# Math 251 notes section 13.6 Center of Mass J. Lewis

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```Math 251 notes section 13.6
Center of Mass
J. Lewis
i  1, 2,3 then the center of mass is at
If three masses m1 , m 2 , m 3 are located at the points ( x i , y i )
_
_ _
_
( x , y ) where x the weighted average of the x - values and y is the weighted average of the y - values.
_
x
1 3
 xi mi ,
M i 1
1 3
_
y
 yi mi ,
M i 1
M  m1  m 2  m 3 .
In general, for a lamina in the shape of region D with continuous density function  ( x, y ),
_ _
_
the center of mass is ( x , y ) with x 
1
M

x ( x , y ) dA ,
_
y
D
1
M
 y ( x, y ) dA ,
D
M 
  ( x, y ) dA
D
Examples
Find the center of mass for the lamina described by the region D with density function  ( x, y ) .
1. D is the rectangle (0,0) and (2,3) as opposite corners,  ( x, y )  y .
2. D is the triangle with corners (0,0), (1,1) and (4,0),  ( x, y )  x .
3. D is the unit disk x 2  y 2  1,
 ( x, y )  K ( x 2  y 2 ) , K a constant
4. Find the moment of inertia about the x-axis which is I x 
 y
2
 ( x , y ) dA for
D
D bounded by y  sin x,
y  0, 0  x   and constant density function,  .
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