<p>As <a href="https://astronomy.stackexchange.com/questions/41244/in-modified-gravity-theories-like-mond-are-galaxies-and-stars-actually-slowl#comment84698_41244">@PeterErwin's comment</a> compassionately points out:</p>
<blockquote>
<p>&quot;<a href="https://en.wikipedia.org/wiki/Acceleration" rel="nofollow noreferrer">Acceleration</a>&quot; means changing the velocity vector, so an object in a circular orbit is always &quot;accelerating&quot; (because the direction of the velocity changes), even if its speed is constant.</p>
</blockquote>

CC BY-SA 4.0
2021-06-02T11:03:50.480Z
There, I made Peter Erwin's 'comment' an 'answer'.... I hope that is okay....
Kurt Hikes
CC BY-SA 4.0
2021-07-02T23:32:53.013Z
It's always okay to answer your own question, even if its by promoting a comment into an answer, and to accept your own answer as well. But it's still important to make it a good Stack Exchange answer post. I've tidied up a bit.
uhoh
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2021-06-02T11:02:51.497Z
2021-07-02T23:34:35.637Z
2021-07-02T23:34:35.637Z
Kurt Hikes
<p>In the <a href="https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics" rel="nofollow noreferrer">Modified Theory of Newtonian Gravity</a> or MOND, they say that massive objects, accelerating slowly, experience this modified gravity.</p>
<p>But are they actually gaining in velocity, albeit very slowly?</p>
<p>Or are the MOND theorists using the term &quot;acceleration&quot; differently, as in the sense that all objects in a gravitational field are accelerating, even if they soon end up traveling at a constant velocity?</p>

CC BY-SA 4.0
2021-02-11T15:55:10.593Z
"Acceleration" means changing the velocity *vector*, so an object in a circular orbit is always "accelerating" (because the direction of the velocity changes), even if its *speed* is constant.
Peter Erwin
<gravity><dark-matter><galactic-dynamics><newtonian-gravity><theories>
In 'Modified gravity' theories like MOND, are galaxies and stars actually, slowly accelerating? In the normal sense?
Kurt Hikes
# In 'Modified gravity' theories like MOND, are galaxies and stars actually, slowly accelerating? In the normal sense?

(Asked by: username_0 on 2021-02-05)

In the [Modified Theory of Newtonian Gravity](https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics) or MOND, they say that massive objects, accelerating slowly, experience this modified gravity.

But are they actually gaining in velocity, albeit very slowly?

Or are the MOND theorists using the term "acceleration" differently, as in the sense that all objects in a gravitational field are accelerating, even if they soon end up traveling at a constant velocity?

**Tags:** gravity, dark-matter, galactic-dynamics, newtonian-gravity, theories
> **Comments:**
> **username_1**: "Acceleration" means changing the velocity *vector*, so an object in a circular orbit is always "accelerating" (because the direction of the velocity changes), even if its *speed* is constant.  

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## 1 answers

### Answer by username_0 on 2021-06-02. Score: 2

As [@PeterErwin's comment](https://astronomy.stackexchange.com/questions/41244/in-modified-gravity-theories-like-mond-are-galaxies-and-stars-actually-slowl#comment84698_41244) compassionately points out:

> "[Acceleration](https://en.wikipedia.org/wiki/Acceleration)" means changing the velocity vector, so an object in a circular orbit is always "accelerating" (because the direction of the velocity changes), even if its speed is constant.

> **Comments:**
> **username_0**: There, I made Peter Erwin's 'comment' an 'answer'.... I hope that is okay....  
> **username_2**: It's always okay to answer your own question, even if its by promoting a comment into an answer, and to accept your own answer as well. But it's still important to make it a good Stack Exchange answer post. I've tidied up a bit.  

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