# Gravity

Discussion in 'Alternative Theories' started by Asexperia, Mar 10, 2019.

1. ### TabbyStarRegistered Member

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Newton's 3rd Law when relativistic effects/limits are considered?

3. ### river

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To the objects , to the mass , not to any law .

Where does the increase in mass come from , physically .

5. ### TabbyStarRegistered Member

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I am a novice on this stuff. Seems too deep for me. I am comfortable believing GR prevents anything physical going faster than c. So at, or near it, the resistance/prevention of that speed limit being broken causes exponential mass increase (via protecting the speed limit being broken). Slow back down and the mass disappears just as quickly.

7. ### river

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To your last statement ; why ?

8. ### TabbyStarRegistered Member

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I probably should have said the resistance (Newton 3rd law on relativity steroids) disappears just as quickly.

9. ### James RJust this guy, you know?Staff Member

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Nowhere. Mass doesn't really increase.

Probably you are thinking of what people call "relativistic mass", a concept which working physicists these days don't really use (they use rest mass).

"Relativistic mass" is sort of a bad term used to describe a combination of rest mass energy and kinetic energy.

10. ### river

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Then what is rest mass energy ?

Last edited: Sep 6, 2019
11. ### James RJust this guy, you know?Staff Member

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Rest mass energy is an energy associated with an object that has mass. The numerical value of the rest mass energy is $E_0 = mc^2$, where $m$ is the rest mass.

The point is that the total mechanical energy of a massive object is $E=\gamma mc^2$, where $\gamma$ is the Lorentz factor, which depends on the speed of the object. When the speed is zero, $\gamma=1$, which means that the object still has some energy even when it is stationary.

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Agreed

13. ### river

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But here is the thing .

Why do the rings of Uranus form as they do ? By gravity .

Does gravity form these rings ? What do you think , JamesR ?

14. ### James RJust this guy, you know?Staff Member

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What relation does that question have to anything that has gone before, in this thread?

The short answer is to your question is "yes", but I don't think you're really interested in the answer.

Stop trolling.

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16. ### James RJust this guy, you know?Staff Member

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Tell me why you think the rings of Uranus would be formed by something other than gravity.

I mean, you must have had some thoughts in mind if you're asking a legitimate question. What are your thoughts? What prompted your question in the first place? And what's the connection between this and anything else in the thread, other than the word "gravity"?

17. ### river

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I see , so rather than answering my question about the rings of Uranus based on gravity , you want my thinking first , on why the rings of Uranus exist .

Rotation .

Last edited: Sep 12, 2019 at 4:30 AM
18. ### James RJust this guy, you know?Staff Member

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Your "thinking" is one single word? That's all?

Explain to me how "rotation" explains the rings of Uranus.

Or stop trolling.

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20. ### James RJust this guy, you know?Staff Member

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You didn't ask for an explanation. You asked whether I thought that gravity formed the rings of Uranus. My answer was yes, it did.

Your turn now. My question to you, as you will recall, is: 'Explain to me how "rotation" explains the rings of Uranus.'

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At best.

22. ### Janus58Valued Senior Member

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No, it's gravity.
You start with a couple of small moons. The moons are compact enough so that they are outside of their Roche limit (That distance where the tidal forces due to differential gravity are stronger than the gravitational forces holding the body together)
The Roche limit is dependent on the relative densities of the bodies involved. When orbiting the same planet a low density object will have a Roche limit distance further from the planet than a higher density object will.
The moons collide, scattering their material. The resulting cloud of debris has, as a whole, a lower density than either of the two moons had. This means that this cloud has a higher Roche limit than the two moons did. So high that it finds itself inside the Roche limit. Tidal forces acting on it are stronger than its own gravity trying to pull it back together and the material gets pulled further apart to form a ring around the planet. The ring is further aided by the various gravitational pulls of other moons orbiting the Planet. Gaps between rings are formed because those regions lay where various gravitational tugs tend to pull objects out. Even then, all rings are temporary and will dissipate over time ( in terms of the age of the Solar system, Uranus' rings are relatively young at ~600 million years old.) From time to time, other objects can collide adding more material to the rings, but eventually you'll run out of source material.

23. ### exchemistValued Senior Member

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Well done for re-introducing some informative science to this train crash of a thread.