Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Harri on 29/05/2017 17:43:09
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Not a scientist, just eager to understand! When standing upright on the ground, am I being pulled down by gravity, or is the ground coming up to meet me?
Hope its okay to add - this question was prompted after I watched a Brian Cox program where a bowling ball and a feather are dropped inside an area that has had the air removed. A vacuum? The ball and feather reach the floor at the same time. I was under the impression that the point being made was the floor was rising to meet the objects as the objects were held relatively still.
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Not a scientist, just eager to understand! When standing upright on the ground, am I being pulled down by gravity, or is the ground coming up to meet me?
Hope its okay to add - this question was prompted after I watched a Brian Cox program where a bowling ball and a feather are dropped inside an area that has had the air removed. A vacuum? The ball and feather reach the floor at the same time. I was under the impression that the point being made was the floor was rising to meet the objects as the objects were held relatively still.
All that can be said is that there's a relative acceleration between you and the floor.
According to the theory of general relativity, when you're in free-fall you are at rest in a locally inertial gravitational field. This frame of reference is accelerating toward the center of the Earth.
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Can we measure the gravitational field acceleration g when in free-falling reference frame?
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Can we measure the gravitational field acceleration g when in free-falling reference frame?
Yes. Merely measure the rate at which the ground is accelerating towards you. As I indicated above what is measureable is the relative acceleration of source and body.
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Can we measure the gravitational field acceleration g when in free-falling reference frame?
Yes. Merely measure the rate at which the ground is accelerating towards you. As I indicated above what is measureable is the relative acceleration of source and body.
Can we measure the acceleration without any signal from the outside?
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Can we measure the acceleration without any signal from the outside?
Absolutely not.
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Can we measure the acceleration without any signal from the outside?
Absolutely not.
The extended thought experiment places a super rifle, capable to shoot its projectile at 7.67km/s, inside the International Space Station firmly horizontally attached to it with the y axis going through the center of the Earth.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Ftheelectromagneticnatureofthings.com%2Fimg%2Fi8%2Ftns_if36.png&hash=ee67264b5b12be107afda1f85c314ea2)
The ISS moves at 7.67km/s in the blue reference frame that is linked to the gravitational field background. The red reference frame is linked to the ISS. The projectile is being shot in the opposite direction of the ISS motion velocity vector. The projectile reaches constant velocity at the end of the barrel that is 7.67km/s in the red ISS reference frame and it is 0km/s in the blue gravitational field background reference frame. At this moment the accelerometer inside the projectile with a wireless connection and a capability to measure in femto seconds is motion less in the blue gravitational field reference frame and it is still inside of the barrel having the support from beneath therefore it is not in a free fall and the accelerometer measures the gravitational acceleration 8.682m/s2.
Is there an error in the thought experiment?
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Is there an error in the thought experiment?
Its not clear what you hope to achieve with this experiment. You say that the barrel is supported from beneath. This support constitutes information from outside the locally inertial frame of reference of the ISS. In fact the rifle is actually not in a locally inertial frame but is at rest in the gravitational field.
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Is there an error in the thought experiment?
Its not clear what you hope to achieve with this experiment. You say that the barrel is supported from beneath. This support constitutes information from outside the locally inertial frame of reference of the ISS. In fact the rifle is actually not in a locally inertial frame but is at rest in the gravitational field.
No, the rifle is firmly attached to the ISS. It is falling with ISS. The rifle and ISS have 7.67km/s velocity in the blue gravitational field reference frame.
The projectile moves at 7.67km/s in the red ISS free-falling reference frame but it has 0km/s velocity in the blue gravitational field reference frame.
Projectile and the accelerometer have no signal from the outside.
Just think about it for a minute or two.
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Why are we thinking gravity pulling us down ?
It should be we are pulling each other together .
By true we human do have gravity too, we also pulling the earth to US .... maybe this is the nature of mass that love to clumps together when they meet each other
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No, the rifle is firmly attached to the ISS. It is falling with ISS. The rifle and ISS have 7.67km/s velocity in the blue gravitational field reference frame.
The projectile moves at 7.67km/s in the red ISS free-falling reference frame but it has 0km/s velocity in the blue gravitational field reference frame.
Projectile and the accelerometer have no signal from the outside.
Just think about it for a minute or two.
Don't worry about me thinking about it. I already did. What is confusing is your assertion that ... it is still inside of the barrel having the support from beneath therefore it is not in a free fall. You made it sound as if the barrel was not in free fall by this statement.
The only acceleration that could possibly be measured by the accelerometer in the projectile is the acceleration due to the slowing down of the projectile as it travels down the barrel due to friction. There is no way that it can determine the acceleration due to gravity because its in a locally inertial frame. It is quite literally impossible to tell that the ISS is in a gravitational field unless one attempts to measure the tidal gradients present in the ISS. However if one does that then one is no longer in a locally inertial frame of reference. And it certainly can't be done by what you described.
What gave you the impression that the projectile wasn't in free fall?
However that will not tell you the acceleration due to gravity as reckoned by observers at rest relative to the Earth's frame of reference.
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am I being pulled down by gravity, or is the ground coming up to meet me?
We can approximate this with Newton's version of physics: If you are falling towards the Earth, it is because there is a force F between you and the Earth.
The amount you accelerate towards the Earth (a) is determined by your mass (m) and the force (F): F=ma.
The amount the Earth accelerates towards you (A) is determined by the Earth's mass (M) and the force (F): F=MA.
Let's call your mass 70kg, so the force (F) is around 700 Newtons.
The Earth's mass is around 6 × 1024 kg, so the acceleration of the Earth A = F/M = 700/6 × 1024 = 1 x 10-22 m/s2, which is microscopic.
In other words, unless you have a significant mass (greater than the mass of a large mountain), you can ignore the acceleration of the Earth, and just pay attention to your own acceleration.
When standing upright on the ground, am I being pulled down by gravity, or is the ground coming up to meet me?
Pedantic answer: If you are standing on the ground, then you are not moving towards the Earth, nor is the ground coming up to meet you - you are already in contact with the ground.
Hopefully, the first answer might be more useful?
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No, the rifle is firmly attached to the ISS. It is falling with ISS. The rifle and ISS have 7.67km/s velocity in the blue gravitational field reference frame.
The projectile moves at 7.67km/s in the red ISS free-falling reference frame but it has 0km/s velocity in the blue gravitational field reference frame.
Projectile and the accelerometer have no signal from the outside.
Just think about it for a minute or two.
Don't worry about me thinking about it. I already did. What is confusing is your assertion that ... it is still inside of the barrel having the support from beneath therefore it is not in a free fall. You made it sound as if the barrel was not in free fall by this statement.
The free-fall is velocity dependent.
The barrel and ISS have the velocity 7.67km/s in the gravitational field reference frame and they fall to the right; +x axis direction.
The projectile with the accelerometer has 0km/s in the gravitational field and its free-fall is down; -y axis direction.
The only acceleration that could possibly be measured by the accelerometer in the projectile is the acceleration due to the slowing down of the projectile as it travels down the barrel due to friction. There is no way that it can determine the acceleration due to gravity because its in a locally inertial frame. It is quite literally impossible to tell that the ISS is in a gravitational field unless one attempts to measure the tidal gradients present in the ISS. However if one does that then one is no longer in a locally inertial frame of reference. And it certainly can't be done by what you described.
What gave you the impression that the projectile wasn't in free fall?
However that will not tell you the acceleration due to gravity as reckoned by observers at rest relative to the Earth's frame of reference.
The projectile free-fall is 90 degrees to the barrel free fall.
The barrel dy/dt = 0 though therefore the barrel supports the projectile, the barrel does not allow the projectile to fall down in the -y direction.
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The free-fall is velocity dependent.
Not exactly. All that is required for an object to be in free-fall is that no force other than gravity act on it. That has nothing to do with velocity. You're referring to the notion that if you're not in a locally inertial frame then you can tell that you're in a gravitational field. That's a very well understood fact in physics. In fact there's a device known as a gradiometer which allows one to make that determination. However your method does not allow you to determine the value of the gravitational acceleration from the ISS. Your method requires that you know the acceleration first. That's the only way to first determine the muzzle velocity required to make your experiment work.
The barrel and ISS have the velocity 7.67km/s in the gravitational field reference frame and they fall to the right; +x axis direction. etc.
Okay. I see what you had in mind. The effect is so small that its extremely difficult to measure. If you use very sensitive equipment than you're no longer in a locally inertial frame of reference and it no longer applies. When I explained above that you cannot measure the acceleration of gravity I explained that it applied only to a locally inertial frame. If you're able to measure the free-fall of the bullet then you're no longer in a locally inertial frame. Please note that a locally inertial frame refers to a limited region of spacetime which means its finite in spatial extent and limited in time.
The exact equivalence of gravity and acceleration only holds in a uniform gravitational field. In fact that's how Einstein originally stated it. This fact is so poorly understood that I wrote a paper on it a while back. See:
Einstein's gravitational field by Peter M. Brown
https://arxiv.org/pdf/physics/0204044
To better understand what I said about locally inertial frames please see Gravitation by Misner, Thorne and Wheeler. I can't find my copy so I can't tell you what page its on. Hopefully you can find it online.
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evan_au
If you are standing on the ground, then you are not moving towards the Earth, nor is the ground coming up to meet you - you are already in contact with the ground.
You are moving toward the ground with an acceleration of g. Insert a scale between the ground and your feet to measure it. The ground is preventing any visible motion and the kinetic energy is transferred to the ground as heat.
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You are moving toward the ground with an acceleration of g. Insert a scale between the ground and your feet to measure it. The ground is preventing any visible motion and the kinetic energy is transferred to the ground as heat.
It's impossible to make that determination as stated since you could very well have a scale accelerating you relative to an inertial frame of reference. This is exactly what the equivalence principle is all about in GR.
You can read about this in Einstein's 1916 review article on GR. See Section two in
hermes.ffn.ub.es/luisnavarro/nuevo_maletin/Einstein_GRelativity_1916.pdf
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The free-fall is velocity dependent.
Not exactly. All that is required for an object to be in free-fall is that no force other than gravity act on it. That has nothing to do with velocity.
Well, the free fall trajectory is velocity dependent. That's why ISS at 7.67km/s falls horizontally and the projectile with 0km/s will fall straight down in the gravitational reference frame.
You're referring to the notion that if you're not in a locally inertial frame then you can tell that you're in a gravitational field. That's a very well understood fact in physics. In fact there's a device known as a gradiometer which allows one to make that determination. However your method does not allow you to determine the value of the gravitational acceleration from the ISS. Your method requires that you know the acceleration first. That's the only way to first determine the muzzle velocity required to make your experiment work.
This is not true. We can have multiple rifles next to each other; 1. shooting at 7.97km/s; 2. shooting at 7.67km/s; 3. shooting at 7.37km/s; ... We can do a sweep and plot the results. We would get a maximum value, that would be the gravitational acceleration. We don not have to know the gravitational acceleration prior to our experiment.
We would shoot in every direction, different velocities, measuring accelerations, doing a sweep and we would be able to map out the gravitational acceleration without any signal from the outside.
The barrel and ISS have the velocity 7.67km/s in the gravitational field reference frame and they fall to the right; +x axis direction. etc.
Okay. I see what you had in mind. The effect is so small that its extremely difficult to measure. If you use very sensitive equipment than you're no longer in a locally inertial frame of reference and it no longer applies. When I explained above that you cannot measure the acceleration of gravity I explained that it applied only to a locally inertial frame. If you're able to measure the free-fall of the bullet then you're no longer in a locally inertial frame. Please note that a locally inertial frame refers to a limited region of spacetime which means its finite in spatial extent and limited in time.
The exact equivalence of gravity and acceleration only holds in a uniform gravitational field. In fact that's how Einstein originally stated it. This fact is so poorly understood that I wrote a paper on it a while back. See:
Einstein's gravitational field by Peter M. Brown
https://arxiv.org/pdf/physics/0204044
To better understand what I said about locally inertial frames please see Gravitation by Misner, Thorne and Wheeler. I can't find my copy so I can't tell you what page its on. Hopefully you can find it online.
But it appears you are missing the major points here.
1. The free-falling inertial reference frame no meter how limited in space and time is not good in predicting instantaneous velocity and acceleration. The inertial reference frame temporarily linked to the gravitational field background is a preferred reference frame. These two reference frames can be at the same point at the same time and one is correct and the other is not correct in prediction of the future motion when dt->0.
2. We can detect the gravitational acceleration without any signal from the outside. Something you said is not possible.
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You are moving toward the ground with an acceleration of g. Insert a scale between the ground and your feet to measure it. The ground is preventing any visible motion and the kinetic energy is transferred to the ground as heat.
It's impossible to make that determination as stated since you could very well have a scale accelerating you relative to an inertial frame of reference. This is exactly what the equivalence principle is all about in GR.
You can read about this in Einstein's 1916 review article on GR. See Section two in
hermes.ffn.ub.es/luisnavarro/nuevo_maletin/Einstein_GRelativity_1916.pdf
Don't assume everyone responding is unfamiliar with the subject. The ep applies to a small interval of space and time since most g-fields are not uniform, which would become evident after larger intervals. The observer in the box can make a hole to the outside world and gain additional information to decide his state. The observer has choices!
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The exact equivalence of gravity and acceleration only holds in a uniform gravitational field. In fact that's how Einstein originally stated it. This fact is so poorly understood that I wrote a paper on it a while back. See:
Einstein's gravitational field by Peter M. Brown
https://arxiv.org/pdf/physics/0204044
To better understand what I said about locally inertial frames please see Gravitation by Misner, Thorne and Wheeler. I can't find my copy so I can't tell you what page its on. Hopefully you can find it online.
Hi Peter,
Thank you for the links, going through the staff...
From the Gravitation book, page 18, 19:
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Ftheelectromagneticnatureofthings.com%2Fimg%2Fi8%2Ftns_if40.png&hash=bc302b952f49faa200c01ea007102c03)
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Ftheelectromagneticnatureofthings.com%2Fimg%2Fi8%2Ftns_if41.png&hash=4271bde5ceeddeaffd76884f9a43d4b6)
"What is direct and simple and meaningful, according to Einstein, is the geometry in every local inertial reference frame. There every particle moves in a straight line with uniform velocity."
What my example is trying to point out is that particles within the local inertial reference frame ISS can start to move at different velocities and therefore their falling trajectories will change because the falling trajectory is a function of the velocity in the gravitational reference frame. Then these particles start to interact and something 'unexpected' is going to happen.
For example if we imagine ideal conditions for the ISS (no drag, ...) and we keep shooting then ISS will start to rotate (the projectile pushing down on the barrel, the barrel attached to the ISS, ...), do you agree?
If we keep shooting and the ISS is in the intergalactic space, almost no gravitational field, g close to 0, then the ISS will not rotate, do you agree?
What happened to the local inertial reference frame? Different physics based on the presence of the gravitational field and the motion through that gravitational field? Wouldn't you agree?
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If you are standing on the ground, then you are not moving towards the Earth, nor is the ground coming up to meet you - you are already in contact with the ground.
You are moving toward the ground with an acceleration of g. Insert a scale between the ground and your feet to measure it. The ground is preventing any visible motion and the kinetic energy is transferred to the ground as heat.
- "Moving towards" Implies a relative velocity. If you are standing upright on the ground, the relative velocity is 0 m/s.
- "acceleration of g" implies a changing velocity. If you are standing upright on the ground, the relative velocity does not change, and the acceleration is 0 m/s2.
- "Insert a scale between the ground and your feet to measure (acceleration)": Bathroom scales do not measure acceleration, they measure force (and then convert that to kilograms or pounds, assuming you are standing on the Earth's surface).
- "the kinetic energy is transferred to the ground as heat": If you are standing upright on the ground, there is no kinetic energy (velocity relative to the ground is 0), so there is no heat to be dissipated.
"The ground is preventing any visible motion": I do agree with this statement.
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What my example is trying to point out is that particles within the local inertial reference frame ISS can start to move at different velocities and therefore their falling trajectories will change because the falling trajectory is a function of the velocity in the gravitational reference frame. Then these particles start to interact and something 'unexpected' is going to happen.
I know precisely what you're example is pointing out. At least when you explained that you were not using a locally inertial frame.
For example if we imagine ideal conditions for the ISS (no drag, ...) and we keep shooting then ISS will start to rotate (the projectile pushing down on the barrel, the barrel attached to the ISS, ...), do you agree?
What happened to the local inertial reference frame?
I referenced that text so you'd learn the definition of a locally inertial frame. Talk to me when you do that. Otherwise I'd just be repeating myself and I choose not to.
Until then please make an attempt to show that if you were in an inertial frame reference in free-fall in a uniform gravitational field that you could measure the strength of the field. Hint: It's impossible.
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What my example is trying to point out is that particles within the local inertial reference frame ISS can start to move at different velocities and therefore their falling trajectories will change because the falling trajectory is a function of the velocity in the gravitational reference frame. Then these particles start to interact and something 'unexpected' is going to happen.
I know precisely what you're example is pointing out. At least when you explained that you were not using a locally inertial frame.
For example if we imagine ideal conditions for the ISS (no drag, ...) and we keep shooting then ISS will start to rotate (the projectile pushing down on the barrel, the barrel attached to the ISS, ...), do you agree?
What happened to the local inertial reference frame?
I referenced that text so you'd learn the definition of a locally inertial frame. Talk to me when you do that. Otherwise I'd just be repeating myself and I choose not to.
Until then please make an attempt to show that if you were in an inertial frame reference in free-fall in a uniform gravitational field that you could measure the strength of the field. Hint: It's impossible.
We were talking about the ISS and not a uniform gravitational field. The ISS is not in a uniform gravitational field.
If I understand you correctly then when an observer is in ISS (an inertial reference frame in free-fall) and he shoots a photon in any direction with frequency ω0 then a receiver a couple of meters away (similar to Figure 1.7) will measure the same frequency ω0?
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If you are standing on the ground, then you are not moving towards the Earth, nor is the ground coming up to meet you - you are already in contact with the ground.
You are moving toward the ground with an acceleration of g. Insert a scale between the ground and your feet to measure it. The ground is preventing any visible motion and the kinetic energy is transferred to the ground as heat.
- "Moving towards" Implies a relative velocity. If you are standing upright on the ground, the relative velocity is 0 m/s.
- "acceleration of g" implies a changing velocity. If you are standing upright on the ground, the relative velocity does not change, and the acceleration is 0 m/s2.
- "Insert a scale between the ground and your feet to measure (acceleration)": Bathroom scales do not measure acceleration, they measure force (and then convert that to kilograms or pounds, assuming you are standing on the Earth's surface).
- "the kinetic energy is transferred to the ground as heat": If you are standing upright on the ground, there is no kinetic energy (velocity relative to the ground is 0), so there is no heat to be dissipated.
"The ground is preventing any visible motion": I do agree with this statement.
Technically and visually it is not moving relative to to the earth, but is moving through the g-field generated by the earth mass, and the g-field is regenerated as the earth moves. Gravity is not turned off because the object is at rest rel to earth. .
F=ma = mg = weight. Where does the F originate?
If you hold a weight off the ground for a long interval, why do our arms get tired?
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We were talking about the ISS and not a uniform gravitational field. The ISS is not in a uniform gravitational field.
The subject of this thread is not the ISS and you know everything you need to about the ISS and the Earth's gravitational field. I also explained that you need to learn what a locally inertial frame is and you have not demonstrated to me that you've learned that yet. Until then you will never correctly understand the equivalence principle and general relativity. Read MTW, The explain it all in there.
If I understand you correctly then when an observer is in ISS (an inertial reference frame in free-fall) and he shoots a photon in any direction with frequency ω0 then a receiver a couple of meters away (similar to Figure 1.7) will measure the same frequency ω0?
Wrong.
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Technically and visually it is not moving relative to to the earth, but is moving through the g-field generated by the earth mass, and the g-field is regenerated as the earth moves. Gravity is not turned off because the object is at rest rel to earth. .
F=ma = mg = weight. Where does the F originate?
If you hold a weight off the ground for a long interval, why do our arms get tired?
Within the context of general relativity it makes no sense to speak of moving relative to the g-field. The gravitational field is observer dependent. That means that in any point in spacetime the g-field can be transformed away be a change in spacetime coordinates. I posted a link to both my paper on the subject and to Einstein's paper. It appears that nobody is reading the references I post which means that you want to be spoon fed and while there are plenty of people here who enjoy doing that I'm not one of them. I post material for a very good reason. If people are going to ignore what I post then there's no good reason for me to keep posting in this thread.
It makes no sense to
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Here is chapter five from the new version of Exploring Black Holes which will be coming out soon.
http://www.eftaylor.com/exploringblackholes/Ch05GlobalLocalMetrics170328v2.pdf
Please read it. It will give you an understanding of what "local" means. If you don't understand the math then just say so and I'll walk you through it.
You can better understand what a locally inertial spacetime coordinate system is by using a sphere, such as an idealized Earth, as an example. People didn't always know that the Earth was a sphere. A very long time ago they thought the Earth was flat (some still do as a matter of fact). If you're small beings on a large sphere then the surface will appear flat to you. But if you either explore the surface or use sensitive measuring equipment then you can detect the curvature of the sphere.
What confuses almost every person attempting to understand GR is that they confuse a local coordinate system with being a completely geometric property and neglect the all important fact that it has to do with the equipment used to make measurements of the coordinate system and how its being measured. In GR its even more complicated because people almost always forget that there is a temporal component to the coordinate system. That means that they neglect to take into account how long the experiment must run in order to make the measurement.
All of this takes a great deal of explanation to get the idea across. Not only is it too much work to do in a thread but not a good idea. Why? Because almost all readers till not read it and instead ask questions which show that they either never read what was posted or don't understand it and chose not to ask about it. I guess that happens due to fear of looking either ignorant or stupid. In either case its unwarranted.
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If you hold a weight off the ground for a long interval, why do our arms get tired?
You can use a brick to hold a weight off the ground for a long interval, and it doesn't get tired or consume energy in the process.
However, animals are optimised to move fairly efficiently (which consumes energy). We are not so efficient at holding weights aloft for long periods; we use the same muscle mechanisms as we use for movement (so it also consumes energy).
Physics describes a brick holding up a weight, which consumes (or dissipates) no energy.
Biology describes a human holding up a weight, which does consume energy.
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If you hold a weight off the ground for a long interval, why do our arms get tired?
That's a question for a biologist. There are internal biological processes which require energy in order to maintain muscle contraction in a given state. This might help in that area: https://courses.kcumb.edu/physio/smoothmuscle/energetics.htm
But think of it this way. Suppose you tied a weight to your arm and let your arm go limp. Would you still get tired? Now think of it in terms of a bronze statue doing the exact same thing, i.e. holding up a weight. Do you have to feed energy to the statue in order for it to maintain the position of its arm holding up the weight? The answer is no.
Have you ever wondered how the Earth can keep revolving around the Sun for billion of years with no input of energy, i.e. nothing to push it to keep it moving in an ellipse?
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What my example is trying to point out is that particles within the local inertial reference frame ISS can start to move at different velocities and therefore their falling trajectories will change because the falling trajectory is a function of the velocity in the gravitational reference frame. Then these particles start to interact and something 'unexpected' is going to happen.
I know precisely what you're example is pointing out. At least when you explained that you were not using a locally inertial frame.
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What does it mean that I am not using a locally inertial frame?
The ISS is considered an inertial reference frame; free-falling, no force acting on the frame.
The projectile moves at a constant velocity in the ISS reference frame when it is at the end of the barrel but still inside and dt->0. Now we have the constant velocity and the locality is covered by dt->0.
Therefore the ISS observer comes to a conclusion that there is no force acting on the projectile hence the projectile is in a local inertial reference frame as well.
We can shoot the projectile at 0.01m/s and the accelerometer will report ≈0m/s2 in all measured directions. It confirms the previous conclusion that the projectile is in a local inertial reference frame.
Now we shoot the projectile at 7.67km/s and what happens?
In a conclusion the ISS as a local inertial reference frame has to be questioned. The LIRF is only an approximation that does not work for all the velocities.
... and there is no signal from the outside. The signal comes from the inside!
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What does it mean that I am not using a locally inertial frame?
You're kidding, right? In posts above I posted you all of the information required to learn what that is and you mean to tell me that you didn't read any of it? I said I won't spoon feed people here. I'll given them the information required to learn what they need to and that's it. If you are still unable to learn what a locally inertial frame is and how you're not using one then send me a PM
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If you hold a weight off the ground for a long interval, why do our arms get tired?
That's a question for a biologist. There are internal biological processes which require energy in order to maintain muscle contraction in a given state. This might help in that area: https://courses.kcumb.edu/physio/smoothmuscle/energetics.htm
But think of it this way. Suppose you tied a weight to your arm and let your arm go limp. Would you still get tired? Now think of it in terms of a bronze statue doing the exact same thing, i.e. holding up a weight. Do you have to feed energy to the statue in order for it to maintain the position of its arm holding up the weight? The answer is no.
Have you ever wondered how the Earth can keep revolving around the Sun for billion of years with no input of energy, i.e. nothing to push it to keep it moving in an ellipse?
Interesting consumption
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Not a scientist, just eager to understand! When standing upright on the ground, am I being pulled down by gravity, or is the ground coming up to meet me?
Hope its okay to add - this question was prompted after I watched a Brian Cox program where a bowling ball and a feather are dropped inside an area that has had the air removed. A vacuum? The ball and feather reach the floor at the same time. I was under the impression that the point being made was the floor was rising to meet the objects as the objects were held relatively still.
Well the ground is not moving and your body wants to free fall. Einstein noticed that staying in a fixed position in constant gravity (say where acceleration is exactly 9.81 m/s^2 and doesn't increase or decrease) is exactly the same physically as accelerating at 9.81 m/s^2 in open space far from a gravity well. This is the Equivalence Principle. That matter-energy causes this effect is written right into General Relativity.
In open space if you had a laser at the front of the spacecraft pointing aft and another aft pointing forward as you accelerate forward the rear facing laser blueshifts and the forward facing laser redshifts. This is the well-known Doppler Shift Effect that police use to measure the speed of cars. If the lasers were equal strength then the aft section will get hit by BLUE shifted HIGHER energy photons and the front gets hit by RED shifted LOWER energy photons. It takes more energy to absorb the rearward moving photons than the forward moving photons leading to an opposite reaction to the acceleration. The photons have momentum also which is E/c = momentum. There is an imbalance of momentum and energy.
The exact same red and blue shift occurs in a gravity well. Light blue shifts heading in and redshifts heading out. In fact if you were to encode a message the higher frequency blue shifted light would contain more cycles per second and you'd send the message FASTER relative to the other location. This shift forces one to conclude that clocks are running at different rates. You also get the same imbalance if you'd setup two lasers on the surface of Earth (one on the ground and one on the top of the tower). The laser slightly further from the gravity well would hit the bottom of the tower harder (higher energy and momentum) than an equal laser pointing upwards would hit the top.
It's this difference in time (and space) that causes the acceleration effect of gravity. If allowed to free fall an object will always move such that the red shift and blue shift equalize (at least when looking local enough). When equalized there is no sensation of acceleration (the effect occurs for more than just light). There is also a shift in wavelength that indicates space also changes.
What's also shown by Einstein's General Relativity is that curvature is locally held and if the Sun suddenly disappeared we wouldn't notice for 8 minutes (the time it takes for light to get to us). Gravity also moves at light speed. The conclusion from this is gravity is a locally held property. The Sun's mass affects the space (and time) directly around it and this affected area in turn affects the area slightly further out. Change in gravity propagates at the speed of light.
The gravity "field" is formed because of the presence of mass-energy (in this case the Earth). However, the mass doesn't "pull you down." The local "curvature of spacetime" (as physicists describe it) both pushes you and pulls you down when you're not free falling. This acceleration effect of gravity (when trying to remain stationary) is identical to the reactive force that occurred for the lasers when accelerating in open space.
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Harri (OP):
"When standing upright on the ground, am I being pulled down by gravity, or is the ground coming up to meet me?"
I see that as follows (within Newton´s Mechanics):
What we experience is not just gravity, but internal compression forces, especially at our feet and legs (if we were hanging from the ceiling by our hands and arms, tensile stresses instead ...)
What causes that?
Each part of our body is pulled down by Earth (gravity). If no other forces acting on them, they would accelerate downwards with "g" acc. (Newton´s 2nd Law)
Solid ground makes that imposible, and by phenomena at microscopic level, it "answers" with an upward push, equal but opposite to Earth´s attraction, to have a null total force acting on us, because acceleration is null (Newton´s 1st and 2nd Laws)
Please kindly note I don´t say that is just the "reaction" to Earth´s downward pull. The Newton´s 3rd Law "couple" of that pull (our body --> Earth) is an equal but opposite pull exerted by us on Earth (Earth --> our body).
Logically that internal compression (when standing) is the lower the area, the bigger. That´s the reason of the complexity and strength of our legs and feet, acquired after a very long evolution.
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Does gravity pull or push you ?
About the gun, the gun would accelerate the station enough for the bullit to leave the barrel. And furthermore the the gun is straight barreled, yet the orbit curved,so the bullet is gaining altitude as it passes down the barrel, probably through the lower end casing of it due to the bullets intense velocity, causing the iss to react by flying off into space. The bullet howeve would continue on it upward tradgectory for a short time and then decend doward the earth on a decaying orbit, hits george clooney and knocks out your sat nav.
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What we experience is not just gravity, but internal compression forces, especially at our feet and legs (if we were hanging from the ceiling by our hands and arms, tensile stresses instead ...)
What causes that?
Each part of our body is pulled down by Earth (gravity). If no other forces acting on them, they would accelerate downwards with "g" acc. (Newton´s 2nd Law)
Solid ground makes that imposible, and by phenomena at microscopic level, it "answers" with an upward push, equal but opposite to Earth´s attraction, to have a null total force acting on us, because acceleration is null (Newton´s 1st and 2nd Laws)
A fanny (curious) thing in relation with that.
We spend most of our time standing, or seating ... Our brain, attracted by Earth and "falling" (together with our skull) with the required centripetal acceleration, has to be pushed up by lower part of the skull. Each area, the lower placed, the higher compressed. Brain upper part is kind of hanging from skull upper part ...
Our body is used to those physical facts.
But what about astronauts after long time in ISS, where that doesn´t happen ? As in the well known problem at bones, I think also lack of gravity and that subsequent change in brain internal stresses is what causes:
"The brain scans revealed that most astronauts who participated in long-duration missions had several key changes to their brain's structure after returning from space: Their brains shifted upward in their skulls, and there was a narrowing of the cerebrospinal fluid (CSF) spaces at the top of the brain".
https://www.livescience.com/60840-space-travel-brain.html?utm_source=ls-newsletter&utm_medium=email&utm_campaign=20171102-ls
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As in the well known problem at bones, I think also lack of gravity and that subsequent change in brain internal stresses is what causes ...
Some people consider quoting oneself kind of not "politically correct" ... For me it´s just a way of giving additional details relative to what already said ...
"Lack of gravity" may be considered an error by some people. They could comment something about inertial reference systems, but I´m not fond of that "cool" expression: not always it is used in a clearly correct way, and understood by everybody.
What happens to the astronaut in ISS is that the hole Earth´s gravity pull exerted on him (on each part of his body, actually) is used to give him the necessary centripetal acceleration to make him rotate (he could also be outside ISS ...). But when on Earth, head up, as the necessary centripetal acceleration is smaller and Earth´s pull bigger, the ground and/or seat have to exert on him an upward push, to avoid any additional movement of his body (Newton´s 2nd Law)
Any imaginary horizontal slice of our body suffers Earth´s pull and internal stresses from contiguous slices. And total forces on each slice, divided by each slice mass, have to be equal to required centripetal acceleration. In other several threads (on centrifugal forces, tides, gravity ...) I´ve already analyzed that in detail.
Now I´ll only say that, in our case, with additional complexities due to the quite anisotropic nature of our body, those slices are mainly compressed, the closer to ground/seat, the more. Inside of our skull, as this is more "solid" than the brain, lower brain parts must be a little compressed, but top of the brain must be even in tension, being somewhat stretched. That happens some 2/3 of day hours, not when at bed.
The brain is physically adapted to those facts, but "suddenly" its environment changes (orbiting with the ISS) and most of those internal stresses disappear (no "spare" gravity to counter, I could have said).
Said that, what happens after some six months:
"Their brains shifted upward in their skulls, and there was a narrowing of the cerebrospinal fluid (CSF) spaces at the top of the brain"
as far as I can see, is quite logical !!
Those internal stresses are paramount, not only in this case, but in MOST OF THE ISSUES I´ve recently been talking about (and last year !)
We should not forget objects´s C.G. are only kind of mathematical tools, that forces (gravity, inertial, contact interactions between objects) are not actually exerted on those C.G., but on each particle of the objects (or in part of them). To forget that when applying basic laws (such us Newton´s) may be utterly misleading.
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"That's why ISS at 7.67km/s falls horizontally and the projectile with 0km/s will fall straight down in the gravitational reference frame." Hmm :)
The point is that there is no "free fall trajectory.. that .. is velocity dependent"
A free fall is following a geodesic in where you ideally have no way to define a 'gravity' acting upon you. If you find a way to define that in a black box scenario you've just upturned relativity. A black box is you enclosed in it, using whatever instruments you have to define whether there is a 'external' gravity acting upon you.
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To see it you need to introduce a new frame of reference (Earth f.ex) from where you now can define that bullet to 'gravitationally accelerate', well, sort of. Without that 'frame' you don't find it. That's a 'black box scenario'
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In several threads, about object dynamics: gravity, tides, centrifugal forces, Newton´s Laws …, I´ve long been showing my stand.
I got some replies, but almost none agreeing with me clearly … I don´t actually know the stand of most readers about “my" ideas ...
There is a detail it would be very interesting for me to receive more comments about, before going any further with other consequences of the issue I have in mind and I´m planing to send. In order to have a cleaner ground to play on …
It´s the question of how we (and somehow objects) feel gravity.
If we (or an object) are pulled by a massive object, and nothing else prevent us to accelerate what required by Newton´s 2nd Law, internal stresses are almost negligible: as if with no gravity, either still or with constant velocity vector.
But if a 3rd object don´t let us move with required 2nd Law acceleration, exerting on us another force, Newton´s 3rd Law “turns up” and we feel internal stresses. THOSE stresses are what actually “tell” us we are in a gravity field (whatever the deep explanation of gravity), NOT GRAVITY itself.
If we are standing, mainly feet and legs compression. If sitting, our bottom compression (especially if a hard seat !!). If in bed, smaller compression along our body. If hanging from our hands and arms, we feel strong tensile stresses there …
And if only part of the theoretically required “g” is allowed to happen, above mentioned stresses will be smaller, proportionally to “not allowed” acceleration … I mean, if e.g. a skyscraper lift fell with g/3 acceleration, and we were standing on its floor, at our feet and legs (and rest of our body) we would feel 2/3 of the compression stresses we were feeling when no movement (or with constant speed movement)
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Another newbie question....
Does anybody know what gravity actually is?
As example, in my classes at Netfilx University :) I see that the moon is racing around the dent in the fabric of space which is being created by the mass of the Earth, or so I vaguely maybe understand it.
What force is causing the Earth to create this dent in the fabric of space?
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Does anybody know what gravity actually is?
In my last post, as usually, I didn´t consider that question ("whatever the deep explanation of gravity").
I think we should be able to clearly understand WHAT actually happens, before trying to understand WHY it happens, in the sense of the deep "nature" of gravity.
By the way, what I said about our way of feeling gravity, may also be said for objects ... They don´t feel as we do, but different internal stresses occur across them, and subsequently they get deformed differently, depending on same factors: forces which prevent them (as a hole or affecting its different parts) to get the required gravity acceleration ... That´s what they "feel", not gravity itself. Their answer to ONLY gravity would be, as ours, just to accelerate with local "g" ...
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Does anybody know what gravity actually is?
I think not, I certainly don't, but here's a thought that might be worth looking at, on its way to the recycle bin.
A body of the mass and density of the sun, for example, will cause relatively gentle curvature of spacetime, over a large area. If this mass were compressed to the size of the Earth, the curvature of spacetime around it would be much more severe.
In terms of the rubber sheet model, the depression in the sheet becomes deeper, and steeper sided, either as a result of an increase of the mass within it, or as a result of the compression of that mass.
Given a situation in which an enormous mass, such as the total mass of the Universe, is compressed into an unthinkably small “speck”, with a diameter no greater than the Planck distance, we might just be forgiven for referring to the resulting curvature of spacetime as “infinite”. In fact, scientists often do just that. This, we are told, approximates to the state of the Universe at the instant of the Big Bang.
If this is the case, it follows that every particle of matter and all the energy in the Universe, at the start of its life – or of this cycle of its life – occupied the same point in spacetime. The energy, whatever its source, that caused this infinitesimal, primordial speck to expand, transforming itself into billions of light years of spacetime, matter and energy would also have caused the curvature of spacetime to expand, and to “soften”, but, it would always remain curved, thus it would always tend to return to its original condition, like a rock, that has been picked up, falling back to Earth once the restraining force has been removed.
This would mean that the energy which drives gravitational attraction is the potential energy imparted to every particle in the Universe by the Big Bang.
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Does anybody know what gravity actually is?
As example, in my classes at Netfilx University I see that the moon is racing around the dent in the fabric of space which is being created by the mass of the Earth, or so I vaguely maybe understand it.
What force is causing the Earth to create this dent in the fabric of space?
Perhaps it would interest you to have a look at:
https://www.thenakedscientists.com/forum/index.php?topic=71799.msg527540#msg527540
where we are discussing, not the actual, deep nature of gravity, but the steps which supposedly lead Einstein to look for something "out of the box" …
A statement that doesn´t make things any easier to understand, as what you quoted, could be:
"… the reason things are going to drop when I trow them, is because there´s a force attracting us down to the center of the earth … relativity tells you that´s not the right way to think … What´s really going on is that YOUR NATURAL PATH (??) in space-time would take you to the center of the earth"