little
> like looking at something in the dark where
> there's no light.
To follow your analogy, your eyes detect light and do not detect an object. One 'sees' an oblject by detecting light bouncing off the object.
> The MKS system has the remarkable feature of
> having the fundamental unit of mass as a thousand
> times what ought to be the fundamental unit, the
> gram.
The basic unit of mass was chosen as most generally convenient for dealing with most applications. There is also the CGS (centimetre/gram/second) system that many of us use for preference as being better scaled to the numbers we work with. And, for old diehards, they can still work with the imperial foot/pound/second system of units. All these systems work equally well - just so long as one is consistent and use only units of measurment from one and the same system in making any given calculation.or a related series of calculations.
2nd Nov 2017 18:52 UTCRonald J. Pellar Expert
My use of the word "meaningless" as applied to an object with no forces applied to it was used to illustrate that an object out in space with no forces applied cannot have its mass determined at all until a force is applied. Just like in the example that you and Rob used of an object in the dark needing light to reflect off of it to see it, a mass needs an accelerating force to act on it to measure its mass. All mass determinations are a comparison between an unknown mass and a known, or predefined, mass. Even the mass of stars in the sky are referred to solar mass. The solar mass can be given in kilograms, but this is a comparison to the standard kilogram. How are all these comparisons made, by observing the effects of accelerating forces upon the unknown masses. This requirement to have an accelerating force to compare masses is why a balance won't work in 0g, but other forces like Rob's example will work in 0g.
The use of the term "weighing" is very loose interpretation for convenience and applies to both balances and scales. It does not invalidate anything that I have said in this thread.
Rob,
A very clever way to measure mass in 0g!!!! But for Owen's benefit, it uses centripetal accelerating force to compare the masses just like a balance uses an accelerating force to compare th masses of an unknown object with a object of known mass.
Your use of total energy to compute mass has ignores all kinetic and potential energies of the object. The E = mc2 equation states the equivalence of mass and energy contained in that mass if it were to be converted into pure energy.
Some might think the momentum could be used to determine mass, but momentum, and kinetic energy, involves the velocity which is a value relative to the observer. As the velocity approaches the speed of light, even mass becomes relative to the observer in the Einstein's rocket ship the acceleration can be measured without reference to anything else but the force acting on your feet. Whereas in the ISS the velocity of the ISS cannot be measured from within the ISS. Thus measurements utilizing velocity cannot be used to determine the mass of an object in the absolute sense that is implied by Newton's second law.
2nd Nov 2017 19:09 UTCJolyon Ralph Founder
2nd Nov 2017 21:09 UTCRob Woodside 🌟 Manager
Ron, energy is a frame dependent concept. It is the time component of the energy momentum vector. So it picks up the Kinetic energy with respect to that frame. A object is heavier when hot! If analytic balances were accurate to half dozen more decimal places, the effect would have been observed long ago. Don't sell energy and momentum short for determining mass. If you know one mass in an elastic collision and have the incoming and out going velocities, you should be able to find the unknown mass. But you are right that force must be involved to determine mass and in this case it is the equal and opposite contact forces in the collision that conserve momentum.
2nd Nov 2017 23:46 UTCRonald J. Pellar Expert
You are correct! The collision of particles involve forces which are necessary to determine, or compare, masses. Now before Owen sinks deeper into rhetoric, I will say that I am done with this topic.
3rd Nov 2017 03:25 UTCDoug Daniels
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Copyright © mindat.org and the Hudson Institute of Mineralogy 1993-2024, except where stated. Most political location boundaries are © OpenStreetMap contributors. Mindat.org relies on the contributions of thousands of members and supporters. Founded in 2000 by Jolyon Ralph.
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