# Difference between centre of mass and centre of gravity

1. Dec 6, 2015

### MBBphys

1. The problem statement, all variables and given/known data
Now, I know the centre of gravity can change depending on how close to uniform the gravitational field is, and it is where the weight appears to act from.
But, what is the centre of mass? Ok, it is a constant; doesn't change; and in an uniform gravitational field the two centres overlap; but how do you define the centre of mass without referencing centre of gravity?
An online source said it is the point from which the mass is 'equally distributed in all directions' for an object; but I can have irregular shapes. What does that source means? Or is the centre of mass defined in another way?
I am really confused!

2. Relevant equations
N/A

3. The attempt at a solution
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2. Dec 6, 2015

### SteamKing

Staff Emeritus
This is a better description of the center of mass:

https://en.wikipedia.org/wiki/Center_of_mass

For the center of gravity:

https://en.wikipedia.org/wiki/Centers_of_gravity_in_non-uniform_fields

For objects in uniform gravity fields, the center of mass and the center of gravity are the same point. When the gravity field is no longer uniform, then things get more complicated.

Basically, it comes down to locating the point at which a force can be applied to a body which causes it to translate in the direction of the applied force without rotation.

3. Dec 6, 2015

### Buzz Bloom

Hi MBBphys:

I suggest you read the Wikipedia article. You can use your browser to search on:
center of mass !wiki​

Hope this helps.

Regards,
Buzz

4. Dec 6, 2015

### MBBphys

Thank you, I think I get it now!

5. Dec 6, 2015

Thanks!