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### Author Topic: Question regarding einstein's theory of gravity  (Read 8774 times)

#### hamza

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##### Question regarding einstein's theory of gravity
« on: 23/08/2007 11:15:32 »
I had a confusion regarding Einstein's theory of gravity.. what confuses me is that if spacetime is warped and every mass curves the space time which causes lighter masses to stick to the heavier mass than why does'nt the lighter masses fall away due to inertia while the heavier mass moves away.. For instance take the example of earth and the gravity.. If we are attached to earth due to gravity which is created because of th earths warping the space. than why dont we fall away due to inertia when the earth orbits arround the sun.. i mean should'nt we be at the initial position when the earth moves away??

#### DoctorBeaver

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##### Question regarding einstein's theory of gravity
« Reply #1 on: 23/08/2007 13:00:41 »
The Earth causes the distortion in space and so drags the distortion with it as it orbits the sun. That means that the distortion remains centred on the Earth and, hence, the Earth's gravity still affects us.

Because we are moving at the same speed as the surface of the Earth, and gravity keeps us rooted to it, inertia isn't a problem.

Is that ok? Or did I misunderstand your question?
« Last Edit: 23/08/2007 13:02:50 by DoctorBeaver »

#### lightarrow

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##### Question regarding einstein's theory of gravity
« Reply #2 on: 23/08/2007 13:02:07 »
I had a confusion regarding Einstein's theory of gravity.. what confuses me is that if spacetime is warped and every mass curves the space time which causes lighter masses to stick to the heavier mass than why does'nt the lighter masses fall away due to inertia while the heavier mass moves away.. For instance take the example of earth and the gravity.. If we are attached to earth due to gravity which is created because of th earths warping the space. than why dont we fall away due to inertia when the earth orbits arround the sun.. i mean should'nt we be at the initial position when the earth moves away??
We orbit around the sun as well as earth.

#### DoctorBeaver

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##### Question regarding einstein's theory of gravity
« Reply #3 on: 23/08/2007 13:05:34 »
I beat you!

#### lightarrow

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##### Question regarding einstein's theory of gravity
« Reply #4 on: 23/08/2007 15:58:40 »
I beat you!
It's not fair: you write (correct) english faster than me!  [B)]

#### syhprum

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##### Question regarding einstein's theory of gravity
« Reply #5 on: 23/08/2007 16:11:07 »
There is a minor problem when we move to the equator of the the rotating earth, at present the rotational speed is rather low but if it was much greater we would certainly fall off.

#### DoctorBeaver

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##### Question regarding einstein's theory of gravity
« Reply #6 on: 23/08/2007 20:06:46 »
There is a minor problem when we move to the equator of the the rotating earth, at present the rotational speed is rather low but if it was much greater we would certainly fall off.

That, of course, is very true. But that would be centrifugal force, would it not?

#### DoctorBeaver

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##### Question regarding einstein's theory of gravity
« Reply #7 on: 23/08/2007 20:07:27 »
I beat you!
It's not fair: you write (correct) english faster than me!  [B)]

I may be faster at typing, but your English is better than a lot of English people I know.

#### syhprum

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##### Question regarding einstein's theory of gravity
« Reply #8 on: 23/08/2007 20:18:44 »
I thought that 'centrifugal force' was a rather muddled concept and that what was really happening was that the body trying to sit on the equator of the fast rotating Earth was trying to move in a straight line as dictated by its inertia and that the Gravity of the Earth was insufficient to deflect it from that path, hence it flies off.

#### DoctorBeaver

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##### Question regarding einstein's theory of gravity
« Reply #9 on: 23/08/2007 20:22:37 »
I thought that 'centrifugal force' was a rather muddled concept and that what was really happening was that the body trying to sit on the equator of the fast rotating Earth was trying to move in a straight line as dictated by its inertia and that the Gravity of the Earth was insufficient to deflect it from that path, hence it flies off.

I have heard that but I don't know what current thinking on it is.

#### syhprum

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##### Question regarding einstein's theory of gravity
« Reply #10 on: 23/08/2007 21:16:44 »
I seem to be in some strange time warp, I am receivng messages time stamped about 25 minutes into the future!

#### lightarrow

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##### Question regarding einstein's theory of gravity
« Reply #11 on: 24/08/2007 15:28:08 »
I may be faster at typing, but your English is better than a lot of English people I know.
(Thank you).

#### lyner

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##### Question regarding einstein's theory of gravity
« Reply #12 on: 24/08/2007 16:16:45 »
Quote
was that the body trying to sit on the equator of the fast rotating Earth was trying to move in a
It is not a good idea to describe anything inanimate as "trying" to do anything. It implies intent.
I remember my Physics teacher being positively 'anal' when people used the term 'centrifugal force'.
In dealing with the initial question, all you need to say is that the gravitational  force of the Earth is a CENTRIPETAL  force - i.e. towards the centre of the rotation. To make an object go in a circle, you need enough centripetal force. For an object in orbit, there is just enough. When you are standing on Earth, there is way more than enough.
If you wanted the Earth to escape from you, someone would have to accelerate it at more than 9.81m/s^2. You'd need quite some rocket engine.
As to the concept of centrifugal force. Whirling a conker on a string provides a centripetal force on the conker to keep it on its curved path but there is an equal and opposite force (or the string wouldn't be stretched). Once you cut the string, both forces disappear and the conker carries on is a straight line (tangential, not outwards). However, if you were standing on the conker, just by the string, you would FEEL the so called centrifugal force, pushing you 'downwards' onto the conker, whilst it was whizzing round.
I think the old Physics teachers threw the baby out with the bathwater in their zeal to show that things aren't 'thrown outwards'.

#### lyner

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##### Question regarding einstein's theory of gravity
« Reply #13 on: 24/08/2007 16:19:20 »
And here's another rant. If people use the term 'inertia', what are its units? I have never seen them defined.
No units means you can't measure it. That means it's not really a part of Physics.
Any views?

#### syhprum

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##### Question regarding einstein's theory of gravity
« Reply #14 on: 24/08/2007 17:13:22 »
A moving body (what ever that means) possesses energy (J) as defined by (mass/2)*V^2 so I guess the units in which inertia is measured are Joules.

Could I have said 'tends'
« Last Edit: 24/08/2007 17:15:56 by syhprum »

#### lightarrow

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##### Question regarding einstein's theory of gravity
« Reply #15 on: 24/08/2007 19:43:57 »
And here's another rant. If people use the term 'inertia', what are its units? I have never seen them defined.
No units means you can't measure it. That means it's not really a part of Physics.
Any views?
According to equivalence principle, a gravitational field is equivalent to an inertial field. Centrifugal force is an inertial field, so it exists as well as a gravitational field.
(Is it me or I have a "Deją vu"?)

#### lyner

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##### Question regarding einstein's theory of gravity
« Reply #16 on: 24/08/2007 20:18:09 »
OK folks - just show me the text book where the units of 'inertia' are listed.
As far as I'm concerned 'inertia' is the thing that keeps me in bed in the mornings.
I just can't see the point of using it when discussing Physics. Don't we have enough well - defined variables?
Surely the definition of an inertial field is one where there is NO acceleration.    Ohmygod here we go again, as you say.

#### DoctorBeaver

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##### Question regarding einstein's theory of gravity
« Reply #17 on: 24/08/2007 21:55:43 »
And here's another rant. If people use the term 'inertia', what are its units? I have never seen them defined.
No units means you can't measure it. That means it's not really a part of Physics.
Any views?
According to equivalence principle, a gravitational field is equivalent to an inertial field. Centrifugal force is an inertial field, so it exists as well as a gravitational field.
(Is it me or I have a "Deją vu"?)

Isn't a gravitational field equivalent to an accelerating inertial frame?

#### JP

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##### Question regarding einstein's theory of gravity
« Reply #18 on: 25/08/2007 04:13:29 »
Inertia is just a concept that something with mass will have a resistance to changes of its velocity.  It's not useful beyond the basic concept, as it's not a measurable quantity.  The closest measurable thing to inertia is "inertial mass," which classically was the "m" in Newton's famous F=ma.

An alternative to defining mass is to define it as the thing that causes and couples to a gravitational field.  For the mathematically inclined, Newton's laws of gravity defined this mass through F=m1*m2/r^2.

Einstein's equivalence principle started by assuming these two concepts of mass were indeed the same, although there's no fundamental reason this should be true.  By doing that, he was able to relate accelerating reference frames to gravitational fields, and by fancy mathematics, linked this all to curved space-time.

#### lightarrow

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##### Question regarding einstein's theory of gravity
« Reply #19 on: 25/08/2007 14:48:14 »
Isn't a gravitational field equivalent to an accelerating inertial frame?
Yes. The forces due to the accelerated frame are also called "inertial field".

#### lightarrow

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##### Question regarding einstein's theory of gravity
« Reply #20 on: 25/08/2007 14:56:33 »
OK folks - just show me the text book where the units of 'inertia' are listed.
As far as I'm concerned 'inertia' is the thing that keeps me in bed in the mornings.
I just can't see the point of using it when discussing Physics. Don't we have enough well - defined variables?
Surely the definition of an inertial field is one where there is NO acceleration.    Ohmygod here we go again, as you say.
No, it's "inertial frame" where there is no acceleration. "Inertial field" means that you experiences forces due to the accelerated frame.
http://en.wikipedia.org/wiki/Inertial_force
http://www.diracdelta.co.uk/science/source/i/n/inertial%20force/source.html
According to  GR, no experiment can distinguish between an accelerated frame and a gravitational field, so, if you want to measure inertial fields you only have to measure the body's mass and acceleration from the inside of the frame and multiply them.
« Last Edit: 25/08/2007 15:11:15 by lightarrow »

#### lyner

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##### Question regarding einstein's theory of gravity
« Reply #21 on: 26/08/2007 23:50:07 »
Yes - you are right. The red mist was coming down and  I used the wrong term.

Inertia seems to be used in hand waving arguments to stand for something half way between mass and momentum.
Certainly, in classical  Physics, you need never use the word at all.  And, before anyone starts to hold forth on SR or GR, they should first have a decent grasp of Newtonian stuff - it's an excellent start and sorts out the men from the boys (or  the gender-free equivalent).

#### lightarrow

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##### Question regarding einstein's theory of gravity
« Reply #22 on: 27/08/2007 16:25:36 »
Yes - you are right. The red mist was coming down and  I used the wrong term.

Inertia seems to be used in hand waving arguments to stand for something half way between mass and momentum.
Certainly, in classical  Physics, you need never use the word at all.  And, before anyone starts to hold forth on SR or GR, they should first have a decent grasp of Newtonian stuff - it's an excellent start and sorts out the men from the boys (or  the gender-free equivalent).
I agree with you.

#### lyner

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##### Question regarding einstein's theory of gravity
« Reply #23 on: 28/08/2007 10:10:22 »
That gives me a warm glow!
We so often have a go at each other but we mostly don't disagree, fundamentally.
We are both 'old school' I think.

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##### Question regarding einstein's theory of gravity
« Reply #23 on: 28/08/2007 10:10:22 »