# The Naked Scientists Forum

### Author Topic: Do we need acceleration to define the concept of mass?  (Read 16359 times)

#### yor_on

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##### Re: Do we need acceleration to define the concept of mass?
« Reply #25 on: 02/06/2012 02:15:56 »
If we refer the arrow to interactions and the time it take for something to decay for example, then that decay will be measured the same in all frames of reference as I see it. Assuming that we do it 'locally'. That constitutes a proof for that the arrow never change - locally - for you, no matter where you go (mass) or how fast you go.

Using that as thumb rule you now need to ask if what you measure, comparing your clock and ruler to other frames of reference, is as real as it can be?  And if you accept the theory of relativity it must be. So depending on 'motion' you can contract the universe, as measured by you locally.

The other thing one need to ask is if there exist any other way of measuring than using your local definitions of 'time and distance'. I can't really see any other types of  'real time' direct measurements possible, except conceptually as in Lorentz transformations of others clocks and rulers.

So your local time is always the same as I understands it, it never change. And that is what you use when comparing 'frames of reference'.

#### yor_on

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##### Re: Do we need acceleration to define the concept of mass?
« Reply #26 on: 02/06/2012 02:35:26 »
To me it becomes a geometrical effect in both ways, as 'time' and as 'distance'. It's as if the arrow, although locally invariant as measured by you, is something that 'distort' in its relations to other frames mass and motion. The next question, if this was right, would be to ask where one can set a limit for calling something 'local', and there I provisionally expect that you will find it at Planck scale. Using that as a definition of a smallest 'locality' as defined by a clock and a ruler you then have to assume that all those Plank 'points' must find time dilations as well as Lorentz contractions relative each other.

But it's somehow geometrical, all of it seems to be so to me. Even the arrow, although the arrows 'origin' (as in what I call 'time') is to be found locally as I suspect, at Planck scale. It becomes a 'inflated' universe, defined from a very small invariant 'point' as I think of it.

#### yor_on

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##### Re: Do we need acceleration to define the concept of mass?
« Reply #27 on: 02/06/2012 02:37:35 »
And what that makes is a universe in where distance and motion both must be limited expressions, working here where we are, but not correct for what creates it. Then to that one can add the question how mass, as in the matter we consist of can exist, if we now constantly are 'exposed' to Planck sized time dilations and Lorentz contractions. There I think it should have to do with equilibriums and 'symmetries' as we go down to particle levels, or if you like the 'forces' we see keeping particles together and also 'adhering' one particle to another creating molecules and the matter we touch.

Whatever the arrow is, it's not as we thought of it historically. Although?
« Last Edit: 02/06/2012 02:53:33 by yor_on »

#### Geezer

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##### Re: Do we need acceleration to define the concept of mass?
« Reply #28 on: 02/06/2012 19:54:23 »

The world line only points forward.  The Earth at any instant is about to enter the next instant of time, not the last.

Actually, I don't believe there is any proof that time is irreversible.

Is this theory of "time acceleration" your theory, or is it supported by some testable proof?

Nor do I.  I believe the arrow of time is double ended but entropy ensures that we only experience the one end of that arrow.  Perhaps in an antimatter universe the arrow points in the opposite direction relative to our universe.

Einstein said that gravity and acceleration are equivalent.
An accelerometer on the Earths surface will register about 1g of acceleration.

The Earth travels through space-time.  That's EQUIVALENT to space-time traveling through (or over) the Earth.  Time is more dilated closer to the Earths surface than in space.  That's been proven by comparing two synchronized atomic clocks.  One on the Earths surface and one in orbit.  So space-time dilates as it reaches the Earth.  That's EQUIVALENT to the Earth accelerating in space-time.

Is it my theory?  I don't think so, "I personally believe" it was what Einstein meant when he said gravity and acceleration are equivalent.

Is it supported by testable proof.  Yes, the accelerometer and time dilation measurements as mentioned.  The accelerometer shows that the Earth is accelerating in space-time.  The Earths diameter is not getting any larger as it accelerates therefore it cannot be accelerating in the three dimensions of space, it can only accelerate in the time dimension of space-time.  The difference in clock times shows that time is relative and passes more slowly near to the surface of the Earth as predicted by GR.

If we put an atomic clock on a rocket and send it into space.  The clock will not only accelerate in the space aspect of space-time but in the time aspect of space-time. (As a second becomes progressively shorter[in comparison to a second on the Earth], the ship covers the same distance in less time from the occupants perspective.)  The Earth essentially does the same but just in the time aspect of space-time.  Again it is a local effect.

If this isn't the explanation of what Einstein meant when he said that gravity and acceleration are equivalent then what other explanation is there?

Mass can be evaluated quite easily without any gravitational field by measuring acceleration. Weight is the measurement of the interaction between mass and gravity.

Gravitational acceleration has not the slightest thing to do with the mass of an object because the acceleration is completely independent of the mass.

#### MikeS

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##### Re: Do we need acceleration to define the concept of mass?
« Reply #29 on: 05/06/2012 10:24:31 »

Mass can be evaluated quite easily without any gravitational field by measuring acceleration. Weight is the measurement of the interaction between mass and gravity.

Gravitational acceleration has not the slightest thing to do with the mass of an object because the acceleration is completely independent of the mass.

Mass creates its own gravitational field therefore you can’t have mass without a gravitational field.  So you can’t measure mass without a gravitational field because a gravitational field is associated with mass.  Likewise, you can’t have acceleration without gravity because they are equivalent, although in the case of a small mass the gravitational component is not so obvious until the acceleration is approaching the speed of light.

This is true but what is your point?

“In science and engineering, the weight of an object is the force on the object due to gravity. Its magnitude (a scalar quantity), often denoted by an italic letter W, is the product of the mass m of the object and the magnitude of the local gravitational acceleration g; thus: W = mg.”
http://en.wikipedia.org/wiki/Weight

As I see it, a weighing machine measures weight because it is in a non-inertial reference frame, that is, it is in an accelerating reference frame.  As the acceleration of that reference frame is constant, so is the measure of weight.  Therefore, a non-inertial reference frame (accelerating) is required to be able to measure weight.
end of edit

This is only true when considering an object in free fall in a gravitational field.  It’s not true for the mass (Earth) that is generating that gravitational field.  See reply #12 in this thread.

Gravitational acceleration of the Earth (gravity) or any other massive gravitating body is entirely dependent upon mass, as it is mass that bends space-time.  (Assuming velocity to be insignificant)

I am talking about the acceleration due to gravity of a massive object like the Earth, not acceleration of an object in free fall.

Where Einstein said that energy and mass are equivalent he meant equivalent not exactly the same.  You can make things from mass (matter) but you need energy to do it.  Although equivalent, they are not the same.  When he said that gravity and acceleration are equivalent, I believe he meant they are the same thing, identical.

« Last Edit: 06/06/2012 08:07:42 by MikeS »

#### Geezer

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##### Re: Do we need acceleration to define the concept of mass?
« Reply #30 on: 06/06/2012 07:54:04 »

I am talking about the acceleration due to gravity of a massive object like the Earth, not acceleration of an object in free fall.

It makes no difference. A massive object like the Earth is in free-fall as it orbits the Sun.

Anyway, as I said earlier, a gravitational field won't allow you to evaluate mass. It will only allow you to evaluate weight. You can infer the mass from the weight if you know the acceleration produced by the gravitational field, but that has nothing to do with the weight.

#### MikeS

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##### Re: Do we need acceleration to define the concept of mass?
« Reply #31 on: 06/06/2012 10:54:23 »

I am talking about the acceleration due to gravity of a massive object like the Earth, not acceleration of an object in free fall.

It makes no difference. A massive object like the Earth is in free-fall as it orbits the Sun.

Anyway, as I said earlier, a gravitational field won't allow you to evaluate mass. It will only allow you to evaluate weight. You can infer the mass from the weight if you know the acceleration produced by the gravitational field, but that has nothing to do with the weight.

That's true but it is still not the same as the gravitational acceleration produced by mass.  An accelerometer in free fall does not register acceleration, an accelerometer on the surface of the Earth does. It's the reference frame that is different.

You say that a gravitational field wont allow you to evaluate mass but then you go on to explain how a gravitational field can be used to evaluate mass?

We seem to be at cross purposes here.  You keep mentioning weight and I am not quite sure why.  You need a non-inertial reference frame (accelerating) to be able to measure weight or mass.  The non-inertial reference frame needed to evaluate weight is due to the gravity (acceleration) of the surface of the Earth.  To evaluate mass in a low gravity environment still requires acceleration (which is equivalent to gravity).  Both require a non-inertial reference frame.  A 1kg mass weighs 1kg on the Earths surface as the Earths surface is accelerating at 1g.  The same mass accelerating at 1g in low gravity (far away from the Earths surface) still weighs 1kg.  That is due to gravity and acceleration being equivalent.
« Last Edit: 06/06/2012 11:28:16 by MikeS »

#### Pmb

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##### Re: Do we need acceleration to define the concept of mass?
« Reply #32 on: 06/06/2012 18:15:49 »
It makes no difference. A massive object like the Earth is in free-fall as it orbits the Sun.

Anyway, as I said earlier, a gravitational field won't allow you to evaluate mass. It will only allow you to evaluate weight. You can infer the mass from the weight if you know the acceleration produced by the gravitational field, but that has nothing to do with the weight.
Nice response Geezer!!

#### Geezer

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##### Re: Do we need acceleration to define the concept of mass?
« Reply #33 on: 06/06/2012 19:07:33 »
The same mass accelerating at 1g in low gravity (far away from the Earths surface) still weighs 1kg.  That is due to gravity and acceleration being equivalent.

Right - which proves that mass and gravity are independent.

#### MikeS

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##### Re: Do we need acceleration to define the concept of mass?
« Reply #34 on: 06/06/2012 21:28:55 »
The same mass accelerating at 1g in low gravity (far away from the Earths surface) still weighs 1kg.  That is due to gravity and acceleration being equivalent.

Right - which proves that mass and gravity are independent.

Could you explain the logic of that please?

#### Geezer

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##### Re: Do we need acceleration to define the concept of mass?
« Reply #35 on: 07/06/2012 08:37:09 »
The same mass accelerating at 1g in low gravity (far away from the Earths surface) still weighs 1kg.  That is due to gravity and acceleration being equivalent.

Right - which proves that mass and gravity are independent.

Could you explain the logic of that please?

You can accelerate a mass without any gravitational field by using, for example, a chemical energy source. A rocket would work.

The point is that mass could care less about gravity. Mass remains with, or without, gravity.

#### MikeS

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##### Re: Do we need acceleration to define the concept of mass?
« Reply #36 on: 07/06/2012 13:53:03 »
The same mass accelerating at 1g in low gravity (far away from the Earths surface) still weighs 1kg.  That is due to gravity and acceleration being equivalent.

Right - which proves that mass and gravity are independent.

Could you explain the logic of that please?

You can accelerate a mass without any gravitational field by using, for example, a chemical energy source. A rocket would work.

The point is that mass could care less about gravity. Mass remains with, or without, gravity.

You can't have mass without a gravitational 'field' but apart from that, this is true and I never said otherwise but it is missing my point, which was acceleration and gravity are equivalent.

That’s not strictly speaking correct.  Mass warps space-time.  That interaction is what we call gravity and it involves time-dilation.  You can’t have mass without gravity.  If you try to accelerate mass in a gravitational field (other than its own), it certainly "cares" as evidenced by the variable amounts of energy required to change velocity (in the sense of direction).

That mass remains is almost certainly true (and I have never said otherwise), or it would cause problems for the conservation of mass/energy but that is not what we have been debating.  The question was “Do we need acceleration to define the concept of mass?"  Whether or not mass remains (and it almost certainly does), it still requires acceleration by way of a non-inertial reference frame to evaluate it.  That is, we can weigh it or use an accelerometer.
« Last Edit: 07/06/2012 13:55:43 by MikeS »

#### Geezer

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##### Re: Do we need acceleration to define the concept of mass?
« Reply #37 on: 08/06/2012 05:30:13 »

which was acceleration and gravity are equivalent.

Who cares! It doesn't help answer the question.

#### MikeS

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##### Re: Do we need acceleration to define the concept of mass?
« Reply #38 on: 08/06/2012 12:56:10 »

which was acceleration and gravity are equivalent.

Who cares! It doesn't help answer the question.

Well I do and Einstein did.

Yes it does.

The question was “Do we need acceleration to define the concept of mass?"  "... it still requires acceleration by way of a non-inertial reference frame to evaluate it.  That is, we can weigh it or use an accelerometer.
Unless you know differently of course.

#### Geezer

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##### Re: Do we need acceleration to define the concept of mass?
« Reply #39 on: 09/06/2012 07:13:05 »

Yes it does.

Other than the fact that you assert that it does, why does it help answer the question?

#### MikeS

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##### Re: Do we need acceleration to define the concept of mass?
« Reply #40 on: 09/06/2012 10:27:21 »

Yes it does.

Other than the fact that you assert that it does, why does it help answer the question?

The question was “Do we need acceleration to define the concept of mass?"  My answer was  "... it still requires acceleration by way of a non-inertial reference frame to evaluate it.  That is, we can weigh it or use an accelerometer."

"For example imagine a big rock floating in space. Give it a slap with a calibrated hand so you know exactly how much energy you gave it. Now measure how fast the rock is moving."
http://education.jlab.org/qa/mass_01.html
That's one way of measuring the mass and it involved acceleration of the mass.
"There are a couple of ways to measure mass. The most common method is to use a balance."  " If you go to a different planet, the balance weights change by the same factor as the object you are measuring. Your mass measured with a balance would be the same on the moon as it is on Earth."
http://education.jlab.org/qa/mass_01.html
This way of measuring mass relies upon comparing a known mass with an unknown mass in a non-inertial (accelerating) reference frame.
A spring balance can also measure mass but strictly speaking only measures weight.  The weight on the Moon would be 1/6 that of Earth although the mass remains the same.  Again it relies upon a non-inertial reference frame.

To the best of my knowledge that is self evident.  I don't know of any other way of evaluating or measuring mass and if you can't measure it you can't fully define it.  In that way I believe it does help to answer the question.

Does anyone know of any way of measuring mass that does not rely upon acceleration?
« Last Edit: 09/06/2012 10:57:27 by MikeS »

#### imatfaal

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##### Re: Do we need acceleration to define the concept of mass?
« Reply #41 on: 09/06/2012 15:37:14 »
1. Put super-critical lump of mixed radioactive Pu and U in large water pool  - measure rise in temperature - do maths - get mass lost.

2. measure energy of photons given off by matter/anti-matter annihilation - do maths - get mass of pair

3. measure volume of ideal gas at stp - do maths - get mass

4. count atoms - do maths - get mass (ok that one is silly)

5. take complex hydrocarbon - burn to buggery - do maths - know mass of result (without measuring energy given off)

(and I know BC or JP will haul me over the coals for the liberties I have taken in the above)
« Last Edit: 09/06/2012 15:41:08 by imatfaal »

#### Geezer

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##### Re: Do we need acceleration to define the concept of mass?
« Reply #42 on: 09/06/2012 18:36:58 »

1. Put super-critical lump of mixed radioactive Pu and U in large water pool  - measure rise in temperature - do maths - get mass lost.

There you go!

#### evan_au

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##### Re: Do we need acceleration to define the concept of mass?
« Reply #43 on: 10/06/2012 04:37:34 »
The Pound-Force was defined in the context of the force/acceleration due to Earth's gravity - which is a problem as the acceleration due to gravity varies slightly at different points on the earth's surface.

There is an interesting article at: http://spectrum.ieee.org/consumer-electronics/standards/the-kilogram-reinvented/0

Summary: At present, the definition of mass in the metric system is a specific lump of metal in Paris. (ie it is not a weight, so it is already independent of the acceleration due to Earth's gravity.)
However, there is currently a project underway to define mass more reproducibly in terms of:
(i) Avogadro's Number: A specific number of atoms of a particular isotope of Silicon
(ii) The Ampere: The force exerted by a certain electric current (which again might be viewed as the cause of an acceleration, from a certain viewpoint)

#### Geezer

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##### Re: Do we need acceleration to define the concept of mass?
« Reply #44 on: 10/06/2012 09:45:15 »

it still requires acceleration by way of a non-inertial reference frame to evaluate it.  That is, we can weigh it or use an accelerometer.

No - that won't work.

If you weigh an object in a gravitational field using the deflection of a spring (as in a bathroom scale or an accelerometer), you cannot properly evaluate the mass because you are only measuring the deflection of a spring, and the deflection will vary according to the intensity of the gravitational field.

If you weigh an object using a comparison with another mass (as in a beam balance) you are using an arbitrary object for comparison, but that's not getting you any closer to "the concept of mass".

On the other hand, if you apply a quantity of energy to an object so that its momentum changes, you can get some idea of the relationship between mass and energy.

#### MikeS

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##### Re: Do we need acceleration to define the concept of mass?
« Reply #45 on: 10/06/2012 11:38:07 »

it still requires acceleration by way of a non-inertial reference frame to evaluate it.  That is, we can weigh it or use an accelerometer.

No - that won't work.

If you weigh an object in a gravitational field using the deflection of a spring (as in a bathroom scale or an accelerometer), you cannot properly evaluate the mass because you are only measuring the deflection of a spring, and the deflection will vary according to the intensity of the gravitational field.

If you weigh an object using a comparison with another mass (as in a beam balance) you are using an arbitrary object for comparison, but that's not getting you any closer to "the concept of mass".

On the other hand, if you apply a quantity of energy to an object so that its momentum changes, you can get some idea of the relationship between mass and energy.

I have already explained above how that can be used to measure mass.

It's not arbitrary as it is comparing an unknown mass with a known mass.

All three cases involve acceleration and acceleration costs energy.  In the case of the spring and triple balance the energy comes from gravitational potential energy.  It makes little difference where the energy comes from it still causes acceleration and gives "some idea of the relationship between mass and energy".

#### MikeS

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##### Re: Do we need acceleration to define the concept of mass?
« Reply #46 on: 10/06/2012 11:49:00 »
1. Put super-critical lump of mixed radioactive Pu and U in large water pool  - measure rise in temperature - do maths - get mass lost.

2. measure energy of photons given off by matter/anti-matter annihilation - do maths - get mass of pair

3. measure volume of ideal gas at stp - do maths - get mass

4. count atoms - do maths - get mass (ok that one is silly)

5. take complex hydrocarbon - burn to buggery - do maths - know mass of result (without measuring energy given off)

(and I know BC or JP will haul me over the coals for the liberties I have taken in the above)

imatfaal
Thanks for the input.  I will try to address all of your points.
1) I need to consider some more.
5) How do you know the starting and final mass without weighing it?  (Without using a non-inertial reference frame)

1) The increase in heat of the water is equivalent to the loss of mass.  So presumably you are referring to the equivalence principle E=mc2.  The 2 in c2 represents acceleration.

2) The energy produced is unknown until it is measured which involves obliterating the photons making them give up their energy as momentum and re-radiating some photons at a lower energy level.  The increase in momentum of the target is acceleration.

3) Presumably this relies upon knowing initially both the volume and mass of 1 molecule.
Knowing the mass of 1 molecule relies upon counting the total number of protons and neutrons and knowing the weight of a proton.
“Because atoms are exceptionally small, scientists typically work with atoms in larger quantities called moles. A mole is the amount of a substance with as many atoms as there would be in 12 grams of the isotope carbon-12. This number is roughly 600 sextillion (6 times 10 to the 23rd power) atoms, and is known as Avogadro's number for the scientist who defined it.”
http://www.wikihow.com/Calculate-Atomic-Mass

This method of calculating the mass depends upon initially knowing the atomic weight of one molecule and that requires ‘weighing’ it.  Weighing it requires a non-inertial (accelerating) reference frame.  It’s a calculation based upon a measurement taken in an accelerating reference frame.

4) Is essentially the same answer as 3 but substituting atom for molecule.
« Last Edit: 10/06/2012 15:06:00 by MikeS »

#### Geezer

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##### Re: Do we need acceleration to define the concept of mass?
« Reply #47 on: 10/06/2012 19:48:54 »

It's not arbitrary as it is comparing an unknown mass with a known mass.

All three cases involve acceleration and acceleration costs energy.  In the case of the spring and triple balance the energy comes from gravitational potential energy.  It makes little difference where the energy comes from it still causes acceleration and gives "some idea of the relationship between mass and energy".

As Evan points out, the known mass is completely arbitrary, so the compared mass is also completely arbitrary. Consequently, any form of balance isn't really telling you anything about the mass.

If you use a spring type scale, or an accelerometer, you are determining weight, not mass. You could use a "known mass" to determine the intensity of a gravitational field by this method then do a comparison to determine the relative mass of another object, but then you are back to only establishing a comparitive arbitrary mass.

On the other hand, if you actually alter the momentum of an object, there are methods of directly quantifying the energy conversion.

#### MikeS

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##### Re: Do we need acceleration to define the concept of mass?
« Reply #48 on: 11/06/2012 08:38:35 »

It's not arbitrary as it is comparing an unknown mass with a known mass.

All three cases involve acceleration and acceleration costs energy.  In the case of the spring and triple balance the energy comes from gravitational potential energy.  It makes little difference where the energy comes from it still causes acceleration and gives "some idea of the relationship between mass and energy".

As Evan points out, the known mass is completely arbitrary, so the compared mass is also completely arbitrary. Consequently, any form of balance isn't really telling you anything about the mass.

If you use a spring type scale, or an accelerometer, you are determining weight, not mass. You could use a "known mass" to determine the intensity of a gravitational field by this method then do a comparison to determine the relative mass of another object, but then you are back to only establishing a comparitive arbitrary mass.

On the other hand, if you actually alter the momentum of an object, there are methods of directly quantifying the energy conversion.

I agree it does not tell you anything about mass itself but it is not really arbitrary if you do the comparison with a known mass. Anyway that argument is beside the point.

The question was “Do we need acceleration to define the concept of mass?"

How do you propose to "alter the momentum of an object" without accelerating it?

If you accept that it accelerates then presumably you accept that you "Do .. need acceleration to define the concept of mass?" Which is what I said in the first place and repeated in post #40 of this thread.
"For example imagine a big rock floating in space. Give it a slap with a calibrated hand so you know exactly how much energy you gave it. Now measure how fast the rock is moving."
http://education.jlab.org/qa/mass_01.html
"That's one way of measuring the mass and it involved acceleration of the mass."
« Last Edit: 11/06/2012 11:28:37 by MikeS »

#### MikeS

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##### Re: Do we need acceleration to define the concept of mass?
« Reply #49 on: 11/06/2012 11:26:16 »

If you use a spring type scale, or an accelerometer, you are determining weight, not mass. You could use a "known mass" to determine the intensity of a gravitational field by this method then do a comparison to determine the relative mass of another object, but then you are back to only establishing a comparitive arbitrary mass.

The acceleration of the Earths surface and hence the scale applies acceleration and hence change of momentum to the mass.  (The Earth pushes the scale.  The scale pushes the mass.  This results in a change of momentum for the mass.  This is acceleration)

A tripple balance compares an unknown mass with a known mass and the difference in gravity (acceleration) on different planets is compensated for by affecting both the known and unknown mass in the same proportion.  The mass remains the same but the weight changes.

Whether you apply acceleration to the mass by the Earth pushing it or anything else pushing it, its the same thing, acceleration.

There is no difference between accelerating a mass by applying a force to it and the Earth accelerating the same mass by applying a pseudo-force to it (gravity).  They are equivalent.

There is no difference in measuring a force applied to a mass and the distance it travels than measuring the pseudo-force the Earth applies to the same mass and the distance it travels.

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##### Re: Do we need acceleration to define the concept of mass?
« Reply #49 on: 11/06/2012 11:26:16 »