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A Closer Look

Gravity Problems
ravity does act on astronauts in space- from the effect of weightlessness. Bones If we lived on a planet with a different gravi-
Gcraft who are in orbit around Earth. lose calcium and other minerals, the heart tational force, we would have muscles and
Since gravity is acting on the astronaut and shrinks to a much smaller size, and leg mus- bones that were adapted to the gravity on
spacecraft, the term zero gravity is not an cles shrink so much on prolonged flights that planet. Many kinds of organisms have
accurate description of what is happening. that astronauts cannot walk when they re- been used in experiments in space to try to
The astronaut, spacecraft, and everything turn to the surface. These changes occur develop a better understanding of how their
in it are experiencing apparent weightless- because on Earth, humans are constantly systems work without gravity.
ness because they are continuously falling subjected to the force of gravity. The na- The problems related to prolonged
toward the surface. Everything seems to ture of the skeleton and the strength of the weightlessness must be worked out before
float because everything is falling together. muscles are determined by how the body long-term weightless flights can take place.
But, strictly speaking, everything still has reacts to this force. Metabolic pathways One solution to these problems might be a
weight, because weight is defined as a gravi- and physiological processes that maintain large, uniformly spinning spacecraft. The
tational force acting on an object (w = mg). strong bones and muscles evolved while astronauts would tend to move in a straight
Whether weightlessness is apparent or having to cope with a specific gravitational line, and the side of the turning spacecraft
real, however, the effects on people are the force. When we are suddenly subjected to a (now the “floor”) would exert a force on
same. Long-term orbital flights have pro- place where gravity is significantly different, them to make them go in a curved path.
vided evidence that the human body chan ges these processes result in weakened systems. This force would act as an artificial gravity.

WEIGHTLESSNESS tional attraction between Earth and an object (mg). The weight
News photos sometimes show astronauts “floating” in the Space of a cup of coffee, for example, can be measured by placing the
Shuttle or next to a satellite (Figure 2.32). These astronauts ap- cup on a scale. The force the cup of coffee exerts against the
pear to be weightless but technically are no more weightless scale is its weight. You also know that the scale pushes back on
than a skydiver in free fall or a person in a falling elevator. Re- the cup of coffee since it is not accelerating, which means the
call that weight is a gravitational force, a measure of the gravita- net force is zero.

FIGURE 2.31 In the Global Positioning System (GPS), each
of a fleet of orbiting satellites sends out coded radio signals that
enable a receiver on Earth to determine both the exact position of
the satellite in space and its exact distance from the receiver. Given
this information, a computer in the receiver then calculates the circle FIGURE 2.32 Astronauts in an orbiting space station may
on Earth’s surface on which the receiver must lie. Data from three appear to be weightless. Technically, however, they are no more
satellites gives three circles, and the receiver must be located at the weightless than a skydiver in free fall or a person near or on the
one point where all three intersect. surface of Earth in a falling elevator.

2-29 CHAPTER 2 Motion 53