Graphics Reference
In-Depth Information
Animal structure has evolved to provide them with the level of stability most
suited to each animal and its particular mode of locomotion. Gazelles have
four slender legs with minimal contact with the ground over a relatively small
area. This allows for agility and the ability to move very quickly. The giant
tortoise also has four legs, relatively far fatter than those of the gazelle and
extended well beyond the body, providing a broader support base. They
move much more slowly and are far less agile. This structure provides more
stability when the tortoise lifts one of its legs during a walk cycle.
When supported by a broad base (four legs spread wide apart), a figure may
maintain its balance, even with a relatively high degree of displacement. A
mass supported at a single narrow point has less tolerance to resist forces
that may displace it; therefore balance becomes more difficult to achieve.
The larger the area covered by the supporting structure, the greater stability
an animal has and the more displacement it can resist. Spiders supported
on eight splayed legs well away from their bodies have more stability than
a horse that is supported on four legs across a relatively lower supporting
area. Displacement is more an issue with the spider than the horse because
of their relative sizes and the relative position of their bodies to the ground.
Spiders are much more likely to be blown over by the wind than a horse is.
Crabs also have a large spread of support with their legs splayed well out and
their bodies very close to the ground; this structure provides the necessary
stability against displacement, which may occur due to water currents. The
body placed nearer the ground in the case of spiders and crabs also offers
additional stability. Heavier animals are able to have the bulk of their weight
supported further from the ground.
Center of gravity
Contact with surface
(a)
Shift in the center of balance
(b)
FIG 4.6
a: The center of gravity on the ball is located directly over its point of contact with the ground; when
this shifts, the ball rolls. b: The stability of the cube is maintained until the point where the center of gravity goes
beyond the point of contact with the surface it sits on.
