I think i am a bit clear now! thank you!
CONCEPT OF RELATIVE MOTION- How Can We Say That Planets revolve around Sun?
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I think i am a bit clear now! thank you!
I think i understand now.
Yes. i was exploring something beyond that, in the hope that i can understand more given in the textbook, if i explore something more.
No. I am not going for it!
OK. The part i think i was wrong is that i was treating acceleration more of as a kinematic problem. I should have thought acceleration in terms of dynamics.
When i think in terms of that it is very clear that acceleration is absolute- every frame would agree that who "causes" the acceleration(change of velocity) to the objects and that's what makes the difference. right?
When i think in terms of that it is very clear that acceleration is absolute- every frame would agree that who "causes" the acceleration(change of velocity) to the objects and that's what makes the difference. right?
In "Relative Motion": velocities of two objects at 'the same instant of time' are subtracted.
In "Acceleration": velocities of the same object at 'two different instants of time' are subtracted.
Yes, if I understand your intent above correctly, this is a good description of acceleration and the relationship between force and acceleration, when dealing with everyday macroscopic objects.
Going further into the issue involves clarifying descriptions or definitions of both mass and inertia. As I mentioned earlier both are generally considered intrinsic characteristics of particles and matter... Though there has been some significant work attempting to describe both as emergent phenomena. Discussions in that direction move into the realm of quantum mechanics and I am not sure that would be productive, at this time.
Since the equivalence principle has been mentioned a few times, without getting into the origins of mass and inertia, keep in mind that it is the inertial resistance of mass to changes in an object's state of motion, that resists the force resulting in acceleration and makes it measureable... And again where gravity is the source of an object's acceleration, it will not be experienced or measureable in the same manner as, within the context of special relativity and everyday experience. — As brucep mentioned earlier the equivalence principle says that inertial mass and gravitational mass are equivalent. The historical problem has been and is, that we have no clear understanding and/or explanation why, just that they are... And that they are involved, both in the equivalence principle and understanding acceleration, at some fundamental level.
Yes. I know that.
The acceleration is NOT zero, just too small to feel as it takes a year to make trip around; but if you go around some central point in a minute, like on a carousel, you would feel the acceleration; and a lot stronger one even with pain and a very distorted face if being spun around on one of the Air Force's centrifuges, at least until you black out - lose consciousness.*
Coming back to the sun case, I believe there are some scales so precise that a weight placed on them is lighter at noon than at midnight. Reason being, of course, that at midnight sun's acceleration of the weight is in same direction as earth's acceleration (of gravity) is, not opposite and cancelling out part of Earth's acceleration of the weight as it is at noon. Mass is a constant, measured in any frame it is at rest, but weight is not, not even in inertial frames.
* If placed on that AF's centrifuge horizontally, with head nearest the central point for say six minutes, I think you would be dead with no blood arriving at the brain but heart would still be beating,** so when centrifuge stops, there would again be blood circulating in the dead brain. - Making an interesting problem for the pathologist, who knows nothing of your body's recent history, trying to tell what was the cause of the deep coma with brain death.
** The heart rate is normally mainly controlled by the brain, but there are some slower back up modes that don't need any brain activity. I once caught and soon cleaned a large fish and noted the heart was still beating in my hand, so quickly made a salt solution that tasted about like sea water and put the beating heart in it. Hours later, just before going to bed, the heart was still slowly beating, but next moring it was still. I should have put some corn syrup in the glass too - heart probably ran out of glucose.
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I hope I'm not being pedantic here but acceleration by gravity is not technically acceleration in the sense that you can feel it. It's a "pseudo-force". If the Earth was a point particle and put on a carousel it would "feel" the centripetal forces but that is different from orbits caused by gravity. I'd imagine you are right about the hyper-sensitive scales but then you're measuring the actual forces caused by the mass pressing against the scale. If you removed the Earth and left the objects and scales they would measure equivalent weights at differing distances from the sun.
Billy while some of the above is true and accurate, it describes things in a manner that is misleading and thus leads to inaccurate conclusions.
Take the earth in orbit around the sun. If the earth were following the same orbital path in the absence of the sun and its gravitational influence, the inertial resistence to the circular path would be measurable. With the sun in place the reason the centrifugal force* of the Earth's path is not felt or measured, is not the result of being so small, it is because it is exactly canceled or balanced by the acceleration resulting from the gravitational influence of the sun. The balancing of the two, the earth as a whole is in a state of free fall around the sun, the sun's gravity resulting in an acceleration toward the sun that is exactly proportional to the exceleration away from the sun resulting from the Earth's curved path.
My point here is that the reason that objects in orbits experience weightlessness or free fall is not because the gravitational force is too small to measure or feel, it is because the gravitational potential present is balanced by an opposing inertial force, due to the accelleration of the curved path of the orbit.
On the second issue, we don't feel the differences in the affects of the sun's and Moon's gravitational influence, because they are too small compared to the Earth's gravitational field where we stand, for us to notice the changes. However, as you mentioned the gravitational variations are detectable or measureable.., and even observable in the tides. The complex interaction between the gravitational fields of the earth, moon and sun lead to our experience of high and low tides.
* Centrifugal force is a ficticiuos force and is actually an object's inertial resistance to the constantly changing motion associated with its circular path. On a carousel there is no sun at the center to counter the "centrifugal force" associate with the circular path. Where it is gravity that results in an object's curved path, gravity and the "centrifugal force" balance oneanother and result in a state of free fall, or in some contexts an object following the local curvature of spacetime.
only time a sensitive detector in contact the mass would not "feel" the force is if it is in free fall. Accelerations are not defined by "feelings" but precisely by changes in velocity (and that includes changes in direction of motion at constant speed). Gravity is an acceleration, not a force but the mass producing the gravity can apply a force to other objects. You can tell that gravity is an acceleration from its units. On Earth surface it is about 10 M/sec^2.
No gravity force is a real force, applied by some mass, but centripetal force and corellas forces are "pseudo-forces", both are related to the use of a rotating coordinate system. Real forces always have an agent applying them with an "equal and opposite" force acting on the agent.
You need to be more specific but if I assume the scale is in contact with the object (mass) on the side of the mass directly away form the sun and still both are in Earth's old orbit, due to a weak rocket accelerating them both towards the sun (Needed for keeping the old orbit) the scale will register the real force acting on it by the mass, which is equal to the mass times the acceleration of the mass. That real force acting on the scale has a real force, equal and opposite, acting on the mass, since the rocket only pushes against the bottom side of the scale.
Such as?
My turn to say: "That is misleading." Here is why and why there are two tides each day: It is only the GRADIENT the gravity fields that makes tides, not some complex interaction of the gravity fields.
Even though the moon's gravity at earth is smaller than that of the sun it dominates the tides as the gradient falls off as the inverse cube, not inverse square. I.e. the gradient of the sun's gravity is greatly weaken by time it gets out to Earth's orbit. What is happening is that due to the gradient, the moon's gravity force on the water side nearest to the moon is stronger than that on the center of the earth and weaker on the side of Earth farther from the moon. The strength of the sun's gravity force does not change much over the diameter of the Earth, so I neglect fact its very slightly different on the two sides of the Earth. lets for simplicity consider when the moon is in the earth's shadow:
Then the net moon force "removes" or cancels some of the sun's force on the side nearest to the moon. - Force needed to keep the mass of water there in orbit about the sun and with inadequate force for the orbit's acceleration, the water on the moon side is pulled up relative to the center of the Earth (by centrifugal force, it you like to think in those terms). On the sunlit side of the earth the moon is not able to keep the water mass there from feeling too much net force for the orbit's acceleration so a bulge is pulled up towards the sun relative to the center of the Earth.
With your mis-leading POV, about gravity and not it gradient, there is no way to explain why there are two high tides each day. If you think you can explain two high tides per day, with some "complex gravity interaction" I'd be very interested to see you try.
I think you are over complicating what was intended only as an observed example of the affect of changes in the affect of gravity, due to location and orientation of the gravitational masses. It was not intended to get into some debate about tides.
This first link is a very simple explanation of how the Moon's gravitational interaction with the Earth's oceans results in two tides. It does not include any reference to centrifugal force, a ficticiuos force...
http://science.howstuffworks.com/environmental/earth/geophysics/tide-cause.htm
The moon pulls water toward it, and this causes the bulge toward the moon. The bulge on the side of the Earth opposite the moon is caused by the moon "pulling the Earth away" from the water on that side.
The moon pulls water toward it, and this causes the bulge toward the moon. The bulge on the side of the Earth opposite the moon is caused by the moon "pulling the Earth away" from the water on that side.
The second is from Wiki, and gets more complicated, involving both the moon and sun, as well as where on the planet you are observing the tide(s). Again no mention of centrifugal force...
http://en.wikipedia.org/wiki/Tide
Some shorelines experience two almost equal high tides and two low tides each day, called a semi-diurnal tide. Some locations experience only one high and one low tide each day, called a diurnal tide. Some locations experience two uneven tides a day, or sometimes one high and one low each day; this is called a mixed tide. The times and amplitude of the tides at a locale are influenced by the alignment of the Sun and Moon, by the pattern of tides in the deep ocean, by the amphidromic systems of the oceans, and by the shape of the coastline and near-shore bathymetry (see Timing)
Some shorelines experience two almost equal high tides and two low tides each day, called a semi-diurnal tide. Some locations experience only one high and one low tide each day, called a diurnal tide. Some locations experience two uneven tides a day, or sometimes one high and one low each day; this is called a mixed tide. The times and amplitude of the tides at a locale are influenced by the alignment of the Sun and Moon, by the pattern of tides in the deep ocean, by the amphidromic systems of the oceans, and by the shape of the coastline and near-shore bathymetry (see Timing)
The POV comment will go unaddressed, as I don't believe there is enough in either of my posts for you to have drawn any real conclussions what my own views about gravity are.., and again a debate on that would not further the purpose or objective of the thread.
This is the portion of your earlier post I was attempting to correct,
The acceleration referred to in the Earth's orbital path around the sun, is experienced as free fall not because it is so small that we cannot feel it, but because the centripetal force or center seeking force, which is not a ficticiuos force (and which in this circumstance is the sun's gravity), is balanced by the centrifugal force (which is a ficticiuos force opposing the centripedal force or gravity.., and is actually the object's inertial resistance to the acceleration associated with the curved path of its orbit).
The planet wants to follow a straight line in space, but the direction component of its velocity is being changed by the gravitational pull of the sun. That change in direction is an acceleration that involves an interial resistance, opposite the sun's gravitational pull, to the constant change in direction.
For most folks on these forms these are minor issues, but from the POV of the threads author and 11th year course work and someone who is attempting to understand, fundamental relationships, even minor issues can be misleading.
But examining changes in direction under the influence of gravity complicates things. Orbiting is free fall, as a geodesic, and is free of external forces. The only exception to this is regarding tidal forces...you are merely measuring tidal forces across the diameter of the Earth at noon and at midnight...which is why I said this would not be the case for Earth as a point particle.
Declaring gravity as a pseudo-force is what allowed Einstein to formulate GR.Billy T said:
Your post is mostly correct but:
(1) No earth is NOT in free fall in the sun's gravity field. If it were it speed would be rapidly increasing towards the sun until it hit the sun.
(2) No these two* force, are not "balanced" (meaning equal and opposite). If they were the earth would be on a straight line trajectory with no acceleration, but it is not.
*if you want to call the centrifugal one which you understand is not even a real force.
(1) above is my reply to RJBeery 's first comment too.
I agree with his second: An alternant to considering gravity as a real force coming from all masses is to consider all masses are distorting space so that other masses do have motion as if a real force were acting on them.
Really. You're going to insist [assert] that acceleration is frame dependent? Your assertion is wrong whether you're going for it or not. Have at it.
Billy, orbiting objects are considered to be in an eternal free fall. This is easily verified by any number of online sources
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