Multiple Integration

# 35 Calculating Centers of Mass and Moments of Inertia

### Learning Objectives

- Use double integrals to locate the center of mass of a two-dimensional object.
- Use double integrals to find the moment of inertia of a two-dimensional object.
- Use triple integrals to locate the center of mass of a three-dimensional object.

We have already discussed a few applications of multiple integrals, such as finding areas, volumes, and the average value of a function over a bounded region. In this section we develop computational techniques for finding the center of mass and moments of inertia of several types of physical objects, using double integrals for a lamina (flat plate) and triple integrals for a three-dimensional object with variable density. The density is usually considered to be a constant number when the lamina or the object is homogeneous; that is, the object has uniform density.

### Center of Mass in Two Dimensions

The center of mass is also known as the center of gravity if the object is in a uniform gravitational field. If the object has uniform density, the center of mass is the geometric center of the object, which is called the centroid. (Figure) shows a point as the center of mass of a lamina. The lamina is perfectly balanced about its center of mass.

To find the coordinates of the center of mass of a lamina, we need to find the moment of the lamina about the and the moment about the We also need to find the mass of the lamina. Then

Refer to Moments and Centers of Mass for the definitions and the methods of single integration to find the center of mass of a one-dimensional object (for example, a thin rod). We are going to use a similar idea here except that the object is a two-dimensional lamina and we use a double integral.

If we allow a constant density function, then give the *centroid* of the lamina.

Suppose that the lamina occupies a region in the and let be its density (in units of mass per unit area) at any point Hence, where and are the mass and area of a small rectangle containing the point and the limit is taken as the dimensions of the rectangle go to (see the following figure).

Just as before, we divide the region into tiny rectangles with area and choose as sample points. Then the mass of each is equal to ((Figure)). Let and be the number of subintervals in and respectively. Also, note that the shape might not always be rectangular but the limit works anyway, as seen in previous sections.

Hence, the mass of the lamina is

Let’s see an example now of finding the total mass of a triangular lamina.

Consider a triangular lamina with vertices and with density Find the total mass.

A sketch of the region is always helpful, as shown in the following figure.

Using the expression developed for mass, we see that

The computation is straightforward, giving the answer

Consider the same region as in the previous example, and use the density function Find the total mass.

Now that we have established the expression for mass, we have the tools we need for calculating moments and centers of mass. The moment about the for is the limit of the sums of moments of the regions about the Hence

Similarly, the moment about the for is the limit of the sums of moments of the regions about the Hence

Consider the same triangular lamina with vertices and with density Find the moments and

Use double integrals for each moment and compute their values:

The computation is quite straightforward.

Consider the same lamina as above, and use the density function Find the moments and

and

Finally we are ready to restate the expressions for the center of mass in terms of integrals. We denote the *x*-coordinate of the center of mass by and the *y*-coordinate by Specifically,

Again consider the same triangular region with vertices and with density function Find the center of mass.

Using the formulas we developed, we have

Therefore, the center of mass is the point

If we choose the density instead to be uniform throughout the region (i.e., constant), such as the value 1 (any constant will do), then we can compute the centroid,

Notice that the center of mass is not exactly the same as the centroid of the triangular region. This is due to the variable density of If the density is constant, then we just use (constant). This value cancels out from the formulas, so for a constant density, the center of mass coincides with the centroid of the lamina.

Again use the same region as above and the density function Find the center of mass.

and

Once again, based on the comments at the end of (Figure), we have expressions for the centroid of a region on the plane:

We should use these formulas and verify the centroid of the triangular region referred to in the last three examples.

Find the mass, moments, and the center of mass of the lamina of density occupying the region under the curve in the interval (see the following figure).

First we compute the mass We need to describe the region between the graph of and the vertical lines and

Now compute the moments and

Finally, evaluate the center of mass,

Hence the center of mass is

Calculate the mass, moments, and the center of mass of the region between the curves and with the density function in the interval

and

Find the centroid of the region under the curve over the interval (see the following figure).

To compute the centroid, we assume that the density function is constant and hence it cancels out:

Thus the centroid of the region is

Calculate the centroid of the region between the curves and with uniform density in the interval

### Moments of Inertia

For a clear understanding of how to calculate moments of inertia using double integrals, we need to go back to the general definition in Section The moment of inertia of a particle of mass about an axis is where is the distance of the particle from the axis. We can see from (Figure) that the moment of inertia of the subrectangle about the is Similarly, the moment of inertia of the subrectangle about the is The moment of inertia is related to the rotation of the mass; specifically, it measures the tendency of the mass to resist a change in rotational motion about an axis.

The moment of inertia about the for the region is the limit of the sum of moments of inertia of the regions about the Hence

Similarly, the moment of inertia about the for is the limit of the sum of moments of inertia of the regions about the Hence

Sometimes, we need to find the moment of inertia of an object about the origin, which is known as the polar moment of inertia. We denote this by and obtain it by adding the moments of inertia and Hence

All these expressions can be written in polar coordinates by substituting and For example,

Use the triangular region with vertices and and with density as in previous examples. Find the moments of inertia.

Using the expressions established above for the moments of inertia, we have

Again use the same region as above and the density function Find the moments of inertia.

and Also,

As mentioned earlier, the moment of inertia of a particle of mass about an axis is where is the distance of the particle from the axis, also known as the radius of gyration.

Hence the radii of gyration with respect to the the and the origin are

respectively. In each case, the radius of gyration tells us how far (perpendicular distance) from the axis of rotation the entire mass of an object might be concentrated. The moments of an object are useful for finding information on the balance and torque of the object about an axis, but radii of gyration are used to describe the distribution of mass around its centroidal axis. There are many applications in engineering and physics. Sometimes it is necessary to find the radius of gyration, as in the next example.

Consider the same triangular lamina with vertices and and with density as in previous examples. Find the radii of gyration with respect to the the and the origin.

If we compute the mass of this region we find that We found the moments of inertia of this lamina in (Figure). From these data, the radii of gyration with respect to the and the origin are, respectively,

Use the same region from (Figure) and the density function Find the radii of gyration with respect to the the and the origin.

and

Follow the steps shown in the previous example.

### Center of Mass and Moments of Inertia in Three Dimensions

All the expressions of double integrals discussed so far can be modified to become triple integrals.

If we have a solid object with a density function at any point in space, then its mass is

Its moments about the the and the are

If the center of mass of the object is the point then

Also, if the solid object is homogeneous (with constant density), then the center of mass becomes the centroid of the solid. Finally, the moments of inertia about the the and the are

Suppose that is a solid region bounded by and the coordinate planes and has density Find the total mass.

The region is a tetrahedron ((Figure)) meeting the axes at the points and To find the limits of integration, let in the slanted plane Then for and find the projection of onto the which is bounded by the axes and the line Hence the mass is

Consider the same region ((Figure)), and use the density function Find the mass.

Follow the steps in the previous example.

Suppose is a solid region bounded by the plane and the coordinate planes with density (see (Figure)). Find the center of mass using decimal approximation.

We have used this tetrahedron before and know the limits of integration, so we can proceed to the computations right away. First, we need to find the moments about the the and the

Hence the center of mass is

The center of mass for the tetrahedron is the point

Consider the same region ((Figure)) and use the density function Find the center of mass.

Check that and Then use from a previous checkpoint question.

We conclude this section with an example of finding moments of inertia and

Suppose that is a solid region and is bounded by and the coordinate planes with density (see (Figure)). Find the moments of inertia of the tetrahedron about the the and the

Once again, we can almost immediately write the limits of integration and hence we can quickly proceed to evaluating the moments of inertia. Using the formula stated before, the moments of inertia of the tetrahedron about the the and the are

and

Proceeding with the computations, we have

Thus, the moments of inertia of the tetrahedron about the the and the are respectively.

Consider the same region ((Figure)), and use the density function Find the moments of inertia about the three coordinate planes.

The moments of inertia of the tetrahedron about the the and the are respectively.

### Key Concepts

Finding the mass, center of mass, moments, and moments of inertia in double integrals:

- For a lamina with a density function at any point in the plane, the mass is
- The moments about the and are

- The center of mass is given by
- The center of mass becomes the centroid of the plane when the density is constant.
- The moments of inertia about the and the origin are

Finding the mass, center of mass, moments, and moments of inertia in triple integrals:

- For a solid object with a density function at any point in space, the mass is
- The moments about the the and the are

- The center of mass is given by
- The center of mass becomes the centroid of the solid when the density is constant.
- The moments of inertia about the the and the are

### Key Equations

**Mass of a lamina**

**Moment about the***x*-axis

**Moment about the***y*-axis

**Center of mass of a lamina**

and

In the following exercises, the region occupied by a lamina is shown in a graph. Find the mass of with the density function

is the triangular region with vertices and

is the triangular region with vertices

is the rectangular region with vertices and

is the rectangular region with vertices and

is the trapezoidal region determined by the lines and

is the trapezoidal region determined by the lines and

is the disk of radius centered at

is the unit disk;

is the region enclosed by the ellipse

is the region bounded by

is the region bounded by and

In the following exercises, consider a lamina occupying the region and having the density function given in the preceding group of exercises. Use a computer algebra system (CAS) to answer the following questions.

- Find the moments and about the and respectively.
- Calculate and plot the center of mass of the lamina.
**[T]**Use a CAS to locate the center of mass on the graph of

**[T]** is the triangular region with vertices and

a. b.

c.

**[T]** is the triangular region with vertices

**[T]** is the rectangular region with vertices

a. b.

c.

**[T]** is the rectangular region with vertices

**[T]** is the trapezoidal region determined by the lines

a. b.

c.

**[T]** is the trapezoidal region determined by the lines and

**[T]** is the disk of radius centered at

a. b.

c.

**[T]** is the unit disk;

**[T]** is the region enclosed by the ellipse

a. b.

c.

**[T]**

**[T]** is the region bounded by and

a. b.

c.

**[T]** is the region bounded by

In the following exercises, consider a lamina occupying the region and having the density function given in the first two groups of Exercises.

- Find the moments of inertia and about the and origin, respectively.
- Find the radii of gyration with respect to the and origin, respectively.

is the triangular region with vertices and

a. b.

is the triangular region with vertices and

is the rectangular region with vertices and

a. b.

is the rectangular region with vertices and

is the trapezoidal region determined by the lines and

a. b. and

is the trapezoidal region determined by the lines and

is the disk of radius centered at

a. b. and

is the unit disk;

is the region enclosed by the ellipse

a. b.

is the region bounded by

a. b.

is the region bounded by

Let be the solid unit cube. Find the mass of the solid if its density is equal to the square of the distance of an arbitrary point of to the

Let be the solid unit hemisphere. Find the mass of the solid if its density is proportional to the distance of an arbitrary point of to the origin.

The solid of constant density is situated inside the sphere and outside the sphere Show that the center of mass of the solid is not located within the solid.

Find the mass of the solid whose density is where

**[T]** The solid has density equal to the distance to the Use a CAS to answer the following questions.

- Find the mass of
- Find the moments about the and respectively.
- Find the center of mass of
- Graph and locate its center of mass.

a. b. c. d. the solid and its center of mass are shown in the following figure.

Consider the solid with the density function

- Find the mass of
- Find the moments about the and respectively.
- Find the center of mass of

**[T]** The solid has the mass given by the triple integral Use a CAS to answer the following questions.

- Show that the center of mass of is located in the
- Graph and locate its center of mass.

a. b. the solid and its center of mass are shown in the following figure.

The solid is bounded by the planes Its density at any point is equal to the distance to the Find the moments of inertia of the solid about the

The solid is bounded by the planes and Its density is where Show that the center of mass of the solid is located in the plane for any value of

Let be the solid situated outside the sphere and inside the upper hemisphere where If the density of the solid is find such that the mass of the solid is

The mass of a solid is given by where is an integer. Determine such the mass of the solid is

Let be the solid bounded above the cone and below the sphere Its density is a constant Find such that the center of mass of the solid is situated units from the origin.

The solid has the density Show that the moment about the is half of the moment about the

The solid is bounded by the cylinder the paraboloid and the where Find the mass of the solid if its density is given by

Let be a solid of constant density where that is located in the first octant, inside the circular cone and above the plane Show that the moment about the is the same as the moment about the

The solid has the mass given by the triple integral

- Find the density of the solid in rectangular coordinates.
- Find the moment about the

The solid has the moment of inertia about the given by the triple integral

- Find the density of
- Find the moment of inertia about the

a. b.

The solid has the mass given by the triple integral

- Find the density of the solid in rectangular coordinates.
- Find the moment about the

Let be the solid bounded by the the cylinder and the plane where is a real number. Find the moment of the solid about the if its density given in cylindrical coordinates is where is a differentiable function with the first and second derivatives continuous and differentiable on

A solid has a volume given by where is the projection of the solid onto the and are real numbers, and its density does not depend on the variable Show that its center of mass lies in the plane

Consider the solid enclosed by the cylinder and the planes and where and are real numbers. The density of is given by where is a differential function whose derivative is continuous on Show that if then the moment of inertia about the of is null.

**[T]** The average density of a solid is defined as where and are the volume and the mass of respectively. If the density of the unit ball centered at the origin is use a CAS to find its average density. Round your answer to three decimal places.

Show that the moments of inertia about the and respectively, of the unit ball centered at the origin whose density is are the same. Round your answer to two decimal places.

### Glossary

- radius of gyration
- the distance from an object’s center of mass to its axis of rotation