I do not understand gravity.

I do not understand gravity! If gravity is a force that pulls you inwards towards it's source then why do the planets 'orbit' the sun! Surely if both objects are pulling at each other then they will inevitably clide into each other! So why haven't all the planets done so already?

Now I think I may understand this to an extent, and I believe it was suggested by Einstein that gravity is merely bends and curves in space and objects such as the Earth will naturally follow these bends in space that are created by the sun. Likewise, the moon will follow the bends in space caused by the Earth!

However, although I may understand that to an extent, if indeed what I've said is actually correct, I don't understand how this could possibly work on a much smaller scale such as when I drop a penny on the floor. That penny, for instance, is not following bends and curves in space, is it?

 

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Planets orbit things because there is gravity, but their also moving to the side, constantly "overshooting" the position/point gravity wants them to fall to. Now if the planet had a little less momentium, it would spiral down into the source of gravity (sun?) sorta like a coin in one of those funnels. I hope this analogy will help you understand how things orbit and not fall straight in.

 

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Consider what happens when u are atop a mountain and shoot a bullet from a gun. Depending how fast it is, it will fall some distance away from u. If u shoot the bullet faster and faster it will eventually fall further and further due to gravities downward effect (its not dropping vertically but in a curve). Now imagine firing it so fast that it drops around the horizon and keeps dropping but never hitting the surface of the earth. The surface of the earth is round so it curves away from the bullet and the bullet keeps going forever (unless it is slowed down by air resistance or something). This is orbital motion and this is how it occurs even though gravity is always directed downwards.

 

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You two have solved my problem in an instance! Now, would I be right in thinking that if an asteriod, lets say, half the size of Earth was to crash into us, and thus slowed down our speed by 50%, would we drop into an orbit closer to the Sun? And presumably, we could reverse this by making the earth faster we could move further away from the Sun.

 

Presumably so, but I wonder if the result would <u>always</u> be spiralling move.....

 

I got a gravity question for you heavies. If the moon is constantly moving away from the Earth because of the Earth's rotation does that mean that all the planets in our solar system are moving away from the sun because of the suns rotation.

 

The moon is moving away from the earth because the tidal bulge of the earth (due to the moon) is slightly ahead of the position it would be if the earth were not rotating at the rate it is.
What is needed for this separation to occur is therefore reliant on the moon having sufficient tidal effect on the earth to create a tidal bulge and for the earth to be spinning faster than the orbit of the moon.

For the planets of our solar system, the tidal bulge they would create on the sun would be negligible.
Also, the sun would need to be spinning at a rate required to advance this bulge enough for a momentum transfer in the direction of the planets orbit.

I can't give a definitive answer, but it is very improbable that these conditions exist.

 

I imagine that the space time geometry around the sun is spinning also at the same rate as the suns rotation, if that's the case then a spinning spacetime geometry would induce kinetic energy in its planets. The additional kinetic energy would boost the planets in higher orbits. Does that make sense?

 

Yes (sort of

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The spacetime geometry around a spinning mass such as the sun, would indeed be affected by the spin component.
This is known as 'frame dragging' and is a gravitomagnetic effect.
A soon to be launched "Gravity Probe B" will attempt to monitor this effect caused by the earth.
Primarily, it will influence the onboard gyroscopes in this satellite, but is predicted to be an extremely weak effect.
The sun's frame dragging should have some influence on the orbital status of the planets ( it is possible that the frame dragging effect of the sun causes the planets to appear to orbit the sun's real time position, instead of the delayed position ), but it wouldn't cause the planets to be boosted into higher orbits.

 

I don't think you should say "spiral down." If an orbiting planet has a sudden, non-orbit related change in momentum, then the eccentricity of the orbit will change, but the orbit will not become a spiral (outside of the Schwarzschild radius, which is only a few km for the sun); it will remain an ellipse. GR predicts a precession of the ellipse, but by no means suggests a spriralling down into the source (outside the Schwarzschild radius).

 

Is the name of that mountain the "Newton Mountain"...?

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Yes have u ever been there?

 

Oh yeah! There's a beautiful view from up there! It's very enchanting...

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Lovely this time of year........!

The only thing was, the last time I was up there I was looking out towards the east when a bullet hit me from behind in the head. Very sore

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The geo-sync sattelites need specific push to follow specific elliptical orbit in sync with earth's rotation. In case of earth being hit by a heavy body when it is farther from the sun and if it reduces its speed considerably, its perigee might possibly get too closer to the sun.

 

Oh! Wasn't it a cannon ball?

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Just because they're not in sync with the earth's rotation does not mean that they will spiral into the earth (or away from).

This would not occur in a spiralling fashion, though.

 

Hi Errandir,

A rotating Earth's gravitationl field is not uniform at a given altitude everywhere & everytime. Drag due to varying gravitional field at a given altitude affects the satellite's non-geosync elliptical/circular orbital speed. Sun has fairly uniform garvitational field around it at a given altitude. Geo-synchronous satellite Vs Earth anology seems more appropriate for Earth Vs Sun. The main point is, instead of push, if earth's orbital speed significantly reduces by the hit, the perigee might fall within a sun's range where earth's orbital velocity would not be sufficient to overcome Sun's grav.pull at periapsis.

If earth goes into sun it would not be a straight entry, it would be a partially spiral trajectory.! I didn't say it would be spiralling down to sun.!

 

In the newer texts, it is a bullet

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Oh! Ok. I didn't know that

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