40
3 Mechanical Aspects of Biosystems
Fig. 3.4 A ‘g-detector’. Two
of six springs are not shown
m
Tiny accelerometers built on silicon and measuring only 0.5 mm across, such as
those in cell phones, are constructed with a movable conductive layer with fingers
interleaved with a fixed set of conductive fingers. The movable layer is suspended
by thin elastic extensions. The movable layer has a mass which reacts to any
acceleration by displacing, while the extensions limit the movement just as a spring
would. Acceleration is detected by measuring the capacitance between the movable
and the fixed fingers. Present devices in phones have an acceleration sensitivity of
about 0.02 g when |a| ≤ 2 g, and a range of ±8 g.
3.3.2 Microgravity and Weightlessness
Near-zero gravity is sometimes referred to as ‘microgravity’. If all masses within a
body have the same force of gravity on them, and there is no opposing external
force, all the mass points in that body will have a single acceleration. As long
as this condition is sustained, then there will be no strain on the material within
the body. If the body is a person, that person will experience ‘zero gravity’. An
astronaut ‘floating’ in orbit around the Earth is such a body. In fact, in Einstein’s
General Theory of Relativity, there is no difference locally between gravity and
acceleration. A person in a free-falling elevator near the Earth will not perceive,
from any local observation or measurement, any difference between that state and
being free floating far from any gravitating mass. Nor could people distinguish their
sense of gravity standing at rest on the Earth from what they would feel while
accelerating in a rocket at a = g in outer space.
A fish or a person in water with the same average density as the water may be able
to move through the water seemingly weightless. In fact, astronauts go into tanks of
water to practice ‘space walks’. However, such astronauts still feel the gravitation
of the Earth when in such tanks, because their flesh and organs are still hanging on
their bones. The buoyant force acts on the surface of the body, not over the body
volume, as does gravity. A person knows this because of proprioceptors which give
information to the brain about the position of muscles.
3 Mechanical Aspects of Biosystems
Fig. 3.4 A ‘g-detector’. Two
of six springs are not shown
m
Tiny accelerometers built on silicon and measuring only 0.5 mm across, such as
those in cell phones, are constructed with a movable conductive layer with fingers
interleaved with a fixed set of conductive fingers. The movable layer is suspended
by thin elastic extensions. The movable layer has a mass which reacts to any
acceleration by displacing, while the extensions limit the movement just as a spring
would. Acceleration is detected by measuring the capacitance between the movable
and the fixed fingers. Present devices in phones have an acceleration sensitivity of
about 0.02 g when |a| ≤ 2 g, and a range of ±8 g.
3.3.2 Microgravity and Weightlessness
Near-zero gravity is sometimes referred to as ‘microgravity’. If all masses within a
body have the same force of gravity on them, and there is no opposing external
force, all the mass points in that body will have a single acceleration. As long
as this condition is sustained, then there will be no strain on the material within
the body. If the body is a person, that person will experience ‘zero gravity’. An
astronaut ‘floating’ in orbit around the Earth is such a body. In fact, in Einstein’s
General Theory of Relativity, there is no difference locally between gravity and
acceleration. A person in a free-falling elevator near the Earth will not perceive,
from any local observation or measurement, any difference between that state and
being free floating far from any gravitating mass. Nor could people distinguish their
sense of gravity standing at rest on the Earth from what they would feel while
accelerating in a rocket at a = g in outer space.
A fish or a person in water with the same average density as the water may be able
to move through the water seemingly weightless. In fact, astronauts go into tanks of
water to practice ‘space walks’. However, such astronauts still feel the gravitation
of the Earth when in such tanks, because their flesh and organs are still hanging on
their bones. The buoyant force acts on the surface of the body, not over the body
volume, as does gravity. A person knows this because of proprioceptors which give
information to the brain about the position of muscles.