# Is centre of mass a factor in space without gravity?

Quick disclaimer; I'm not that well informed on many of the mechanics on space in general so sorry if this question is rather easy/basic.

Is the center of mass in a object a factor in space when there is no gravity? By gravity I mean planets or any other large body of mass that could interfere. For example if I had a rocket in space where the bottom was significantly heavier than the top and exerted force at the top would it rotate around the center of mass or the center of the object shape/volume wise?

This is all assuming its an enclosed system and the rocket had no velocity prior to the addition of the force exerted.

## 2 Answers

The c.g./center of mass is the center of rotation even for a free-falling spacecraft.

Many spacecraft are rather symmetrical and so the difference between the c.g. and the volumetric center isn't too pronounced. A notable exception was the shuttle orbiter, especially when its payload bay was empty. You can see in this diagram from an old training manual how the maneuvering engines are gimbaled to point through the center of gravity, to keep the orbiter from rotating during orbital adjustment burns.

You have a number of misunderstandings.

First, there is no place anywhere, in space or not, where there is no gravity. Secondly, when you say "the rocket has no velocity" what does that mean? Velocity is a relative thing, so you can only define it relative to some other object, e.g. a planet.

Finally, Newtons laws of motion are completely independent of the location where the force happens. In other words, the presence of gravity is irrelevant. As a result, yes the centre of mass is equally important in space and on earth. Any force applied to an object will cause it to rotate, unless that force is directed through the centre of mass.

In fact, this is exactly how attitude control jets work: their force does not pass through the centre of mass of the spaceship, hence they apply a force that causes it to rotate around its centre of mass.

• 1) Although you are correct in space you are usually interacted by gravity there are places like the CMB cold spot. Besides this question was to further my understanding of the basics of velocity. 2) Although I did not mention it (perhaps I should have), the rotation was relative to the origin of the craft at the start of the question. Your last point did help me grasp the concept more, in any case cheers. Commented Dec 18, 2017 at 0:23
• re: 1). You can't assume there IS a CMB cold spot just because there apparently was one 14 billion years ago, a time when your problem can't exist. Commented Dec 21, 2017 at 6:00