How do gravity and buoyancy relate?

  • 37 Replies
  • 4998 Views

0 Members and 1 Guest are viewing this topic.

*

Offline gazza711

  • Sr. Member
  • ****
  • 144
    • View Profile
How do gravity and buoyancy relate?
« on: 08/11/2014 19:29:05 »
wow.Thats so kool what replies I got.Its all in the SUBJECT words.I too believe that we are pushed to the earth,not pulled.what are your thoughts anyone?

*

Offline chiralSPO

  • Global Moderator
  • Neilep Level Member
  • *****
  • 1917
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #1 on: 08/11/2014 19:47:14 »
wow.Thats so kool what replies I got.Its all in the SUBJECT words.I too believe that we are pushed to the earth,not pulled.what are your thoughts anyone?


If gravity is a push-type force, why does it depend so much on the masses of the bodies involved? The force of gravity exerted by the Earth on my body is much more than the force that would be exerted by Mars, if I were on the surface of Mars. If it were something from outside pushing down, presumably the forces would be essentially the same since the "outside" of Earth is essentially the same as the "outside" of Mars.
« Last Edit: 13/06/2015 16:15:56 by chiralSPO »

*

Offline gazza711

  • Sr. Member
  • ****
  • 144
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #2 on: 08/11/2014 20:06:42 »
Uhuh.I like people like you.Im eager to discuss this.

Why does the moon keep us on its surface?Same reason as mars situation.But were still attracted somehow.The force exerted is earths atmosphere,not earth itself.So on mars,there is a small atmosphere-just like on the moon-yes,the moon has a sodium atmosphere.Why is there more pressure the deeper we go in water.Its buoyancy that's keeps all that we see pushed to the ground like a helium balloon is pushed to ur ceiling in ur living room.

Have I lost you yet?

If a feather and bowling ball were dropped at the same time in a near perfect vacuum,99.99999% vacuum,the objects fall extremely slowly towards the ground.why.The other .001111111% gases are doing the pushing on the objects maybe.Gallileo experiment.

Thanks for ur response dude.

*

Offline gazza711

  • Sr. Member
  • ****
  • 144
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #3 on: 08/11/2014 21:41:43 »
why would it depend so much on the masses of the bodies involved?

*

Offline chiralSPO

  • Global Moderator
  • Neilep Level Member
  • *****
  • 1917
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #4 on: 13/06/2015 16:31:09 »

the moon has a sodium atmosphere.

No it doesn't. Where did you get this (mis)information? The moon does have an atmosphere, but the pressure is about 3x10–15 times the pressure on Earth 99.999999999997% vacuum compared to Earth's atmosphere. Mostly it is made of helium, neon and hydrogen.

Why is there more pressure the deeper we go in water.Its buoyancy that's keeps all that we see pushed to the ground like a helium balloon is pushed to ur ceiling in ur living room.

There is more pressure under deep water because there is more water above you, and all of it is being pulled down by gravity.

Buoyancy has to do with relative densities in a gravitational field. If the helium balloon has less mass than an equal volume of the air around it, the air around it will fall as it rises. This process will continue until the balloon reaches a point where the air around it has the same density, or until the balloon bursts.


If a feather and bowling ball were dropped at the same time in a near perfect vacuum,99.99999% vacuum,the objects fall extremely slowly towards the ground.why.The other .001111111% gases are doing the pushing on the objects maybe.Gallileo experiment.

No. The Gallileo experiment has been performed in near perfect vacuum, and the objects fall faster in vacuum than in air.

https://www.youtube.com/watch?v=5C5_dOEyAfk
https://www.youtube.com/watch?v=7eTw35ZD1Ig
https://www.youtube.com/watch?v=E43-CfukEgs (note the vacuum drop starts at about 2:50, and is shown in slow motion, this is not how fast they actually fell; also shown at 4:15)

*

Offline evan_au

  • Neilep Level Member
  • ******
  • 4261
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #5 on: 15/06/2015 22:25:52 »
I think the hypothesis here is backwards:
  • It is proposed that atmospheric pressure causes gravity
  • When in fact gravity causes atmospheric pressure 

Atmospheric pressure is caused by gravity, but it is a very non-linear relationship, since
  • a very light body (like the Moon) will lose virtually all its atmosphere
  • while a heavier body (like the rocky core of Jupiter) will retain almost all its atmosphere, including hydrogen & helium
  • So this hypothesis badly underestimates the gravity on the Moon, and overestimates the gravity on Jupiter

Atmospheric pressure is also affected by multiple other factors, like:
  • Strength of the Solar Wind: This is very strong near Mercury, and much weaker out past Jupiter. The solar wind can tear away at the upper atmosphere.
  • Strength of the planetary magnetic field: A strong magnetic field (like Earth or Jupiter) tends to deflect the more energetic outbursts of the solar wind around the planet.
  • Temperature: A hot planet like Venus will drive all liquids and gases into the atmosphere, producing an enormous atmospheric pressure. On the other hand, on Mars, liquid water is frozen out under the surface, producing a very low atmospheric pressure. However, the surface gravity of both bodies is similar.
  • History: The position in the solar system where the planet formed, which determines how it reached its current mass. The orbits of planets in the Solar System does change over astronomical time periods.

So any relationship between atmospheric pressure and gravity is very indirect, with lots of confounding factors.
 
However, as Newton showed, mass causes gravity, and there is quite a simple relationship between the mass of the bodies, the distance between them, and the gravitational force of attraction between them.


*

Offline gazza711

  • Sr. Member
  • ****
  • 144
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #6 on: 20/06/2015 07:22:53 »
I think the hypothesis here is backwards:
  • It is proposed that atmospheric pressure causes gravity
  • When in fact gravity causes atmospheric pressure 

Atmospheric pressure is caused by gravity, but it is a very non-linear relationship, since
  • a very light body (like the Moon) will lose virtually all its atmosphere
  • while a heavier body (like the rocky core of Jupiter) will retain almost all its atmosphere, including hydrogen & helium
  • So this hypothesis badly underestimates the gravity on the Moon, and overestimates the gravity on Jupiter

Atmospheric pressure is also affected by multiple other factors, like:
  • Strength of the Solar Wind: This is very strong near Mercury, and much weaker out past Jupiter. The solar wind can tear away at the upper atmosphere.
  • Strength of the planetary magnetic field: A strong magnetic field (like Earth or Jupiter) tends to deflect the more energetic outbursts of the solar wind around the planet.
  • Temperature: A hot planet like Venus will drive all liquids and gases into the atmosphere, producing an enormous atmospheric pressure. On the other hand, on Mars, liquid water is frozen out under the surface, producing a very low atmospheric pressure. However, the surface gravity of both bodies is similar.
  • History: The position in the solar system where the planet formed, which determines how it reached its current mass. The orbits of planets in the Solar System does change over astronomical time periods.

So any relationship between atmospheric pressure and gravity is very indirect, with lots of confounding factors.
 
However, as Newton showed, mass causes gravity, and there is quite a simple relationship between the mass of the bodies, the distance between them, and the gravitational force of attraction between them.
Hi.What are the chances anyones going to answer to a post from 18/11/15.I am very honoured as I thought I was mearly a blip on your radio.
Mars and venus have way different gravity's.
A lot of 'facts' in planetology might involve a lot of assumptions.Cavendish experiment predicted a lot of things.
So if air we breath has a weight(mass),it must have a downward force of some sort?
So a trillion cazillion atoms in the atmosphere with mass can(entrophic idea maybe)exert a force.water is in most things.without the water,most things would break up and disappear.So when we talk about gravities on other planets and moons for example,you might find that temps on most planets cause the gas,liquid and solid formations.when a gas in the atmosphere is cooled-what happens-gravity or density displacement?
A bit like a bucket of sand.The sand is the atmosphere pressurised by the outside vacuum.you are a spec of something at the bottom of that bucket.
With regards to gravities on other planets and moons-I think the atmosphere is cleverer than we think.
The problem I have in all of this as you know is-what modern proof do we have that objects attract?using lead or metal objects to do experiments is biased-I believe.
I believe that you might have an interest in my findings?or just bored to scroll through peoples old posts-HMMMM.Thanks for the reply.Really enjoying ur comments.Thank you

*

Offline gazza711

  • Sr. Member
  • ****
  • 144
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #7 on: 20/06/2015 07:27:10 »
wow.Thats so kool what replies I got.Its all in the SUBJECT words.I too believe that we are pushed to the earth,not pulled.what are your thoughts anyone?


If gravity is a push-type force, why does it depend so much on the masses of the bodies involved? The force of gravity exerted by the Earth on my body is much more than the force that would be exerted by Mars, if I were on the surface of Mars. If it were something from outside pushing down, presumably the forces would be essentially the same since the "outside" of Earth is essentially the same as the "outside" of Mars.
1.well unless you could weigh planets-which we cant-then you cannot assume their mass.
2.thus we cannot say gravity depends on the bodies involved.
3.how can the outside of earth be the same as the outside of mars?
4.if it was,we wouldn't be going on holiday on earth-mars is the same isn't it.cant see that one happening.

*

Offline PmbPhy

  • Neilep Level Member
  • ******
  • 2794
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #8 on: 20/06/2015 12:05:59 »
Quote from: gazza711
wow.Thats so kool what replies I got.Its all in the SUBJECT words.I too believe that we are pushed to the earth,not pulled.what are your thoughts anyone?
I don't think you understand the different between something being pushed or pulled. It really only has meaning for macroscopic objects with a finite length, width and height.

Push - To push something means that a force is exerted on the surface of the body directed into the body. Such a force acting in this way acts to compress the body. So you know something is being pushed if the body is being compressed along its length parallel to the direction of the force.

Pull - A body is being pulled if a force is exerted on the surface of a body directed outward from the inside of the body. Such a force acts to extend the body. There will be a tension in the object under such a force.

You could also drill a hole through the body and insert a rod and then exert a force on the rod. Then some parts of the body will be pushed while other parts will be pulled depending on where the rod is inserted. You could also grasp a rod with your hand and exert a force parallel to the rod's length. The part of the rod above the hand in the direction of the force is being pushed while the other part is being pulled.

Gravity does neither of those since it exerts a force not simply on the surface of the object but on each point of the body at the same time.

*

Offline gazza711

  • Sr. Member
  • ****
  • 144
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #9 on: 20/06/2015 12:52:53 »
Quote from: gazza711
wow.Thats so kool what replies I got.Its all in the SUBJECT words.I too believe that we are pushed to the earth,not pulled.what are your thoughts anyone?
I don't think you understand the different between something being pushed or pulled. It really only has meaning for macroscopic objects with a finite length, width and height.

Push - To push something means that a force is exerted on the surface of the body directed into the body. Such a force acting in this way acts to compress the body. So you know something is being pushed if the body is being compressed along its length parallel to the direction of the force.

Pull - A body is being pulled if a force is exerted on the surface of a body directed outward from the inside of the body. Such a force acts to extend the body. There will be a tension in the object under such a force.

You could also drill a hole through the body and insert a rod and then exert a force on the rod. Then some parts of the body will be pushed while other parts will be pulled depending on where the rod is inserted. You could also grasp a rod with your hand and exert a force parallel to the rod's length. The part of the rod above the hand in the direction of the force is being pushed while the other part is being pulled.

Gravity does neither of those since it exerts a force not simply on the surface of the object but on each point of the body at the same time.
So a balloon submerged deep in water.is it being pulled or pushed to the surface?surely the inside of the balloon is too being affected-as a stone drops in water due to......gravity?
Sure,so if water has the same characteristics as the air we breath,then we are dealing with the same thing at different temperatures.

*

Offline alancalverd

  • Global Moderator
  • Neilep Level Member
  • *****
  • 4816
  • life is too short to drink instant coffee
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #10 on: 20/06/2015 13:21:57 »
Sure,so if water has the same characteristics as the air we breath,then we are dealing with the same thing at different temperatures.

Speak for yourself, but I think most of our correspondents are mammals, not fish.
helping to stem the tide of ignorance

*

Offline PmbPhy

  • Neilep Level Member
  • ******
  • 2794
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #11 on: 20/06/2015 13:22:56 »
Quote from: gazza711
So a balloon submerged deep in water.is it being pulled or pushed to the surface?
Since neither definition applies it's neither pushed nor pulled, just like gravity.

*

Offline jeffreyH

  • Global Moderator
  • Neilep Level Member
  • *****
  • 4072
  • The graviton sucks
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #12 on: 20/06/2015 14:39:34 »
Gazza, PmbPhy told you what the distinction was between gravity and other types of forces. This is a very important point and you should take it on board. Unless you grasp the concept that gravity acts on every part of a body all at once you will be stuck in your incorrect way of viewing things. Forum members such as Alan, PmbPhy, Evan and Chiral, that are bothering to reply to you, know what they are talking about. By all means ask questions but make no assumptions that anything you believe is right simply because it has popped into your head.

*

Offline gazza711

  • Sr. Member
  • ****
  • 144
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #13 on: 20/06/2015 20:22:19 »
Gazza, PmbPhy told you what the distinction was between gravity and other types of forces. This is a very important point and you should take it on board. Unless you grasp the concept that gravity acts on every part of a body all at once you will be stuck in your incorrect way of viewing things. Forum members such as Alan, PmbPhy, Evan and Chiral, that are bothering to reply to you, know what they are talking about. By all means ask questions but make no assumptions that anything you believe is right simply because it has popped into your head.
Really-are you a scientist?

*

Offline gazza711

  • Sr. Member
  • ****
  • 144
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #14 on: 20/06/2015 20:23:29 »
Sure,so if water has the same characteristics as the air we breath,then we are dealing with the same thing at different temperatures.

Speak for yourself, but I think most of our correspondents are mammals, not fish.
and the others?Arent whales mammals?

*

Offline gazza711

  • Sr. Member
  • ****
  • 144
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #15 on: 20/06/2015 20:28:30 »

the moon has a sodium atmosphere.

No it doesn't. Where did you get this (mis)information? The moon does have an atmosphere, but the pressure is about 3x10–15 times the pressure on Earth 99.999999999997% vacuum compared to Earth's atmosphere. Mostly it is made of helium, neon and hydrogen.

Why is there more pressure the deeper we go in water.Its buoyancy that's keeps all that we see pushed to the ground like a helium balloon is pushed to ur ceiling in ur living room.

There is more pressure under deep water because there is more water above you, and all of it is being pulled down by gravity.

Buoyancy has to do with relative densities in a gravitational field. If the helium balloon has less mass than an equal volume of the air around it, the air around it will fall as it rises. This process will continue until the balloon reaches a point where the air around it has the same density, or until the balloon bursts.


If a feather and bowling ball were dropped at the same time in a near perfect vacuum,99.99999% vacuum,the objects fall extremely slowly towards the ground.why.The other .001111111% gases are doing the pushing on the objects maybe.Gallileo experiment.

No. The Gallileo experiment has been performed in near perfect vacuum, and the objects fall faster in vacuum than in air.

https://www.youtube.com/watch?v=5C5_dOEyAfk
https://www.youtube.com/watch?v=7eTw35ZD1Ig
https://www.youtube.com/watch?v=E43-CfukEgs (note the vacuum drop starts at about 2:50, and is shown in slow motion, this is not how fast they actually fell; also shown at 4:15)
Yes.I felt a bit of an idiot when I wrote that.I thought I was onto something whilst watching Brian Cox do the Gallileo experiment. Didn't think it through.Was obvious once I pressed the POST button.OOPS

*

Offline PmbPhy

  • Neilep Level Member
  • ******
  • 2794
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #16 on: 20/06/2015 21:18:14 »
Gazza, PmbPhy told you what the distinction was between gravity and other types of forces. This is a very important point and you should take it on board. Unless you grasp the concept that gravity acts on every part of a body all at once you will be stuck in your incorrect way of viewing things. Forum members such as Alan, PmbPhy, Evan and Chiral, that are bothering to reply to you, know what they are talking about. By all means ask questions but make no assumptions that anything you believe is right simply because it has popped into your head.
Really-are you a scientist?
Yes. Among other things, I consider Jeff to be a scientist. Do you actually know what a scientist is? Please look it up.

*

Offline gazza711

  • Sr. Member
  • ****
  • 144
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #17 on: 20/06/2015 22:11:12 »
Gazza, PmbPhy told you what the distinction was between gravity and other types of forces. This is a very important point and you should take it on board. Unless you grasp the concept that gravity acts on every part of a body all at once you will be stuck in your incorrect way of viewing things. Forum members such as Alan, PmbPhy, Evan and Chiral, that are bothering to reply to you, know what they are talking about. By all means ask questions but make no assumptions that anything you believe is right simply because it has popped into your head.
Really-are you a scientist?
Yes. Among other things, I consider Jeff to be a scientist. Do you actually know what a scientist is? Please look it up.
Just asked. I have never made an assumption. I just don't ask in a scientific way. I simply ask-why do you believe all tings attract. Theres more posts on the internet disproving newton and saying GR is far fetched as well. Actually newton disproved many times. they said the world was flat not long before that....So if your a scientist-prove attraction not repulsion(my argument)using more than silly examples like Cavendish and schiehallion.how do we find the mass of planets without formulas,equations etc. astronomy is based on assumptions everyday. The fact is that im stating repulsion and the "CASIMIR EFFECT" looks plausible maybe.

*

Offline alancalverd

  • Global Moderator
  • Neilep Level Member
  • *****
  • 4816
  • life is too short to drink instant coffee
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #18 on: 22/06/2015 23:25:31 »

and the others?Arent whales mammals?

To the best of my knowledge, all whales and all correpondents to this forum, know that water and air do not "have the same characteristics".
helping to stem the tide of ignorance

*

Offline PmbPhy

  • Neilep Level Member
  • ******
  • 2794
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #19 on: 23/06/2015 04:51:50 »
Quote from: gazza711
Theres more posts on the internet disproving newton and saying GR is far fetched as well. Actually newton disproved many times.
And not one of them by anybody who knows what they're talking about. Only crackpots make those claims. Newton's laws and Einstein's theories have withstood the test of time and their validity have been borne out by experiment countless times. So what you read was absolute total garbage. What else is new?

*

Offline rmolnav

  • Sr. Member
  • ****
  • 113
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #20 on: 24/06/2015 10:42:45 »
Gazza711, #9
You ask:
"So a balloon submerged deep in water.is it being pulled or pushed to the surface?"
Any submerged object experiences water pressure all around its surface. That pressure is due and equal to the weight of a column of water with a surface unit section, and going from water surface to the considered point of the object surface (water pressure PUSHES object surface ...)
Integration of all forces due to that pressure (pressures multiplied by surfaces) gives an upward push. Arquimedes discovered that: net total push is equal to the weight of the liquid previously filling the space where the object is situated.
If that upward push is bigger than object own weight, the push not compensated by object weight will make it move up.
If object weight is bigger (it would mean its average density is higher than water´s), then the object would further sink (not compensated weight would PULL it down: gravity is attraction between massive objects, kind of "tele-pull").   

 

*

Offline PmbPhy

  • Neilep Level Member
  • ******
  • 2794
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #21 on: 24/06/2015 11:27:09 »
Quote from: gazza711
is it being pulled or pushed to the surface?
I can't believe that you asked me this question in reply #9 since I just answered it in reply #8, i.e. the part that you quoted. So why on Earth id you ignore it?

Do you even know what those terms, i.e. push and pull mean?
« Last Edit: 24/06/2015 11:28:40 by PmbPhy »

*

Offline gazza711

  • Sr. Member
  • ****
  • 144
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #22 on: 25/06/2015 14:10:44 »
Quote from: gazza711
is it being pulled or pushed to the surface?
I can't believe that you asked me this question in reply #9 since I just answered it in reply #8, i.e. the part that you quoted. So why on Earth id you ignore it?

Do you even know what those terms, i.e. push and pull mean?
Ok.are attracted or repelled then as the other words are incorrect.dont answer without self evidence.

*

Offline gazza711

  • Sr. Member
  • ****
  • 144
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #23 on: 25/06/2015 14:19:58 »

and the others?Arent whales mammals?

To the best of my knowledge, all whales and all correpondents to this forum, know that water and air do not "have the same characteristics".
Ok.so air and water have similar characteristics.theyre breathable,use convection to help create movement,you could swim in air(drag),water is part oxygen.any suggestions on more similarities?

*

Offline chiralSPO

  • Global Moderator
  • Neilep Level Member
  • *****
  • 1917
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #24 on: 25/06/2015 14:33:24 »

Ok.so air and water have similar characteristics.theyre breathable,use convection to help create movement,you could swim in air(drag),water is part oxygen.any suggestions on more similarities?

Every fluid has drag and can convect. Oxygen is the most common element on Earth (by mass).
You're really grasping at straws here. This is not at all scientific.

Water and air are also both transparent in the visible region (but so is glass, diamond, polycarbonate plastic, hydrofluoric acid, etc. etc. etc.), but that has nothing to do with gravity... Water and air both have mass (so does pretty much all matter), and that does have to do with gravity, but isn't very helpful. Water and air are both fairly nontoxic (you can drink enough water to kill you, enough nitrogen will result in narcosis, coma or death, too much oxygen is neurotoxic and carcinogenic...)

*

Offline gazza711

  • Sr. Member
  • ****
  • 144
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #25 on: 25/06/2015 14:55:48 »

Ok.so air and water have similar characteristics.theyre breathable,use convection to help create movement,you could swim in air(drag),water is part oxygen.any suggestions on more similarities?

Every fluid has drag and can convect. Oxygen is the most common element on Earth (by mass).
You're really grasping at straws here. This is not at all scientific.

Water and air are also both transparent in the visible region (but so is glass, diamond, polycarbonate plastic, hydrofluoric acid, etc. etc. etc.), but that has nothing to do with gravity... Water and air both have mass (so does pretty much all matter), and that does have to do with gravity, but isn't very helpful. Water and air are both fairly nontoxic (you can drink enough water to kill you, enough nitrogen will result in narcosis, coma or death, too much oxygen is neurotoxic and carcinogenic...)
Well said.just that it was said they don't have same characteristics.nuthin to do with gravity at all.i know.but surely 7lbs/sq inch of downforce with an atmisphere that is 100k feet high that has mass must contribute to something.and all that in a perfect vacuum of ether or something like that.

*

Offline chiralSPO

  • Global Moderator
  • Neilep Level Member
  • *****
  • 1917
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #26 on: 25/06/2015 15:25:54 »
surely 7lbs/sq inch of downforce with an atmisphere that is 100k feet high that has mass must contribute to something.

But it can also be a 7 lbs per square inch of an up force (or a sideways force). Suction cups (https://en.wikipedia.org/wiki/Suction_cup) can stick to an appropriate surface in any direction as long as there is pressure on them from the outside.

*

Offline PmbPhy

  • Neilep Level Member
  • ******
  • 2794
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #27 on: 25/06/2015 17:34:14 »
Quote from: gazza711
Ok.are attracted or repelled then as the other words are incorrect.dont answer without self evidence.
So, you think that the difference between push and pull is the direction of the force. Suppose that we use the electric force as an example so as to illustrate your point about being pushed and pulled.

Let there be two large sheets of charge parallel to the xz-plane. One has a positive charge density +[tex]\sigma[/tex] that passes through the y-axis at y = +d, the other having a negative charge density -[tex]\sigma[/tex] that passes through the y-axis at y = -d. There is an electric field in the region -d < y < -d. Place a positively charged particle in the region -d < y < +d. In between the plates there is a uniform electric field directed in the +y direction. Outside the region -d < y < +d the electric field is zero.

Now place a positively charged particle at y = 0. In this instance is the particle pushed or pulled?

Now take away the sheet of positive charge. There is still a uniform electric field in the region -d < y < +d. The only difference being the magnitude of the particle. Is the positive particle at y = 0 being pushed or pulled?

Now put the sheet of positive charge back at y = +d and take away the sheet of negative charge. There is still a uniform electric field in the region -d < y < +d. Again, the only difference being the magnitude of the particle. Is the positive particle at y = 0 being pushed or pulled?
« Last Edit: 26/06/2015 01:48:57 by PmbPhy »

*

Offline gazza711

  • Sr. Member
  • ****
  • 144
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #28 on: 25/06/2015 20:07:14 »
Quote from: gazza711
Ok.are attracted or repelled then as the other words are incorrect.dont answer without self evidence.
So, you think that the difference between push and pull is the direction of the force. Suppose that we use the electric force as an example so as to illustrate your point about being pushed and pulled.

Let there be two large sheets of charge parallel to the xz-plane. One has a positive charge density [tex]+\sigma[/tex] that passes through the y-axis at y = +d, the other having a negative charge density [tex]-\sigma[/tex] that passes through the y-axis at y = -d. There is an electric field in the region -d < y < -d. Place a positively charged particle in the region -d < y < +d. In between the plates there is a uniform electric field directed in the +y direction. Outside the region -d < y < +d the electric field is zero.

Now place a positively charged particle at y = 0. In this instance is the particle pushed or pulled?

Now take away the sheet of positive charge. There is still a uniform electric field in the region -d < y < +d. The only difference being the magnitude of the particle. Is the positive particle at y = 0 being pushed or pulled?


Now put the sheet of positive charge back at y = +d and take away the sheet of negative charge. There is still a uniform electric field in the region -d < y < +d. Again, the only difference being the magnitude of the particle. Is the positive particle at y = 0 being pushed or pulled?
I'm done with the push/pull convo.

*

Offline gazza711

  • Sr. Member
  • ****
  • 144
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #29 on: 25/06/2015 20:19:22 »
surely 7lbs/sq inch of downforce with an atmisphere that is 100k feet high that has mass must contribute to something.

But it can also be a 7 lbs per square inch of an up force (or a sideways force). Suction cups (https://en.wikipedia.org/wiki/Suction_cup) can stick to an appropriate surface in any direction as long as there is pressure on them from the outside.
Well a massive suction cup would have more force upon it.air moves at about 500 miles/hour.when u pour a drink,air is displacing it.suction is using air pressure.there is a force on everything.when you spin a 1kg metal disc for example at sat 2k revs a minute,you could throw it in the air and it might not come down providing it's still spinning at that speed.this would mean that no force is influencing it except the spinning.now,if I was correct,gravity would cease to exist as an attractive force?i could be wrong but it's a standard science experiment of making something lighter.no experiment has been done the way I say.if a force isn't moving,we can only measure what is moving so 7lbs could be more like a force equal to the density of the object.its clear water is lighter than air if not bonded.

*

Offline chiralSPO

  • Global Moderator
  • Neilep Level Member
  • *****
  • 1917
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #30 on: 25/06/2015 21:11:19 »
spinning a disk does not make it lighter (actually, if you can spin it really, really fast, it would get slightly heavier--but that's beside the point).

spinning a disk can make it easier to lift, but that has to do with torque, not gravity. (https://www.youtube.com/watch?v=GeyDf4ooPdo)

*

Offline PmbPhy

  • Neilep Level Member
  • ******
  • 2794
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #31 on: 26/06/2015 01:51:10 »
Quote from: chiralSPO
(actually, if you can spin it really, really fast, it would get slightly heavier--but that's beside the point).
By this he's referring to a relativistic effect that I derive here: http://home.comcast.net/~peter.m.brown/sr/rotating_cylinder.htm

*

Offline rmolnav

  • Sr. Member
  • ****
  • 113
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #32 on: 26/06/2015 06:21:51 »
English is not my mother tongue, so I could be wrong in relation to most precise definition of PULL. But the way I put it in #20 regarding gravity attraction is very widely used in scientific literature:
https://www.google.es/?gfe_rd=cr&ei=pJmGVfP0JMyA8Qf0rICICg&gws_rd=ssl#q=sun´s+pull

PD: Sorry I said Archimedes in Spanish, with "qu" instead of "ch" ...

 

*

Offline PmbPhy

  • Neilep Level Member
  • ******
  • 2794
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #33 on: 26/06/2015 06:55:38 »
English is not my mother tongue, so I could be wrong in relation to most precise definition of PULL. But the way I put it in #20 regarding gravity attraction is very widely used in scientific literature:
https://www.google.es/?gfe_rd=cr&ei=pJmGVfP0JMyA8Qf0rICICg&gws_rd=ssl#q=sun´s+pull

PD: Sorry I said Archimedes in Spanish, with "qu" instead of "ch" ...
It's best to forget all this nonsense about "push" and "pull" because those terms are rarely, if ever, used in physics. In fact in all the physics texts that I've read in the last 35 years I've never once seen it used. The only related term with any meaning is force.

*

Offline gazza711

  • Sr. Member
  • ****
  • 144
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #34 on: 26/06/2015 11:01:43 »
spinning a disk does not make it lighter (actually, if you can spin it really, really fast, it would get slightly heavier--but that's beside the point).

spinning a disk can make it easier to lift, but that has to do with torque, not gravity. (https://www.youtube.com/watch?v=GeyDf4ooPdo)
Well the effect is wherever you influence the direction.the point is weightlessness in a sense.the weight changes.why?if air moves incredibly fast,preventing its molecules from applying a force on an object because the object is moving faster and thus breaking its grip on the object.i bet the brown experiment could cause anti gravity or indeed does!
Lexus reckons it's got ah overboard on concrete apparantly.lol.wont use push and pull words again,that's for sure!

*

Offline chiralSPO

  • Global Moderator
  • Neilep Level Member
  • *****
  • 1917
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #35 on: 26/06/2015 14:51:54 »
Well the effect is wherever you influence the direction.the point is weightlessness in a sense.the weight changes.

No, this experiment has nothing to do with weightlessness. It is not so difficult to lift the 19 kg flywheel if you are holding on to the wheel itself. Putting the wheel at the end of a stick and trying to lift from the other end introduces a lot of torque, and that is why it is difficult to lift. Again, lifting from the end of the stick close to the flywheel is not so hard, but lifting from the far end is hard. This torque is eased when the flywheel is spinning, but the mass and weight remain (effectively) constant.

*

Offline gazza711

  • Sr. Member
  • ****
  • 144
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #36 on: 26/06/2015 15:01:59 »
Well the effect is wherever you influence the direction.the point is weightlessness in a sense.the weight changes.

No, this experiment has nothing to do with weightlessness. It is not so difficult to lift the 19 kg flywheel if you are holding on to the wheel itself. Putting the wheel at the end of a stick and trying to lift from the other end introduces a lot of torque, and that is why it is difficult to lift. Again, lifting from the end of the stick close to the flywheel is not so hard, but lifting from the far end is hard. This torque is eased when the flywheel is spinning, but the mass and weight remain (effectively) constant.
It could have been 50kg.i think u missed the point.

*

Offline chiralSPO

  • Global Moderator
  • Neilep Level Member
  • *****
  • 1917
    • View Profile
Re: How do gravity and buoyancy relate?
« Reply #37 on: 29/06/2015 13:33:56 »
No, you have missed the point. watch the video I posted again, it gives an excellent explanation of the phenomenon.