Distributed Forces: Center of Mass, Center of Gravity, Centroid of Areas and Lines by Integration Introduction o In previous discussions, all forces were treated as concentrated along their lines of action. o This treatment provided a reasonable model for those forces. o “CONCENTRATED” forces do not exist in the exact sense, since every external force applied mechanically to a body is distributed over a finite contact area, however small. Introduction o when forces are applied over a region whose dimensions are not negligible compared with other pertinent dimensions, the actual manner in which the force is distributed must be accounted for o This is done by summing the effects of the distributed force over the entire region using mathematical integration o This requires that we know the intensity of the force at any location o There are three categories of such problems Introduction o Line distribution – when a force is distributed along a line, the intensity w of the loading is expressed as force per unit length of line (N/m). o Area distribution – when a force is distributed over an area, the intensity is expressed as force per unit area (N/m2). o Volume distribution - when a force is distributed over the volume of a body (body force), the intensity is expressed as force per unit area (N/m3). Centers of mass and centroids o Consider a 3D body of any size and shape, having a mass m. o If this body is suspended as shown from any point such as A, the body will be in equilibrium under the action of tension in the cord and the resultant W of the gravitational forces acting on all particles of the body o The lines of action will be concurrent at a single point G, which is called the center of gravity of the body. Determining the center of gravity • Center of gravity of a plate M y x W xW x dW M y yW yW y dW • Center of gravity of a wire M y x W xW x dW M y yW yW y dW Centroids and First Moments of Areas and Lines o Centroid of an area o Centroid of a line x W x dW x At x t dA x A x dA Q y first moment with respect to y yA y dA Qx first moment with respect to x x W x dW x La x a dL x L x dL yL y dL First Moments of Areas and Lines o An area is symmetric with respect to an axis BB’ if for every point P there exists a point P’ such that PP’ is perpendicular to BB’ and is divided into two equal parts by BB’. o The first moment of an area with respect to a line of symmetry is zero. o If an area possesses a line of symmetry, its centroid lies on that axis o If an area possesses two lines of symmetry, its centroid lies at their intersection. o An area is symmetric with respect to a center O if for every element dA at (x,y) there exists an area dA’ of equal area at (-x,-y). o The centroid of the area coincides with the center of symmetry. Practice Problem Centroid of a circular arc: Locate the centroid of the circular arc as shown in the figure. Solution Practice Problem Centroid of a triangular area: Determine the distance h from the base of a triangle of altitude h to the centroid of its area. Practice Problem Centroid of the area of a circular sector: Locate the centroid of the area of a circular section with respect to its vertex. Solution Practice Problem Locate the centroid of the area under the curve x = ky3 from x = 0 to x = a. Solution Centroids of Common Shapes of Areas Centroids of Common Shapes of Areas Centroids of Common Shapes of Areas Composite Plates and Areas o Composite plates X W x W Y W y W o Composite area X A xA Y A yA Sample Problem For the plane area shown, determine the first moments with respect to the x and y axes and the location of the centroid. SOLUTION: • Divide the area into a triangle, rectangle, and semicircle with a circular cutout. • Calculate the first moments of each area with respect to the axes. • Find the total area and first moments of the triangle, rectangle, and semicircle. • Subtract the area and first moment of the circular cutout. • Compute the coordinates of the area centroid by dividing the first moments by the total area. Sample Problem • Find the total area and first moments of the triangle, rectangle, and semicircle. Subtract the area and first moment of the circular cutout. Qx 506.2 103 mm 3 Q y 757.7 103 mm 3 Sample Problem • Compute the coordinates of the area centroid by dividing the first moments by the total area. x A 757.7 103 mm 3 X A 13.828 103 mm 2 X 54.8 mm y A 506.2 103 mm 3 Y A 13.828 103 mm 2 Y 36.6 mm Determination of Centroids by Integration x A xdA x dxdy xel dA yA ydA y dxdy yel dA Double integration to find the first moment may be avoided by defining dA as a thin rectangle or strip. x A xel dA x A xel dA x A xel dA yA yel dA ax a x dy 2 yA yel dA x ydx y ydx 2 y a x dy 2r 1 2 cos r d 3 2 yA yel dA 2r 1 sin r 2 d 3 2 Sample Problem Determine by direct integration the location of the centroid of a parabolic spandrel. SOLUTION: • Determine the constant k. • Evaluate the total area. • Using either vertical or horizontal strips, perform a single integration to find the first moments. • Evaluate the centroid coordinates. Sample Problem SOLUTION: • Determine the constant k. y k x2 b b k a2 k 2 a b a y 2 x 2 or x 1 2 y1 2 a b • Evaluate the total area. A dA 3 a b b x y dx 2 x 2 dx 2 a 3 0 0a ab 3 a Sample Problem o Using vertical strips, perform a single integration to find the first moments. a b Q y xel dA xydx x 2 x 2 dx 0 a a b x4 a 2b 2 4 a 4 0 2 a y 1 b Q x yel dA ydx 2 x 2 dx 2 02a a b2 x5 ab 2 4 2a 5 0 10 Sample Problem o Or, using horizontal strips, perform a single integration to find the first moments. b 2 ax a x2 a x dy Q y xel dA dy 2 2 0 1 b 2 a 2 a 2 0 b 2 a b y dy 4 a Q x yel dA y a x dy y a 1 2 y1 2 dy b a 3 2 ab 2 ay 1 2 y dy 10 b 0 b Sample Problem o Evaluate the centroid coordinates. xA Q y ab a 2b x 3 4 3 x a 4 yA Q x ab ab 2 y 3 10 y 3 b 10 Practice Problem The figure shown is made from a piece of thin, homogeneous wire. Determine the location of its center of gravity. x = 10 in, y = 3 in. C 10 in. A B 24 in. Practice Problem Determine the x- and ycoordinates of the centroid of the 𝑎 shaded area. 𝑥 = 6 𝜋7𝑎− 1 𝑦= 𝜋−1 Practice Problem Locate the centroid of the shaded area. 𝑥 = 7.50 𝑖𝑛. 𝑦 = 5.08 𝑖𝑛. Solution 𝑦 𝑥𝐴 𝑦𝐴 in in3 in3 6 5 720 600 30 14 10/3 420 100 3 -14.14 6 1.273 -84.8 -18 4 -8 12 4 -96 -32 TOTALS 127.9 969 650 A 𝑥 Part in2 in 1 120 2 Practice Problem Determine the coordinates of the mass center of the bracket, which is constructed from sheet metal of uniform thickness. 𝑥 = 2.48 𝑖𝑛. 𝑦 = 2.71 𝑖𝑛. 𝑧 = −0.882 𝑖𝑛. Solution A 𝑥 𝑧 𝑥𝐴 Part 𝑦 in2 in in in in3 in3 in3 1 16 2 2 0 32 32 0 2 6 5 4/3 0 30 8 0 3 12 2 4 -1.5 24 48 -18 4 2p 2 4 -3.85 12.57 25.1 -24.2 5 -p 2 4 -3 -6.28 -12.56 9.42 92.3 100.6 -32.8 TOTALS 𝑦𝐴 𝑧𝐴 MecMovies http://web.mst.edu/~mecmovie/