Balance
A number of toys use a low centre of gravity to make a system stable which at first sight looks top heavy.
-
The skier
Extra weight at the end of the ski poles lowers the centre of gravity below the pivotal point.
There are many variations on this theme. -
The eagle
A rather unlikely balancing feat made possible by weights in the wing tips. The resulting centre of gravity is below the level of the beak.
-
The rocking dog
A weight low down in the hemispherical base causes the animal to spring back up when pushed over.
There are many variations of this idea.
This particular toy was sold in a pet shop as a budgie toy! -
The rocking pen
An “executive” version of the “cannot knock it over toy”.
The Physics
The centre of gravity of an object is the point at which the total weight of the object can be considered to act. It is therefore the point at which the object will balance.
The balancing (or equilibrium) is stable if when you nudge the object it comes back again. In physics jargon “a small displacement creates a restoring couple”.
With a fixed pivot (i.e. The skier or eagle) stability is achieved by distributing the weight so the centre of gravity is below the pivot.
With a moving pivot – the point of contact with the ground as the hemispherical base of the pen or dog rolls – the centre of gravity must be low enough within the hemisphere to provide a restoring force in the opposite direction to the displacement.