Q: Can a body with only one force acting on it be in equilibrium? A: No, the condition of equilibrium is F_{net} = 0. 
Q: Can a body in motion be in equilibrium? A: Yes, the condition of equilibrium is F_{net} = 0. For an object in motion, F_{net} = 0 means acceleration a_{net} = 0 and velocity v_{net} = constant. So, the only possible motion an object in equilibrium is the uniform motion (motion in straight line with constant speed.) 
Q: How far must the Tower of Pisa lean before it topples? A: The
leaning Tower of Pisa is 55 meters high and 7.0 meters in
diameter. The top of the tower is 5.2 meters from the vertical (as
of 1997). The tower will topple when the center of mass is no
longer above the structure's base. If we treat the tower as a
cylinder of uniform mass, its center of mass will be in the middle of
the tower (where the line between the middle of the bases crosses the
line that joins the middle of two opposite walls. The center
of mass will no longer be above the base when the top of the tower has
moved 7.0 meters from the vertical (another 1.8 meters). To find
what angle this is calculate = sin^{1} (7.0 m/ 55m) = 7.3°. You can learn more about the Tower of Pisa and the
attempts to save it at this site: 
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Static Equilibrium 

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