# The Naked Scientists Forum

### Author Topic: Does gravitational pull on planets also cause them to turn?  (Read 375 times)

#### Thebox

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##### Does gravitational pull on planets also cause them to turn?
« on: 28/07/2016 09:52:54 »
A title question I did not know how to word correctly sorry, so hopefully you understand my question from explanation.  There is something I am not quite understanding or can ''see'' in my thoughts  is possible about the explanation of gravity and the Earths orbit around the Sun.

The Sun ''pulls'' the Earth and likewise the Earth Pulls the Sun towards each other, our basic understanding of the centripetal action of gravity.  This I have no problem visualising in my thinking, mass is attracted to mass .

Now we come to the second part of the explanation the ''centrifugal''  (note centrifugal is in quotation),

It is said that the body is travelling a straight line, but the gravity of the Sun curves the bodies path of the straight line to create the  orbit,

Ok, I can ''see'' how this applies, a bit like the bat and ball game where the tennis ball is attached to a pole by a string and once the ball is ''hit'' it rotates around the pole until it loses velocity, the string being a linkage.

However, when I ''see'' this , I also ''see'' a problem which in experiment I think we see a problem visually. (note ''see'' is my inner brain picture/thought,  see is the visual experience we perceive).

The problem I ''see'' which I think we would see in experiment is this -  If I was to retract the string of the pole centripetally, regardless of the speed of the ball orbiting the pole , the ball would also retract to the pole, so I can not 'see'' how the Sun does not retract the Earth if it is continually retracting the Earth and winding in.

????????????

added- My thoughts on this are watching a plug hole in a bath, things are attracted to the plug hole, orbit the plug hole but eventually go down the plug hole,

added- I have done a diagram to explain the question, yes it does resemble the inner of a worm hole, a worm hole is missing the disk in the middle.

« Last Edit: 28/07/2016 11:38:42 by chris »

#### evan_au

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##### Re: Does gravitational pull on planets also cause them to turn?
« Reply #1 on: 28/07/2016 22:43:12 »
Quote from: TheBox
the bat and ball game where the tennis ball is attached to a pole by a string and once the ball is ''hit'' it rotates around the pole... the ball would also retract to the pole
I think your game is broken.

The string is supposed to attach to the pole via a swivel. If the swivel seizes up, the string will wind around the pole and retract the ball into the pole.

Gravity (according to Isaac Newton's approximation) only acts along a straight line between the two masses, and is perfectly elastic (unlike your string). Lets say that the Earth is at it's farthest point from the Sun (this occurs in July). The Earth is moving at the slowest speed of its orbit, and Earth's centrifugal force is insufficient to hold it against the Sun's gravity, and Earth starts to get closer to the Sun.

When Earth is at it's closest distance from the Sun (this occurs in January), the Earth is moving the fastest speed of its orbit. Earth's centrifugal force is greater than the Sun's gravity, and Earth starts to get farther from the Sun.

The Earth cyclically trades distance from the Sun (gravitational potential energy) with speed (kinetic energy)

There is an analogy to this with your ball on a string game: When you hit the ball downwards at an angle, instead of horizontally. The ball goes up and down as it spins around, cyclically trading height (gravitational potential energy) with speed (kinetic energy) - at least until air resistance takes its inevitable toll (not to mention the effects of a dodgy swivel).

Note that "Ball's distance from the pole" is not the analogy to "Earth's distance from the Sun"; the correct analogy is "Ball's distance from the center of the Earth".

#### jerrygg38

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##### Re: Does gravitational pull on planets also cause them to turn?
« Reply #2 on: 04/08/2016 10:56:35 »

The problem I ''see'' which I think we would see in experiment is this -  If I was to retract the string of the pole centripetally, regardless of the speed of the ball orbiting the pole , the ball would also retract to the pole, so I can not 'see'' how the Sun does not retract the Earth if it is continually retracting the Earth and winding in.

To rephrase your question "Why doesn't the Earth spiral toward the sun? Thus the Earth if only acted upon by gravity would cause the sun to get closer and closer to the Sun for every revolution. this is a simple engineering problem of forces. It is true that gravity exerts a vector force which pushes the earth toward the sun and the sun toward the Earth. If this was the only force then we would be in the sun right now.
However the force of inertia is such that the Earth wants to go in a straight line. So you have a velocity vector tangential to the Earths motion and the gravity vector perpendicular to the motion. These two force vectors causes the earth to move around the sun and not spiral toward it.

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##### Re: Does gravitational pull on planets also cause them to turn?
« Reply #2 on: 04/08/2016 10:56:35 »