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STATICS - THEORY

   

Recall, the moment of a force about a point is given by the magnitude of the force times the perpendicular distance from the point to the force.

Using the same definition, the moment of an area about a point is the magnitude of the area times the perpendicular distance to the point. When the moment of an area about a point is zero, that point is called the centroid of the area.

The same method can be used to determine the centroid of a line or the centroid of a volume by taking the moment of the line or the moment of the volume.

Finding the location of a centroid is very similar to finding the location of the force resultant of a distributed force as covered in the moment chapter.

     

Example Problem Solution Steps
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  Centroid of a Line

 

The coordinates for the centroid of a line can be determined by using three scalar equations,

     

 

    Centroid of an Area


Centroid of an Area
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The centroid of an area can be determined by using three similar equations:

     

 

    Centroid of a Volume


Centroid of a Volume
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Similarly, centroid of a volume can be determined by

     

 

    Use of Symmetry


Centroid and Planes of Symmetry
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Finding the centroid of a body is greatly simplified when the body has planes of symmetry. If a body has a single plane of symmetry, then the centroid is located somewhere on that plane. If a body has more than one plane of symmetry, then the centroid is located at the intersection of the planes.

 

    Center of Mass


Three Planes of Symmetry
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The centroid of a volume defines the point at which the total moment of volume is zero. Similarly, the center of mass of a body is the point at which the total moment of the body's mass about that point is zero. The location of a body's center of mass can be determined by using the following equations,

     

Here ρ is the density of the body. If ρ is constant throughout the body, then the center of mass is exactly the same as the centroid.

     
    Center of Gravity

   

The center of gravity of a body is the point at which the total moment of the force of gravity is zero. The coordinates for the center of gravity of an object can be determined with

     

Here g is the acceleration of gravity (9.81m/s2 or 32.2 ft/s2). If g is constant throughout the body, then the center of gravity is exactly the same as the center of mass.