<p>The Newtonian gravitational acceleration of a test object (a small object with negligible mass) toward a point mass is given by</p>

<p>$$\vec g =\frac {GM}{||\vec r||^3} \vec r \tag{1}$$</p>

<p>where $\vec g$ is the acceleration vector, $G$ is the Newtonian gravitational constant, $M$ is the mass of the point mass, and $\vec r$ is the displacement vector directed from the test object toward the point mass.</p>

<p>Equation (1) cannot be used to compute the net gravitation acceleration of a test object toward a pair of point masses by pretending that the pair of point masses is equivalent to a single point mass located at the barycenter of the two point masses. Instead, what needs to be done is to apply equation (1) to the point masses individually and then form the vector sum of these individual accelerations. This in general will not point toward the barycenter.</p>

<p>There are two classes of points where the net gravitational acceleration vector to a pair of point masses is directed toward the barycenter of the point masses:</p>

<ul>
<li>Points along the line connecting the point masses, less for two regions where the net gravitational acceleration vector points away from the point mass, and</li>
<li>Points away from this line and whose projection onto this line is exactly midway between the point masses.</li>
</ul>

<p>For a test object located anywhere else, the net gravitational acceleration vector does not point toward the barycenter.</p>

CC BY-SA 3.0
2015-05-20T18:06:39.007Z
2015-05-20T18:06:39.007Z
David Hammen
<p>If an object was a distance from a planet away that equaled the distance from the object to a moon of the said planet, would it be drawn into the direction of the barycenter as it is the common center of mass?</p>

CC BY-SA 3.0
2015-05-20T15:43:27.427Z
In general, no.
David Hammen
CC BY-SA 3.0
2015-05-20T16:05:27.977Z
why? where to if not the barycenter?
Astrony
CC BY-SA 3.0
2015-05-20T17:13:09.053Z
Even that's not true, @barrycarter.
David Hammen
CC BY-SA 3.0
2015-05-20T18:07:27.170Z
Which Lagrangian point? There are five of them, and in general, none of them is at the barycenter.
David Hammen
<newtonian-gravity>
Attraction to barycenter
Astrony
# Attraction to barycenter

(Asked by: username_0 on 2015-05-20)

If an object was a distance from a planet away that equaled the distance from the object to a moon of the said planet, would it be drawn into the direction of the barycenter as it is the common center of mass?

**Tags:** newtonian-gravity

> **Comments:**
> **username_1**: In general, no.  
> **username_0**: why? where to if not the barycenter?  
> **username_1**: Even that's not true, @barrycarter.  
> **username_1**: Which Lagrangian point? There are five of them, and in general, none of them is at the barycenter.  

---

## 1 answers

### Answer by username_1 on 2015-05-20. Score: 2 | Accepted answer

The Newtonian gravitational acceleration of a test object (a small object with negligible mass) toward a point mass is given by

$$\vec g =\frac {GM}{||\vec r||^3} \vec r \tag{1}$$

where $\vec g$ is the acceleration vector, $G$ is the Newtonian gravitational constant, $M$ is the mass of the point mass, and $\vec r$ is the displacement vector directed from the test object toward the point mass.

Equation (1) cannot be used to compute the net gravitation acceleration of a test object toward a pair of point masses by pretending that the pair of point masses is equivalent to a single point mass located at the barycenter of the two point masses. Instead, what needs to be done is to apply equation (1) to the point masses individually and then form the vector sum of these individual accelerations. This in general will not point toward the barycenter.

There are two classes of points where the net gravitational acceleration vector to a pair of point masses is directed toward the barycenter of the point masses:

* Points along the line connecting the point masses, less for two regions where the net gravitational acceleration vector points away from the point mass, and
* Points away from this line and whose projection onto this line is exactly midway between the point masses.

For a test object located anywhere else, the net gravitational acceleration vector does not point toward the barycenter.


---

