# The Difference Between Centre of Mass and Centre of Gravity

Many people assume that the terms “centre of mass” and “centre of gravity” are synonymous, but this is not the case.

Centre of mass is the point at which the distribution of mass is equal in all directions, and does not depend on gravitational field. Centre of gravity is the point at which the distribution of weight is equal in all directions, and does depend on gravitational field.

A toy bird balances when a pivot is placed at its centre of gravity.

The centre of mass and the centre of gravity of an object are in the same position if the gravitational field in which the object exists is uniform. In most cases this is true to a very good approximation: even at the top of Mount Everest (8848 metres) the gravitational field strength is still 99.6% of its standard value. You are unlikely ever to experience a difference between centre of mass and centre of gravity, as the gravitational field in which you find yourself is extremely uniform.

The object on the left, in a uniform gravitational field, has overlapping centres of gravity and mass. For the object on the right, in which the gravitational field is stronger towards its base, the centre of gravity is below the centre of mass. Approaching a black hole the gradient of the gravitational field would be infinitely “steep”, leading to an incredible difference in gravitational field and death by spaghettification for anyone falling into a black hole.

## 30 thoughts on “The Difference Between Centre of Mass and Centre of Gravity”

1. Mark says:

The toy bird is not balancing at its centre of gravity, the pivot point is in line (vertically, in the direction of gravitational force) with the models centre of gravity which could be above, below or at the pivot point. at least that was what I was taught. Could be wrong I guess.
Ps – most interesting info on your site, good stuff.

2. Anonymous says:

Good exlpanation but need simple words.

3. Anonymous says:

The toy bird is balancing with its centre of gravity below its pivot. That is why equilibrium is restored after it wobbles. It is in a state of stable equilibrium. If the centre of gravity were at the pivot, it would be in a state of neutral equilibrium and it will remain in that new position if if the toy bird is left at rest. If the centre of gravity were above the pivot, it would be in a state of unstable equilibrium and the toy bird will rotate until it achieves stable equilibrium.

4. Ajay kumar says:

I am aman raj ok!

5. vibhor says:

6. SATYAM SHUKLA says:

Good briefing.Thanks.

7. Anonymous says:

the centre of mass and centre of gravity of toy bird coincide at that point where it is pivoted.

9. Anonymous says:

10. Anonymous says:

11. Faiz says:

Very helpful thank u so much for wonder ful explanation

12. Anonymous says:

Best explanation sir

13. Anonymous says:

What about ice bergs COM AND COG

14. That is a really interesting point. That needs some thought.

15. Anonymous says:

It is very useful and I have understood briefly

16. MrJanus says:

It always amazes me that people don’t understand the difference between COM and COG. As someone who works in game development, the separation of the two terms is critical when it comes to making your physics engine work properly…. excellent post.

17. Abdul Rauf says:

Sir if COG is above or below from the pivot than how Toy bird will rotate???
Because in this case moment arm will zero and the torque will also zero as
Torque=moment arm ? force
As
Moment arm=0
Force=weight
Then
Torque= 0 ? weight
Torque=0

18. Yes, the bird will not rotate. It will just stay where it is.

19. Anonymous says:

Best explination sir

20. Anonymous says:

Good explanation

21. Anonymous says:

Nice explanation

22. Anonymous says:

very good

23. Anonymous says:

Very useful

24. Gopakumar says:

Of an object is cut into two pieces exactly at its centre of gravity, whether both pieces weigh equal?

25. If the object is uniform, yes.

26. Anonymous says:

Why does the center of gravity shifts?

27. Because the gravitational field is not uniform.