O Level Physics Notes: Centres of Gravity

Every body has a point through which the whole mass of the body can be considered to act. If the body is held tat the centre of gravity it will not move or tilt. If the body is suspended from some arbitrary point, the centre of gravity will hang below the point of suspension. This is illustrated below for the triangular piece of card CDE suspended by a string AB. The centre of gravity is at O, vertically below AB.

The centre of gravity of an object can be found by suspending an object from two different points.

The centre of gravity can be found from the intersection of the lines PQ and XY when the pictures of the chair are superimposed. Notice the centre of gravity does not have to be inside the chair.

If the vertical through the centre of gravity lies outside the base then an object will topple. This is illustrated below. The objects in pictures 1 to 3 are stable but the object in picture 4 will topple.

Notice the difference between pitures 3 and 4. The centre of gravity of the object on the right is higher than for the one on the kleft. When they are both tilted through an ever increasing angle, the object on the right will topple before the object on the left. It's centre of gravity is higher so it is less stable.