How to Find a Moment of Two Functions in Calculus

A moment in calculus is related to the center of mass of an object at a given point in space. The center of mass of an object, with a given density, is the point where the object would be perfectly balanced if it were suspended from that point in relationship to a defined axis. Moments can be applied to a lamina of variable density and are given by taking the double integral over the region occupied by the object and under the function of a given density multiplied by the distance from either the x or y-axis.

Instructions

  1. Defining a Moment About the X-Axis

    • 1

      Evaluate the bounds of the x and y-coordinates to define the limits of the area of the region occupied by a specific lamina. If either the x or y bound involve a function, use it as your inner bound and use the constant values of the other as your outer bound. If both x and y limits are constants, the order of bounds is irrelevant.

    • 2

      Multiply the given density at the point (x,y) on the lamina by the distance of the point (x,y) from the y-axis. Set-up a double integral using the density multiplied by the y-coordinate as the function to be integrated and the area using the respective x and y-coordinates as the limitation bounds.

    • 3

      Integrate the function once by taking its anti-derivative and solving for the inner bound limits. Solve by integrating the function again by taking its anti-derivative and solving for the outer bound limits.

    Defining a Moment About the Y-Axis

    • 4

      Evaluate the bounds of the x and y-coordinates to define the limits of the area of the region occupied by a specific lamina. If either the x or y bound involve a function, use it as your inner bound and use the constant values of the other as your outer bound. If both x and y limits are constants, the order of bounds is irrelevant.

    • 5

      Multiply the given density at the point (x,y) on the lamina by the distance of the point (x,y) from the x-axis. Set-up a double integral using the density multiplied by the x-coordinate as the function to be integrated and the area using the respective x and y-coordinates as the limitation bounds.

    • 6

      Integrate the function once by taking its anti-derivative and solving for the inner bound limits. Solve by integrating the function again by taking its anti-derivative and solving for the outer bound limits.

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