EQUILIBRIUM

STABILITY

501

Equilibrium.—When solid objects are supported at their centre of
gravity they are in equilibrium; it may be stable, neutral or unstable
equilibrium.

Take an oblong block of wood as in Figure 23. Draw diagonals on
one of its faces; their intersection gives the axis of its centre of gravity
which, of course, will be situated within the block at its centre. Drive
a small tack in the block at the point where the diagonals cross and
tie the string of a plumb bob to it.

Fig 23.
(i) Stand the block of wood on its end It is in equilibrium and
the plumb line bisects the base. The weight of the block acting down-
wards through its centre of gravity is counteracted by the upward
force of the table and being broad of base the block is firmly established
in stable equilibrium.
(ii) Tilt the block a little to one side. It is supported on one edge,
but it is still in stable equilibrium because, if the tilting pressure of the
hand were removed, it would return to its original upright position as
indicated by the plumb line intersecting the base.
(iii) Tilt the block over a little more until the plumb line passes
through its corner. The block is then in neutral equilibrium, it cannot
remain balanced on its edge but will either return to the upright
position again or topple over on its side when the hand is removed.
(iv) Tilt it over still further until the plumb line lies outside the
base of the block. It will now be in unstable equilibrium and will
topple over on its side.
The block is in stable equilibrium when the vertical line through its
centre of gravity falls within its base, but its equilibrium is unstable