The centre of gravity of an object is defined as the point through which its whole weight appears to act for any orientation of the whole object. The centre of gravity is sometimes confused with the centre of mass. The centre of mass of an object is that point in a body which moves as if the entire mass of the body existed at that point. The centre of gravity coincides with the centre of mass provided the object is placed in a uniform gravitational field. Therefore these two points are treated as the same point.
The centre of gravity (centre of mass) is at the geometrical centre of the object and the centre of gravity can even lie outside the object (for regular shaped objects). The centres of gravity of irregular shaped objects or objects of non-uniform thickness can be determined by means of a plumb line experiment.
Experiment
To determine the centre of mass of a plane lamina with an irregular shape or non-uniform thickness or density. Apparatus: retort stand, plumb line, cork and pin.
1. Make three small holes near the edge of the lamina. The holes should be as far apart as possible from each other. 2. Suspend the lamina through one of the holes using a pin. 3. Hang a plumb line on the pin in front of the lamina. 4. When the plumb line is steady, draw a line on the lamina over the plumb line. 5. Repeat the above for the next two holes. 6. The point of intersection of the three lines is the position of the centre of mass.