Activity 4 — the lamina

With a thin lamina and a thread, a striking fact about the lines of action of three non-parallel forces can be shown.

Activity 4 — first part

You will need: a thin rectangular plane lamina, a thread. A piece of thin metal sheet or a piece of cardboard can be used as the lamina.

Hang the lamina from three different points, on three separate occasions.
On each occasion mark on the lamina the vertical line that passes through the thread.

Figure to be added

Figure 12.13 — finding where the centre of gravity of a plane lamina lies: the lamina hung from three points, with the vertical line through the thread marked on each occasion.

Observation

All three lines meet at a single point. That point can be taken as the centre of gravity of the lamina.

Activity 4 — second part

Using two threads attached to two points on the lamina, hang it the way the framed picture of figure 12.12 hangs, and hold it in equilibrium in a vertical plane.
On a sheet of paper placed behind the lamina, mark the line each thread runs along. Mark the vertical line through the centre of gravity as well.

What you find

The three lines marked above all lie in one plane, and they meet at one and the same point.

Now, holding the direction of one of the three forces fixed, turn the lamina so that the plane the other forces act in changes. Turn it and let go, and you will see the system return to equilibrium only in the arrangement where all three forces lie in one plane again.