Offline edby

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Re: Increased gravity at the poles?
« Reply #20 on: November 12, 2018, 07:59:00 PM »
This link provides a pretty good explanation of buoyancy and points out a few things that may be confusing:
True, but in his original reply to the OP, Tom claimed (1) that the atmosphere has weight, and atmospheric pressure "pushes down" on us and (2) The barometric pressure is higher at the poles, therefore (3) we will weigh more at the poles.

This argument is totally fallacious, as I pointed out above. It is true that the atmosphere has great weight, and pushes down on us, but it pushes down with the same weight on the bathroom scales even when I am not standing on them. When I do stand on them, I remove the weight from the scales themselves, but the weight remains pushing on my head. Net difference is my weight alone, atmospheric weight makes no difference.
« Last Edit: November 12, 2018, 08:01:20 PM by edby »

Offline edby

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Re: Increased gravity at the poles?
« Reply #21 on: November 12, 2018, 08:00:39 PM »
Tom, in a nutshell, Archimedes Principle says:  "Every object is buoyed upwards by a force equal to the weight of the fluid the object displaces."  This means that dense objects will always become lighter as the density of the fluid displaced increases.

That is not what that quote means. It means that if the weight of the object is equal to the fluid it displaces it will pulled neigher lower or higher.

But if the object is lighter than the fluid it displaces it will be pulled higher. Just the same, if the object is heavier than the fluid it displaces it will be pushed lower.
Very true, but in your original reply you claimed that the increase in our weight at the poles was due to the greater weight pushing down on our head. This is fallacious.

What is the true reason for the increased weight at the poles?

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Offline markjo

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Re: Increased gravity at the poles?
« Reply #22 on: November 12, 2018, 08:21:52 PM »
Tom, in a nutshell, Archimedes Principle says:  "Every object is buoyed upwards by a force equal to the weight of the fluid the object displaces."  This means that dense objects will always become lighter as the density of the fluid displaced increases.

That is not what that quote means. It means that if the weight of the object is equal to the fluid it displaces it will pulled neither lower or higher.

But if the object is lighter than the fluid it displaces it will be pulled higher. Just the same, if the object is heavier than the fluid it displaces it will be pushed lower.
Gravity always pushes the object down and buoyancy always pushes the object up.  Whether the object rises, floats or sinks depends on whether the buoyant force is greater than, equal to or less than the gravitational force.
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Offline Tom Bishop

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Re: Increased gravity at the poles?
« Reply #23 on: November 12, 2018, 09:07:05 PM »
Very true, but in your original reply you claimed that the increase in our weight at the poles was due to the greater weight pushing down on our head. This is fallacious.

What is the true reason for the increased weight at the poles?

Air, like water, does have weight to it. The thread just got nitpicky about buyancy.

Take a glass jar and fill it 1/4th of the way with sand.

Now fill the rest of the glass with water.

Are you to say that the sand does not feel the weight of the water upon it?

How is it possible to argue, as markjo does above, that this water is not pushing the sand down?
« Last Edit: November 12, 2018, 09:11:28 PM by Tom Bishop »

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Offline Bobby Shafto

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Re: Increased gravity at the poles?
« Reply #24 on: November 12, 2018, 09:11:54 PM »
Because downward force is not isolated from the upward buoyant force.

if the object is lighter than the fluid it displaces it will be pulled higher. Just the same, if the object is heavier than the fluid it displaces it will be pushed lower.

If the object doesn't change its volume, it's displacing the same amount of air regardless of pressure.

So, say location A has low pressure atmosphere and an object displaces X cubic whatevers of rarer, lower pressure air.

Location B has high pressure atmosphere and the object there displaces the same X cubic whatevers of denser air.

Which locations' displaced volume of air weighs more?

Location B, right?

At which location does the object with X volume experience a greater buoyant force?

Same answer. Location B.  Right?

Yes, higher pressure air "pushes down" more from above, but the buoyant force also "pushes up" more.

This is a key principle in scuba diving. Buoyancy compensators adjust the diver's volume to manage the amount of water being displaced as water density/pressure changes with depth. Great pressure with greater depth squeezes the diver and his neoprene, making him less voluminous and less buoyant. But adding air to the BCD increases volume and increases buoyancy by displacing more of the higher pressure water. If an object is non-compressible, its buoyancy will increase with increase in water pressure. The same is true of the atmosphere. High pressure air increases buoyancy, as long as volume remains constant.

The error is believing that fluid pressure is just a vector force in one direction. Bouyancy isn't "something else." It's part of the pressure force equation.

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Offline Dr Van Nostrand

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Re: Increased gravity at the poles?
« Reply #25 on: November 12, 2018, 09:19:10 PM »
Tom, in a nutshell, Archimedes Principle says:  "Every object is buoyed upwards by a force equal to the weight of the fluid the object displaces."  This means that dense objects will always become lighter as the density of the fluid displaced increases.

That is not what that quote means. It means that if the weight of the object is equal to the fluid it displaces it will pulled neigher lower or higher.

But if the object is lighter than the fluid it displaces it will be pulled higher. Just the same, if the object is heavier than the fluid it displaces it will be pushed lower.
Very true, but in your original reply you claimed that the increase in our weight at the poles was due to the greater weight pushing down on our head. This is fallacious.

What is the true reason for the increased weight at the poles?

Air, like water, does have weight to it. The thread just got nitpicky about buyancy.

Take a glass jar and fill it 1/4th of the way with sand.

Now fill the rest of the glass with water.

Are you to say that the sand does not feel the weight of the water upon it?

How is it possible to argue, as markjo does above, that this water is not pushing the sand down?


Ambient pressure is distributed in all directions. As someone pointed out upthread, it has no vector.

The weight of the water surrounds each sand grain from all directions.  It does not push 'down.'


The barometric pressure does not affect how much we weigh otherwise we'd see our weight fluctuate when the barometer changes.


However, we can be affected by the buoyancy of the atmosphere. We are less dense than air so we sink in air. We are comparable to the density of water so we float or sink based on how much fat (floats) or how much muscle (sinks) we have.  We are much less dense than liquid mercury so we float high is a body of mercury. However, the difference in buoyancy for a human in a low pressure atmosphere and a high pressure atmosphere is negligible.   
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Offline markjo

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Re: Increased gravity at the poles?
« Reply #26 on: November 12, 2018, 09:31:53 PM »
Very true, but in your original reply you claimed that the increase in our weight at the poles was due to the greater weight pushing down on our head. This is fallacious.

What is the true reason for the increased weight at the poles?

Air, like water, does have weight to it. The thread just got nitpicky about buyancy.

Take a glass jar and fill it 1/4th of the way with sand.

Now fill the rest of the glass with water.

Are you to say that the sand does not feel the weight of the water upon it?

How is it possible to argue, as markjo does above, that this water is not pushing the sand down?

Tom, the effect of buoyancy on dense objects is easy enough to test for yourself.
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If you can't demonstrate it, then you shouldn't believe it.

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Offline Tom Bishop

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Re: Increased gravity at the poles?
« Reply #27 on: November 12, 2018, 09:42:45 PM »
Ambient pressure is distributed in all directions. As someone pointed out upthread, it has no vector.

The weight of the water surrounds each sand grain from all directions.  It does not push 'down.'

You are basically suggesting that if you were burried two feet under the sand of the deepest ocean that you would not be crushed, since as you argue, that sand is not feeling the downward weight of the ocean.

I think you are mistaken. My position is that you would be crushed.
« Last Edit: November 12, 2018, 09:44:20 PM by Tom Bishop »

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Offline Dr Van Nostrand

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Re: Increased gravity at the poles?
« Reply #28 on: November 12, 2018, 09:45:52 PM »
Ambient pressure is distributed in all directions. As someone pointed out upthread, it has no vector.

The weight of the water surrounds each sand grain from all directions.  It does not push 'down.'

You are basically suggesting that if you were burried two feet under the sand of the deepest ocean that you would not be crushed, since that sand is not feeling the weight of the ocean.

I think you are mistaken. My position is that you would be crushed.


You would be crushed by the ambient pressure crushing you in all directions. You would not be crushed flat.
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Offline RonJ

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Re: Increased gravity at the poles?
« Reply #29 on: November 12, 2018, 10:06:00 PM »
You guys are a bit off.  Take a look at some back of the envelope calculations.  Forget water for now.  It's mostly considered to be non-compressible, but it slightly is.  Now consider AIR.  At an air pressure of 1018 hPa the density of air is about 1.2260 kg/cubic meter.  If you increase air pressure the density of air increases because air IS highly compressible.  Air at a pressure of 3000 hPa is 3.622 kg/cubic meter.  Now if you were to take a cubic meter block of balsa wood to the north pole and assumed it was in a vacuum it would weigh about 160 kg.  Now if you took that block of balsa wood out of the vacuum and put it in the air at 1018 hPa the effect of buoyancy would lighten the block by 1.226 kg.  Now the starting weight of the block would be about 158.774 kg.  If you INCREASE the air pressure to an unreasonable 3000 hPa then the same block would actually weigh less because the more dense area would 'float' a little more of the balsa block and it would then weigh 156.378 kg.  Of course the density of the air depends on the temperature and the dew point as well, but you get the point.  An increase in air pressure will actually cause something to weigh a little LESS because air compresses leading to a higher density and a little more buoyant effect.  As far as the argument that air doesn't have any mass, I better tell all the wind farm folks in the area that fact.  They seem to think that the horizontal flow of air decelerating against the fan blades produces a force that can be turned into electrical power.  Newton was kind enough to give us the equation F=MA.  Of course that also means that water has mass as well, otherwise a ship's propeller just wouldn't work either.  Shall we argue that the whole earth must have mass? 
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Offline Baby Thork

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Re: Increased gravity at the poles?
« Reply #30 on: November 13, 2018, 12:23:19 AM »
This thread is a bloodbath of scientific illiteracy.

Watch the video below. You should understand the difference between something in water and something on water.

If something is in water it displaces its volume of water.
If something is on water, it displaces its weight in water.

A scuba diver is in water. The atmosphere is on water. They are not analogous.



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Offline Tom Bishop

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Re: Increased gravity at the poles?
« Reply #31 on: November 13, 2018, 01:20:17 AM »
Very true, but in your original reply you claimed that the increase in our weight at the poles was due to the greater weight pushing down on our head. This is fallacious.

What is the true reason for the increased weight at the poles?

Air, like water, does have weight to it. The thread just got nitpicky about buyancy.

Take a glass jar and fill it 1/4th of the way with sand.

Now fill the rest of the glass with water.

Are you to say that the sand does not feel the weight of the water upon it?

How is it possible to argue, as markjo does above, that this water is not pushing the sand down?

Tom, the effect of buoyancy on dense objects is easy enough to test for yourself.
https://www.youtube.com/watch?v=ROXYr_SzNW4

Marjo, it depends how you weigh it. If you have a jar of sand on a scale and add water to the jar, it will weigh more than just the jar and sand alone. The sand now has more weight on top of it.

This link provides a pretty good explanation of buoyancy and points out a few things that may be confusing:
True, but in his original reply to the OP, Tom claimed (1) that the atmosphere has weight, and atmospheric pressure "pushes down" on us and (2) The barometric pressure is higher at the poles, therefore (3) we will weigh more at the poles.

This argument is totally fallacious, as I pointed out above. It is true that the atmosphere has great weight, and pushes down on us, but it pushes down with the same weight on the bathroom scales even when I am not standing on them. When I do stand on them, I remove the weight from the scales themselves, but the weight remains pushing on my head. Net difference is my weight alone, atmospheric weight makes no difference.

Detecting the difference in atmospheric weight depends on the type of scale and calibration.

If you take an old fashioned bathroom scale and drill holes through the exterior and put it on the bottom of a swimming pool so that it's interior is filled, then it will read zero. If you wrap the scale it in seran wrap and put it on the bottom of the pool then it is feeling the tremendous weight of the water above it.

Subjectively, the water is much heavier than the air, but it is possible to calibrate the difference to zero. Depending on the type of scale and the procedure, the difference may be felt.
« Last Edit: November 13, 2018, 04:08:28 AM by Tom Bishop »

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Offline RonJ

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Re: Increased gravity at the poles?
« Reply #32 on: November 13, 2018, 03:59:08 AM »
For many years there have been many different entities that have been surveying and mapping the gravity field of the earth.  There has been established a reference gravity field called the 'Geodetic Reference System' and that reference is being constantly updated with corrections as conditions change on the earth.  Gravity is a force and is a vector quantity.  Things inside the earth and on top of the earths surface can change the gravity force vector.  You can be sure that with all the volcanic activity in Hawaii a short time ago there has been changes to the local gravitational field in that area as a bunch of lava flowed out of the earth onto the earth's surface.  The Geodetic Survey folks call the earth an ellipsoid and have detailed equations that show the nominal variation of the gravity vector at different latitudes.  You would expect that as the distance to the earth's center is a bit further at the equator than at the poles.  It is interesting to note that as part of the measurement system a correction is made for the buoyancy due to air.  The answer to the original question on this thread is yes, you can expect a difference in weight the further North you go.  The survey folks have well calibrated instruments that are used on land, at sea and in the air to map the gravity field of the earth.  Unfortunately, since 'gravity' on the earth isn't an "FE belief" I don't know what else to say.  There are countless scientists and engineers working for universities and governments, worldwide, that study, survey, and chart the gravity of the earth.  Are they all just wasting time & money?
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Offline Tom Bishop

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Re: Increased gravity at the poles?
« Reply #33 on: November 13, 2018, 04:41:44 AM »
As a follow up to my last post, look at this:

http://physicstasks.eu/930/spring-scale-on-the-pole-and-on-the-equator

Quote
We calibrated a spring scale on the North Pole and then we moved the scale to the Equator.

Does the spring scale give the same readings as on the pole?

The experiment is about taking a spring scale that is calibrated at the North Pole and moving it to the Equator.

This is a different experiment than weighing a mass in two different areas with scales that have been calibrated for their local areas.

Offline edby

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Re: Increased gravity at the poles?
« Reply #34 on: November 13, 2018, 08:42:21 AM »
As a follow up to my last post, look at this:

http://physicstasks.eu/930/spring-scale-on-the-pole-and-on-the-equator

Quote
We calibrated a spring scale on the North Pole and then we moved the scale to the Equator.

Does the spring scale give the same readings as on the pole?

The experiment is about taking a spring scale that is calibrated at the North Pole and moving it to the Equator.

This is a different experiment than weighing a mass in two different areas with scales that have been calibrated for their local areas.
The conclusion is that the difference in weight is nothing to do with atmospheric pressure, but rather of difference in centrifugal force. This answers my question.

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Offline Tom Bishop

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Re: Increased gravity at the poles?
« Reply #35 on: November 13, 2018, 01:11:10 PM »
As a follow up to my last post, look at this:

http://physicstasks.eu/930/spring-scale-on-the-pole-and-on-the-equator

Quote
We calibrated a spring scale on the North Pole and then we moved the scale to the Equator.

Does the spring scale give the same readings as on the pole?

The experiment is about taking a spring scale that is calibrated at the North Pole and moving it to the Equator.

This is a different experiment than weighing a mass in two different areas with scales that have been calibrated for their local areas.
The conclusion is that the difference in weight is nothing to do with atmospheric pressure, but rather of difference in centrifugal force. This answers my question.

If this were the case, why do they specifically not calibrate the scale again at the equator or use a different scale that is calibrated? If the weight were truly heavier in one location, this difference should appear on calibrated scales.

The experiment is specifically about taking a scale calibrated for the North Pole to the Equator, and so the idea that this must be measuring weight difference of the mass and nothing more is fallacious.

Where are the controls in this experiment to show what is and is not being measured? Since you are presenting this as fact, you should be expected to defend these experiments.
« Last Edit: November 13, 2018, 01:32:51 PM by Tom Bishop »

Re: Increased gravity at the poles?
« Reply #36 on: November 13, 2018, 01:33:46 PM »
This is confusing. Are you now claiming that weight doesn’t vary between the pole and equator?
You’ve spent the last page saying that it does because of air pressure difference.
Wrongly of course, but are you now saying it doesn’t?
Far as I understand the weight variation is more to do with the earth’s rotation, not because of earth’s oblateness
If you are making your claim without evidence then we can discard it without evidence.

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Offline Tom Bishop

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Re: Increased gravity at the poles?
« Reply #37 on: November 13, 2018, 01:38:48 PM »
This is confusing. Are you now claiming that weight doesn’t vary between the pole and equator?
You’ve spent the last page saying that it does because of air pressure difference.
Wrongly of course, but are you now saying it doesn’t?
Far as I understand the weight variation is more to do with the earth’s rotation, not because of earth’s oblateness

Different areas have different air pressures and therefore different weights to their atmosphere. They took a scale calibrated for an area of higher pressure to an area of lower pressure and are measuring the difference seen on that scale.

Offline edby

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Re: Increased gravity at the poles?
« Reply #38 on: November 13, 2018, 01:38:58 PM »
This is confusing. Are you now claiming that weight doesn’t vary between the pole and equator?
You’ve spent the last page saying that it does because of air pressure difference.
Wrongly of course, but are you now saying it doesn’t?
Far as I understand the weight variation is more to do with the earth’s rotation, not because of earth’s oblateness

Different areas have different air pressures and therefore different weights to their atmosphere. They took a scale calibrated for an area of higher pressure to an area of lower pressure and are measuring the difference seen on that scale.

Quote
https://en.wikipedia.org/wiki/Gravimetry
An instrument used to measure gravity is known as a gravimeter, or gravinometer. For a small body, general relativity predicts gravitational effects indistinguishable from the effects of acceleration by the equivalence principle. Thus, gravimeters can be regarded as special-purpose accelerometers. Many weighing scales may be regarded as simple gravimeters. In one common form, a spring is used to counteract the force of gravity pulling on an object. The change in length of the spring may be calibrated to the force required to balance the gravitational pull. The resulting measurement may be made in units of force (such as the newton), but is more commonly made in units of gals.

Researchers use more sophisticated gravimeters when precise measurements are needed. When measuring the Earth's gravitational field, measurements are made to the precision of microgals to find density variations in the rocks making up the Earth. Several types of gravimeters exist for making these measurements, including some that are essentially refined versions of the spring scale described above. These measurements are used to define gravity anomalies.

Besides precision, stability is also an important property of a gravimeter, as it allows the monitoring of gravity changes. These changes can be the result of mass displacements inside the Earth, or of vertical movements of the Earth's crust on which measurements are being made: remember that gravity decreases 0.3 mGal for every metre of height. The study of gravity changes belongs to geodynamics.

The majority of modern gravimeters use specially-designed metal or quartz zero-length springs to support the test mass. Zero-length springs do not follow Hooke's Law, instead they have a force proportional to their length. The special property of these springs is that the natural resonant period of oscillation of the spring-mass system can be made very long - approaching a thousand seconds. This detunes the test mass from most local vibration and mechanical noise, increasing the sensitivity and utility of the gravimeter. Quartz and metal springs are chosen for different reasons; quartz springs are less affected by magnetic and electric fields while metal springs have a much lower drift (elongation) with time. The test mass is sealed in an air-tight container so that tiny changes of barometric pressure from blowing wind and other weather do not change the buoyancy of the test mass in air.

Spring gravimeters are, in practice, relative instruments which measure the difference in gravity between different locations. A relative instrument also requires calibration by comparing instrument readings taken at locations with known complete or absolute values of gravity. Absolute gravimeters provide such measurements by determining the gravitational acceleration of a test mass in vacuum. A test mass is allowed to fall freely inside a vacuum chamber and its position is measured with a laser interferometer and timed with an atomic clock. The laser wavelength is known to ±0.025 ppb and the clock is stable to ±0.03 ppb as well. Great care must be taken to minimize the effects of perturbing forces such as residual air resistance (even in vacuum), vibration, and magnetic forces. Such instruments are capable of an accuracy of about two parts per billion or 0.002 mGal[1] and reference their measurement to atomic standards of length and time. Their primary use is for calibrating relative instruments, monitoring crustal deformation, and in geophysical studies requiring high accuracy and stability. However, absolute instruments are somewhat larger and significantly more expensive than relative spring gravimeters, and are thus relatively rare.

Re: Increased gravity at the poles?
« Reply #39 on: November 13, 2018, 01:41:03 PM »
This is confusing. Are you now claiming that weight doesn’t vary between the pole and equator?
You’ve spent the last page saying that it does because of air pressure difference.
Wrongly of course, but are you now saying it doesn’t?
Far as I understand the weight variation is more to do with the earth’s rotation, not because of earth’s oblateness

Different areas have different air pressures and therefore different weights to their atmosphere. They took a scale calibrated for an area of higher pressure to an area of lower pressure and are measuring the difference seen on that scale.
Can you show evidence the North Pole is an area of higher pressure? This air pressure map (at least during July) suggest this is not necessarily the case.

https://www.mapsofworld.com/world-maps/image/wether/wind-and-pressure-july-enlarge.jpg