262 Chapter 10 Force and Force-Related Parameters
Example 10.4 The space shuttle orbits the Earth at altitudes from as low as 250 km (155 miles) to as high as
965 km (600 miles), depending on its missions. Determine the value ofgfor an astronaut in a
space shuttle. If an astronaut has a mass of 70 kg on the surface of Earth, what is her weight
when in orbit around Earth?
When the space shuttle is orbiting at an altitude of 250 km above Earth’s surface:At an altitude of 965 km:
Note that at these altitudes an astronaut still has a significant weight. The near weightless con-
ditions that you see on TV are created by the orbital speed of the shuttle. For example, when
the space shuttle circles Earth at an altitude of 935 km at a speed of 7744 m/s, it creates a nor-
mal acceleration of 8.2 m/s
2. It is the difference betweengand normal acceleration that creates
the condition of weightlessness.
10.3 Moment, Torque–Force Acting at a Distance
As we mentioned earlier, the two tendencies of an unbalanced force acting on an object are to
translate the object (i.e., to move it in the direction of the unbalanced force) and to rotate or
bend or twist the object. In this section, we will focus our attention on understanding the ten-
dency of an unbalanced force to rotate, bend, or twist objects. Being able to calculate moments
created by forces about various points and axes is important in many engineering analyses. For
example, all of you have noticed that the street light poles or the traffic light poles are thicker
near the ground than they are on the top. One of the main reasons for this is that wind loading
creates a bending moment, which has a maximum value about the base of the pole. Thus, a big-
ger section is needed at the base to prevent failure. Nature understands the concept of bend-
ing moments well; that is why trees have big trunks near the ground to support the bending
moment created by the weight of the branches and the wind loading. Understanding moments,
or torques, is also important when designing objects that rotate, such as a shaft, gear, or wheel.
To better grasp the concept of moment, let us consider a simple example. When you open
a door you apply a pulling or a pushing force on the doorknob (or handle). The application ofW 1 70 kg2a7.38
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