Centre of gravity

The centre of gravity is the point through which the whole weight of object seems to act.  

Centre of gravity is sometimes called the centre of mass. 

If you were carrying a ladder, which part of the ladder would you rest on your shoulder to carry it most comfortably?

Depending on the object's shape, its centre of gravity can be inside or outside it.

Regular shapes

A metre rule is a uniform and regular shape, therefore its centre of gravity, G, is at its centre i.e. at the 50 cm mark.

Do you notice that for a regular shaped object, the centre of gravity is also the middle of the object?

Activity

What you will need:

A mass piece

string 

a retort stand

a pin

a piece of card

a pencil and a ruler

What you will do:

1.  Try and balance the ruler on the pencil.

Where is its centre of gravity?

2. Tie the weight to a small piece of string to form a plumbline. Let it hang from your hand. 

Why does it always hang vertically?

Finding the centre of gravity of a piece of card using a plumbline. 

3.  Pin the card to the retort stand but allow it to hang freely.

4. hand the plumbline from the same pin.

Mark the position of the plumbline by two crosses on the card. Join the crosses with the ruler. 

Just as the plumbline hangs with its centre of gravity vertically below the pivot, so will the card. This means that the centre of gravity of the card is somewhere on the line you have marked.  

5.  To find where the centre of gravity is on the line, re-hang the card with the pin through another hole, and again mark the vertical line.  

The only point that is on both lines is where they cross, so this point is the centre of gravity.

6.  Repeat the same with an L-shaped piece of card.