Theory:

Centre of gravity:
  
The centre of gravity of an object is the point where the entire weight of the object appears to act.
 
Let's understand this with an example:
 
Try to balance a pen on your finger tip. What do we observe? We can see that there is only one point on which the pen is balanced. The point at which the pen is balanced is called the center of gravity of the pen.
 
Center of gravity on a regular - shapes objects:  
Generally the center of gravity of the geometrically shaped object lies in the geometric center of the object.
In below figures you can see where the centre of gravity lies on regular shaped objects.
 
post card1.JPG
 
Ring1.JPG
 
Thus, in this exercise we learned that,
  • The centre of gravity of an object is the point where the entire weight of the object appears to act.
  • The center of gravity of the geometrically shaped object lies in the geometric center of the object.