# CONCEPT OF RELATIVE MOTION- How Can We Say That Planets revolve around Sun?

Discussion in 'Physics & Math' started by ash64449, Sep 14, 2013.

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1. ### ash64449Registered Senior Member

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Modern Concept of motion tells that motion is a combined effect of the state of object and the observer.

We Know that Galileo was punished for supporting the Heliocentric Theory instead of Geocentric Theory.

So the Motion being relative say that both theories are not equally wrong?

So still Heliocentric Theory Being followed? Isn't this theory fully correct only if the concept of absolute motion prevails?

3. ### GrumpyCurmudgeon of LucidityValued Senior Member

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Both objects orbit around the balance point between the two objects, neither orbits the other exclusively. Between two identical objects the balance point is mid way between them, when one is significantly heavier than the other the balance is closer to the heavier one, it's all RELATIVE to their weight differences. With the huge disparity in the masses of the Earth and the Sun the point the Earth is orbiting is very near the center of the sun, so near as to be able to say with some accuracy that the Earth orbits the Sun(Heliocentric). Higher accuracy than that requires we say that both the sun and Earth orbit the epicenter of the Earth/Sun system. All motion is relative, there is no absolute motion.

Grumpy

5. ### ash64449Registered Senior Member

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You can't say that. You are viewing in the spacial reference frame. A good example is our reference frame. Can't we see that Sun is like Orbiting earth and earth as stationary? This is relative motion. But from the viewpoint outside the solar system it the earth that revolving around the sun? Got the point? So heliocentric theory is not possible.

7. ### Russ_WattersNot a Trump supporter...Valued Senior Member

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You can construct a mathematical model that works for both, but it would be cumbersome and just reduce to Newton's laws anyway.

Also, while constant speed motion is relative, acceleration is not.

8. ### ash64449Registered Senior Member

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Isn't this sentence too based on the concept of relative motion? Can't this change if the reference frame is changed? i am sure it will. as it is based on our thinking of absolute motion.

Unnoticed we always mean events only of what is seen in our reference frame.

Last edited: Sep 14, 2013
9. ### ash64449Registered Senior Member

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No. I can show you that Acceleration is relative. But i don't know how to type with Latex. So i can't show.

Acceleration is also relative.

Actually, i have carried that exercise Trying to plot the position of an object A with respect to another object B

End result after solving: in our frame object A is accelerating but in that object's(B) frame, Object A is not accelerating.

10. ### ash64449Registered Senior Member

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Here is a simple mathematical exercise to show that acceleration is too relative. I have learned LaTex a bit.

Consider two objects $A$ and $B$ accelerating uniformly with both objects accelerating at a constant rate $a$. Both have same acceleration.

Consider that the reference frame that i was viewing this observation to be of our's.

Now Position of $A$ in our reference frame is given by :

$x_t(A) = V_0 (A)t + 0.5at^2(A)$

And position of $B$ is given by :
$x_t(B) = V_0 (B)t + 0.5at^2(B)$.

Now position of B as seen from A is given by:
$x_t(B) - x_t(A) = {V_0(B) - V_0(A)}t + 0.5at^2(B) - 0.5at^2(A)$

Therefore:
$x_t(B) - x_t(A) = {V_0(B) - V_0(A)}t$

inother words, the position of B as seen from A changes according to the rate ${V_0(B) - V_0(A)}$ in each unit of time. That is the velocity of B with respect to A.

Note that object B is not accelerating with respect to A but accelerating with respect to us. Therefore acceleration is relative.

[Note: assumed that initial position of both objects are assumed to start from origin of our reference frame.]

11. ### PandaemoniValued Senior Member

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If all motion and acceleration is relative, then imagine yourself spinning around once. Say it takes you one second to go through one 360 degree revolution. From most people's perspective, you spun, but--if you are correct--that acceleration is relative.

From your perspective, the whole universe just spun around you (or at least your axis of spin), including distant galaxies, most of which would have vastly exceeded the speed of light to do so if this particular frame of reference were indeed equally valid.

12. ### eugene381Registered Member

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If a spaceship fires its rockets and accelerates, then in order for you to believe that it is the spaceship which is standing still, and that it is the rest of the Universe which is accelerating, then you need to believe that there is an external gravitational force causing the rest of the universe to accelerate, and that this gravitational force is exactly balancing out the force from the rockets, causing the spaceship to stay in one place.

According to General Relativity, this view that the spaceship is standing still, and that there is an external gravitational field causing the Universe to accelerate, is just as valid as the view that it is the spaceship which is accelerating, and that there is no external gravitational field present.

In reply to your question, for you to believe that the Earth is standing still, and that it is the Sun which is moving, you have to believe that there is a time varying gravitational field affecting the entire Universe, and that this time varying gravitational field just happens to cancel out the gravitational pull of the sun on the Earth, so that the Earth is standing still.

So, you could believe that the Earth is standing still. But then you would need to believe that there is a very complicated time varying gravitational field affecting the entire Universe in order to achieve this.

13. ### eugene381Registered Member

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Also, as in interesting trivia point about history, the Catholic church initially told Galileo that he could publish a book which compares the evidence for the Heliocentric Theory and the Geocentric Theory, so long as he concluded that there is no way to know for sure which one is valid. Galileo then went ahead and published the book, but in the sections where he presented the evidence in a Geocentric Theory, he did so in a very sarcastic and mocking way, and this is what got him in in trouble with the Catholic church.

14. ### wellwisherBannedBanned

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If you do an energy balance, each reference sees a different energy balance. Conservation of energy implies energy is not relative but absolute. If we do an energy balance with respect to each reference, since energy is conserved and absolute, then reference is absolute.

Einstein wrote special relativity with three parameters, mass, distance and time. The mass is downplayed since this is the gateway to the energy balance. if we leave that out, we can pretend relative and make room for perpetual motion tricks.

Say we have two rockets, one with mass M and the other with mass 2M. They move with relative velocity V. The energy will not be the same depending on who we assume is moving. If M is moving its kinetic energy is 1/2MV2, if 2M is moving its kinetic energy is MV2 or double. Say we cannot do an energy balance, if we pick wrong, because we assume motion is relative and not absolute, we have added or taken away energy from the universe. In this scenario the energy balance defines these two references in an absolute way since there will only be one scenario that allows the energy to balance out.

15. ### eugene381Registered Member

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The example of the spaceship and the gravitational field is not from Special Relativity but from General Relativity, and the equivalence of gravitation and accelerating reference frames is a founding principle of the General Theory of Relativity.

The conservation of energy works in both reference frames in General Relativity.

16. ### ash64449Registered Senior Member

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According to Einstein's Theory Of Relativity No Object can exceed the light speed limit. So even if i spun,whole galaxies won't exceed faster than the speed of light. You cannot just state it. You have to provide the calculations.

Well, Make sure you use Einstein's Velocity Transformation Formula while making the calculation.

I have actually stated the mathematics of acceleration being relative? Why should you reject that?

17. ### ash64449Registered Senior Member

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Well... Then i have accept that!!

18. ### ash64449Registered Senior Member

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That's the point.Both of them are equally valid. So we cannot say in reality if rocket is the one accelerating or the universe is the one that is accelerating.

The Fact that we feel we should be the one who is accelerating even if we see universe is the one is accelerating is just the subjective impression or because we have long rooted the concept of absolute motion.

I don't understand why the concept of absolute motion should be introduced. The word "motion" itself says that Motion is Relative!!!

19. ### wellwisherBannedBanned

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If you assume the earth is standing still and the sun moves around the earth, then the energy balance of the solar system becomes higher that it is in reality. We have the mass of the sun moving around the earth once per day. That adds a lot of extra energy to the solar system.

If the earth moves around the sun, the energy balance is the mass of the earth moving around the sun once per year, which is much less in terms of energy compared to the sun moving. The ancients had more energy in their heavens, so they added gods to account for it. Once we changed to the earth around the sun, the energy of the local heavens decreased. The real energy balance makes these references absolute; earth moves round the sun.

20. ### eugene381Registered Member

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I am not sure I fully understand what you just said, but it is OK for different observers to believe that the total energy of the Universe is different. For example, if there is a very massive object, and it is standing still relative to one observer, but moving relative to another observer, then the two observers will disagree about the amount of kinetic energy in the object. Similarly, they will disagree about the total kinetic energy of the entire Universe.

However, they will both still agree about the fact that energy is conserved. Whatever they think the total energy of the Universe is, it will always stay constant.

21. ### ash64449Registered Senior Member

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According to me,that's just impression.

As I already said "motion" doesn't make sense if you don't present itself the observer. How can Motion be absolute if there are many reference frame?

22. ### ash64449Registered Senior Member

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Do you still Believe Motion is absolute?

23. ### eugene381Registered Member

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Well, the fact that there is no absolute motion is not intuitively obvious. That is why the concept of relative motion needs to be introduced.