We have weight because of the force of gravity of the Earth on us. Each part of our body – arms, legs, head, for example – experiences a force, caused by the force of gravity. However, it is much simpler to picture the overall effect of gravity as acting at a single point. This is our centre of gravity – the point where all the weight of the object may be considered to act.
For a person standing upright, the centre of gravity is roughly in the middle of the body, behind the navel.
For a sphere, it is at the centre. It is much easier to solve problems if we simply indicate an object’s weight by a single force acting at the centre of gravity, rather than a large number of forces acting on each part of the object. Figure 4.14 illustrates this point. The athlete performs a complicated manoeuvre.
However, we can see that his centre of gravity follows a smooth, parabolic path through the air, just like the paths of projectiles we discussed in Chapter 2.

PRACTICAL ACTIVITY 4.1

 

Finding the centre of gravity

The centre of gravity of a thin sheet, or lamina, of cardboard or metal can be found by suspending it freely from two or three points (Figure 4.15).

Small holes are made round the edge of the irregularly shaped object. A pin is put through one of the holes and held firmly in a clamp and stand so the object can swing freely. A length of string is attached to the pin. The other end of the string has a heavy mass attached to it. This arrangement is called a plumb line.
The object will stop swinging when its centre of gravity is vertically below the point of suspension. A