<p><a href="https://astronomy.stackexchange.com/questions/8254/do-planets-repel">Gravity does not repel</a>, but there is another "force" at work here. The angular momentum of the original protoplanetary disk is conserved. In other words, just because the cloud condensed into planets doesn't mean it is going to stop spinning. </p>

<p>When we orbit artificial objects, we speed them up until they reach a velocity that sustains orbit--so again there is another force at work, just not gravity. Gravity keeps objects in orbit (rather than letting them fly off into space). Imagine if you spin a <a href="https://en.wikipedia.org/wiki/Yo-yo" rel="nofollow noreferrer">yo-yo</a> around in the air. The tension of the string is like gravity, keeping the yo-yo in orbit, but if you let go of the string then the other "force" (its momentum) will cause it to fly off.</p>

<p>(Technically, momentum is not a force. It is the product of the mass and velocity of the system--a conserved quantity, but the object required a force to bring it up to that velocity. In the case of a planet, it is the astronomical forces that acted on the protoplanetary disk. In the case of the yo-yo, it was your applied force to spin it.)</p>

CC BY-SA 3.0
2016-06-15T19:32:36.390Z
2016-06-15T19:32:36.390Z
2017-04-13T12:59:07.017Z
called2voyage
<p>It's simply that</p>
<h1>we are moving fast.</h1>
<p>It's that simple.  The Earth is moving VERY fast around the sun, so it is not &quot;dragged in&quot;.</p>
<p>If for some bizarre reason the Earth <strong>slowed down</strong> ... it would move in closer to the Sun.  If it stopped moving - it would fall directly in to the sun.  It's that simple.</p>
<p>Note that this is <em>exactly</em> how satellites work.  (I mean the satellites we launch to use for TV communications and the like.)  If they need the satellite to move down closer, they simply slow it down a bit. To go up higher, you just speed it up a bit.</p>
<p>So it's that simple.</p>
<p>&quot;I think there must be a repulsive force.&quot;</p>
<p>There's no repulsive force ... we are quite simply moving very fast!</p>
<p>Note that, quite simply, say you want a rocket to &quot;get away from&quot; the gravitational power of the Earth ... what do you do? Simply make it go very fast!  It's that simple.</p>

CC BY-SA 3.0
2016-06-18T12:39:27.197Z
2016-10-26T18:33:20.027Z
2020-06-17T09:47:40.653Z
Fattie
<p>If the gas that the solar sustem was created from had been  standing completely still, then yes, everything would fall to the center and become a big huge star. But the direction that the gas was moving is not zero, or random from temperature. The cloud as a whole has angular momentum. So more molecules are  moving, say, left than right.</p>
<p>Sure, most of the stuff in the universe wasn't moving fast enough, so it got dragged in to a nearby star. That's why stars are so big and the planets are so small and there's a supermassive black hole at the center of galaxies and most of the visible universe is stars.</p>
<p>But stuff moving fast enough in the right direction can fall together, and then fall into an orbit, like the Earth.</p>

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2024-11-17T01:08:39.780Z
"*most of the visible universe is stars*" -- if I'm reading this correctly, it is very false. Less than 10% of the ordinary matter (by mass) is in stars in the present epoch (e.g., [Li & White 2009](https://doi.org/10.1111/j.1365-2966.2009.15268.x) estimated 3.5%).
Sten
CC BY-SA 4.0
2024-11-17T01:12:42.287Z
Also, about the broader point: the tendency for star mass to dominate over planet mass is not due to the particles having too low velocities *initially* (you would need a crazy low velocity to hit something as small as a star) but rather because a protostellar gas cloud cools over time by emitting radiation.
Sten
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2024-11-17T02:33:40.550Z
@Sten Fascinating, but why would the temperature have anything to do with that?
Miss Understands
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2024-11-17T04:13:40.510Z
Lower temperature means lower velocities, which are what you need to gravitationally condense into a star. (Or for collisional gas, it might be clearer to see that the pressure drops as the temperature drops, and the reduced pressure support lets the star form.)
Sten
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2024-11-17T04:50:43.210Z
@MissU As I said in https://physics.stackexchange.com/a/460909/123208 when gas is compressed it heats up, and that heat tends to make the gas expand and resist whatever is compressing it. So gas undergoing gravitational collapse needs to radiate that excess energy away. Also see https://physics.stackexchange.com/a/413603/123208
PM 2Ring
CC BY-SA 4.0
2024-11-17T05:48:03.710Z
@Sten Gee, they call it the Schechter function Instead of the luminosity function. That's pretty hardcore. I did that in a poem once. I  called Lorenz transformation "the Fitzgerald contraction" because I needed it for prosidy.
Miss Understands
CC BY-SA 4.0
2024-11-17T06:05:49.253Z
@PM2Ring The way you explain stuff reminds me of Mike A'Hearn. Of all the teachers I had at UM, he most made complicated things simple. Not by condensing information out, but by revealing the underlying simplicity, like Feynman. === He had a black beard then and was young. When I saw a picture of him as a wrinkled old man with grey hair, it really bummed me out. In the back of my mind, I feel like some people are so cool that they ought to be given an exemption from death.
Miss Understands
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2024-11-16T22:49:57.933Z
2024-11-16T23:01:00.480Z
2024-11-16T23:01:00.480Z
Miss Understands
<p>Gravity is related to mass, and gravity is an attracting force on every body. Then what happens when bodies only attract each other..without repulsion they may strike the sun. I think there must be a repulsive force. </p>

<p>Another question arises, if only attraction is there, then why does the moon rotate elliptically around the earth?</p>

<p>And Mars comes closer to earth after 3 years?</p>

CC BY-SA 4.0
2018-08-28T09:30:15.543Z
Possible duplicate of [Why does earth not collide with the sun?](https://astronomy.stackexchange.com/questions/27419/why-does-earth-not-collide-with-the-sun)
peterh
<planet><solar-system>
If the planets only attract, then how can they stay in their orbit?
swapnil dhondge
# If the planets only attract, then how can they stay in their orbit?

(Asked by: username_0 on 2016-06-15)

Gravity is related to mass, and gravity is an attracting force on every body. Then what happens when bodies only attract each other..without repulsion they may strike the sun. I think there must be a repulsive force.

Another question arises, if only attraction is there, then why does the moon rotate elliptically around the earth?

And Mars comes closer to earth after 3 years?

**Tags:** planet, solar-system
> **Comments:**
> **username_1**: Possible duplicate of [Why does earth not collide with the sun?](https://astronomy.stackexchange.com/questions/27419/why-does-earth-not-collide-with-the-sun)  

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## 3 answers

### Answer by username_2 on 2016-06-15. Score: 10

[Gravity does not repel](https://astronomy.stackexchange.com/questions/8254/do-planets-repel), but there is another "force" at work here. The angular momentum of the original protoplanetary disk is conserved. In other words, just because the cloud condensed into planets doesn't mean it is going to stop spinning.

When we orbit artificial objects, we speed them up until they reach a velocity that sustains orbit--so again there is another force at work, just not gravity. Gravity keeps objects in orbit (rather than letting them fly off into space). Imagine if you spin a [yo-yo](https://en.wikipedia.org/wiki/Yo-yo) around in the air. The tension of the string is like gravity, keeping the yo-yo in orbit, but if you let go of the string then the other "force" (its momentum) will cause it to fly off.

(Technically, momentum is not a force. It is the product of the mass and velocity of the system--a conserved quantity, but the object required a force to bring it up to that velocity. In the case of a planet, it is the astronomical forces that acted on the protoplanetary disk. In the case of the yo-yo, it was your applied force to spin it.)


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### Answer by username_3 on 2016-06-18. Score: 8

It's simply that

# we are moving fast.

It's that simple. The Earth is moving VERY fast around the sun, so it is not "dragged in".

If for some bizarre reason the Earth **slowed down** ... it would move in closer to the Sun. If it stopped moving - it would fall directly in to the sun. It's that simple.

Note that this is *exactly* how satellites work. (I mean the satellites we launch to use for TV communications and the like.) If they need the satellite to move down closer, they simply slow it down a bit. To go up higher, you just speed it up a bit.

So it's that simple.

"I think there must be a repulsive force."

There's no repulsive force ... we are quite simply moving very fast!

Note that, quite simply, say you want a rocket to "get away from" the gravitational power of the Earth ... what do you do? Simply make it go very fast! It's that simple.


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### Answer by username_4 on 2024-11-16. Score: 1

If the gas that the solar sustem was created from had been standing completely still, then yes, everything would fall to the center and become a big huge star. But the direction that the gas was moving is not zero, or random from temperature. The cloud as a whole has angular momentum. So more molecules are moving, say, left than right.

Sure, most of the stuff in the universe wasn't moving fast enough, so it got dragged in to a nearby star. That's why stars are so big and the planets are so small and there's a supermassive black hole at the center of galaxies and most of the visible universe is stars.

But stuff moving fast enough in the right direction can fall together, and then fall into an orbit, like the Earth.

> **Comments:**
> **username_5**: "*most of the visible universe is stars*" -- if I'm reading this correctly, it is very false. Less than 10% of the ordinary matter (by mass) is in stars in the present epoch (e.g., [Li & White 2009](https://doi.org/10.1111/j.1365-2966.2009.15268.x) estimated 3.5%).  
> **username_5**: Also, about the broader point: the tendency for star mass to dominate over planet mass is not due to the particles having too low velocities *initially* (you would need a crazy low velocity to hit something as small as a star) but rather because a protostellar gas cloud cools over time by emitting radiation.  
> **username_4**: @Sten Fascinating, but why would the temperature have anything to do with that?  
> **username_5**: Lower temperature means lower velocities, which are what you need to gravitationally condense into a star. (Or for collisional gas, it might be clearer to see that the pressure drops as the temperature drops, and the reduced pressure support lets the star form.)  
> **username_6**: @MissU As I said in https://physics.stackexchange.com/a/460909/123208 when gas is compressed it heats up, and that heat tends to make the gas expand and resist whatever is compressing it. So gas undergoing gravitational collapse needs to radiate that excess energy away. Also see https://physics.stackexchange.com/a/413603/123208  
> **username_4**: @Sten Gee, they call it the Schechter function Instead of the luminosity function. That's pretty hardcore. I did that in a poem once. I  called Lorenz transformation "the Fitzgerald contraction" because I needed it for prosidy.  
> **username_4**: @PM2Ring The way you explain stuff reminds me of Mike A'Hearn. Of all the teachers I had at UM, he most made complicated things simple. Not by condensing information out, but by revealing the underlying simplicity, like Feynman. === He had a black beard then and was young. When I saw a picture of him as a wrinkled old man with grey hair, it really bummed me out. In the back of my mind, I feel like some people are so cool that they ought to be given an exemption from death.  

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