Difference between revisions of "Center of mass"

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[[File:com icon.png|framed|left|Icon]]
 
[[File:com icon.png|framed|left|Icon]]
 
[[File:Com and cot.png|thumb|The thrust points away from the CoM]]
 
[[File:Com and cot.png|thumb|The thrust points away from the CoM]]
The '''center of mass''', abbreviated '''CoM''', or '''center of gravity''', is the location of an object where all mass is equally distributed around it. It is important to balance a [[craft]] to prevent it from getting out of control. [[Plane]]s should have a center of mass slightly in front of the [[center of lift]]. For all craft the thrustvector should point to, point away from or go through the center of mass.
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The '''center of mass''' (abbreviated '''COM''' or '''CoM''') is the point within an object where all mass is equally distributed around it. This is sometimes wrongly called its ''center of gravity'' (see below).  
  
It is possible to show the center of mass in the [[Vehicle Assembly Building]] and [[Space Plane Hangar]], but not while in flight. Also it is not possible to view the center of masses of the different stages. This is the only information about a center which is only a point and not also a vector.
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Along with the [[center of thrust]] (COT), and the [[center of lift]] (COL) while within an [[atmosphere]], it is one of the key physics properties determining a [[craft]]'s behavior while in motion. Display of the COM can be toggled in the [[editor]] where it is seen as a large yellow and black checkered sphere.
  
Unlike in real life, the center of mass of two celestial bodies called the barycenter, e.g. [[Kerbin]] and [[Mun]], is always in the center of the orbited body, which is in the example Kerbin.
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== Usage ==
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The center of mass is distinct in being a point without a vector, and more so for acting as the fulcrum for the forces of [[w:Thrust|thrust]] and [[w:Lift (force)|lift]]. In short, the position of a craft's center of thrust and the thrust vector should be in an axial alignment with the COM; otherwise it will cause the craft to turn off course. The center of lift is more complex, but in most cases it should be near the COM but on the tailward side and centered side-to-side, and possibly top-to-bottom, while in flight; otherwise the craft may pitch or yaw more uncontrollably the faster it goes. For more detail, see their respective articles.
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The center of mass will shift as fuel is consumed or stages detached. It is not possible to see the COM, or the other centers, outside the editor. However, temporarily taking stages off or using [[tweakable]]s to alter the fuel level, or both, will alter the position of the COM and other centers. This can reveal issues that could result in nonstandard flight profiles before actually launching.
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[[SAS]] modules and [[reaction wheel]]s are better at smoothly stabilizing a craft the closer they are to the COM.
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{{quote|The placement does matter for [[reaction wheel]]s. Generally speaking they can cause some problems if placed far from the center of mass. Imagine you are grabbing that point and rotating it. That is what the reaction wheels will try to do. You'll get offcenter rotation anywhere other then near the COM.|[http://forum.kerbalspaceprogram.com/entries/740-Updated-Information-on-SAS-in-0-21-1#comment_4254 comment] by C7, in his blog entry “[http://forum.kerbalspaceprogram.com/entries/740-Updated-Information-on-SAS-in-0-21-1 Updated Information on SAS in 0.21.1]”}}
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== Center of gravity ==
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Sometimes, instead of center of mass, the term center of gravity is used, which describes a similar but distinct concept. Because gravity weakens with the distance to the gravity source, a long rod pointing away from the source would experience different gravitational pull at different points along its length. The "lower" end is being pulled harder than the "high" end; therefore, its center of gravity would be nearer to the gravity source than its center of mass.
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In Kerbal Space Program, this difference of gravity is ignored, so the gravity is always the same on all parts of a craft, and thus both centers are in the same place.
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Unlike in real life, the center of mass of two celestial bodies, called the barycenter, e.g. [[Kerbin]] and [[Mun]], is always in the center of the orbited body, which is in the example Kerbin.
  
 
== See also ==
 
== See also ==
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* [[Center of thrust]]
 
* [[Center of thrust]]
 
* {{Wikipedia}}
 
* {{Wikipedia}}
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[[Category:Physics]]

Latest revision as of 18:46, 17 March 2017

Icon
The thrust points away from the CoM

The center of mass (abbreviated COM or CoM) is the point within an object where all mass is equally distributed around it. This is sometimes wrongly called its center of gravity (see below).

Along with the center of thrust (COT), and the center of lift (COL) while within an atmosphere, it is one of the key physics properties determining a craft's behavior while in motion. Display of the COM can be toggled in the editor where it is seen as a large yellow and black checkered sphere.

Usage

The center of mass is distinct in being a point without a vector, and more so for acting as the fulcrum for the forces of thrust and lift. In short, the position of a craft's center of thrust and the thrust vector should be in an axial alignment with the COM; otherwise it will cause the craft to turn off course. The center of lift is more complex, but in most cases it should be near the COM but on the tailward side and centered side-to-side, and possibly top-to-bottom, while in flight; otherwise the craft may pitch or yaw more uncontrollably the faster it goes. For more detail, see their respective articles.

The center of mass will shift as fuel is consumed or stages detached. It is not possible to see the COM, or the other centers, outside the editor. However, temporarily taking stages off or using tweakables to alter the fuel level, or both, will alter the position of the COM and other centers. This can reveal issues that could result in nonstandard flight profiles before actually launching.

SAS modules and reaction wheels are better at smoothly stabilizing a craft the closer they are to the COM.

The placement does matter for reaction wheels. Generally speaking they can cause some problems if placed far from the center of mass. Imagine you are grabbing that point and rotating it. That is what the reaction wheels will try to do. You'll get offcenter rotation anywhere other then near the COM.

comment by C7, in his blog entry “Updated Information on SAS in 0.21.1

Center of gravity

Sometimes, instead of center of mass, the term center of gravity is used, which describes a similar but distinct concept. Because gravity weakens with the distance to the gravity source, a long rod pointing away from the source would experience different gravitational pull at different points along its length. The "lower" end is being pulled harder than the "high" end; therefore, its center of gravity would be nearer to the gravity source than its center of mass.

In Kerbal Space Program, this difference of gravity is ignored, so the gravity is always the same on all parts of a craft, and thus both centers are in the same place.

Unlike in real life, the center of mass of two celestial bodies, called the barycenter, e.g. Kerbin and Mun, is always in the center of the orbited body, which is in the example Kerbin.

See also