# 35 Lecture

## Volume by Cylindrical Shells

The cylindrical shells method is particularly useful when the solid of revolution is formed by rotating a curve around a vertical or horizontal axis, and the integration can be set up using the cylindrical coordinates.

## Important Mcq's Midterm & Finalterm Prepration Past papers included

What is the formula for finding the volume of a solid using the cylindrical shells method?

a) V = 2?rh

b) V = 2?rh + 2?r^2

c) V = ?r^2h

d) V = ?r^2

When using the cylindrical shells method, what shape are the "shells" that are added up to find the volume of the solid?

a) Cylinders

b) Rectangles

c) Triangles

d) Spheres

When using the cylindrical shells method, what axis is typically used to form the cylinders?

a) x-axis

b) y-axis

c) z-axis

d) None of the above

Which of the following is a necessary step when using the cylindrical shells method to find the volume of a solid?

a) Find the limits of integration

b) Take the derivative of the function

c) Solve for the area under the curve

d) None of the above

Answer: a) Find the limits of integration

When using the cylindrical shells method, what is typically the function used to find the height of the shells?

a) The function that defines the curve rotated about the axis

b) The function that defines the axis of rotation

c) The function that defines the radius of the shell

d) None of the above

Answer: a) The function that defines the curve rotated about the axis

What is the typical range of the radius when using the cylindrical shells method?

a) 0 to the length of the curve

b) 0 to infinity

c) 0 to the height of the curve

d) None of the above

Answer: a) 0 to the length of the curve

When using the cylindrical shells method, what is the typical range of the height of the shells?

a) 0 to the length of the curve

b) 0 to infinity

c) 0 to the height of the curve

d) None of the above

Answer: c) 0 to the height of the curve

What is the formula for finding the volume of a cylindrical shell?

a) V = 2?rh

b) V = 2?rh + 2?r^2

c) V = ?r^2h

d) V = ?r^2

What is the main advantage of using the cylindrical shells method over other methods for finding volumes?

a) It is easier to set up

b) It is more accurate

c) It works for any solid of revolution

d) None of the above

Answer: c) It works for any solid of revolution

What is the typical shape of the cross-sections of the solid when using the cylindrical shells method?

a) Circles

b) Rectangles

c) Triangles

d) Spheres

## Subjective Short Notes Midterm & Finalterm Prepration Past papers included

What is the formula for finding the volume of a solid using cylindrical shells?

Answer: The formula is V = ?2?rh dx, where r is the radius of the shell, h is the height of the shell, and dx is the width of the shell.

What is the difference between cylindrical shells and disks/washers for finding volume?

Answer: Cylindrical shells are used to find the volume of a solid with a curved boundary, while disks/washers are used to find the volume of a solid with a flat boundary.

How do you determine the height of a cylindrical shell for volume calculation?

Answer: The height of the cylindrical shell is determined by subtracting the function value of the upper curve from the function value of the lower curve.

Can you use cylindrical shells to find the volume of a solid with a circular cross-section?

Answer: Yes, cylindrical shells can be used to find the volume of a solid with a circular cross-section, as long as the axis of rotation is perpendicular to the circular cross-section.

What is the formula for finding the radius of a cylindrical shell?

Answer: The radius of a cylindrical shell is determined by the distance between the axis of rotation and the curve being rotated.

Can you use cylindrical shells to find the volume of a solid with a non-uniform cross-section?

Answer: Yes, cylindrical shells can be used to find the volume of a solid with a non-uniform cross-section, as long as the axis of rotation is perpendicular to the cross-section.

How do you determine the width of a cylindrical shell for volume calculation?

Answer: The width of the cylindrical shell is determined by the size of the intervals used in the integration process.

What is the general process for finding the volume of a solid using cylindrical shells?

Answer: The general process involves identifying the axis of rotation, determining the limits of integration, finding the radius and height of each shell, calculating the volume of each shell using the cylindrical shell formula, and then summing the volumes of all the shells.

Can you use cylindrical shells to find the volume of a solid with a slanted boundary?

Answer: Yes, cylindrical shells can be used to find the volume of a solid with a slanted boundary, as long as the axis of rotation is perpendicular to the slanted boundary.

How does the volume calculation using cylindrical shells differ from the volume calculation using disks/washers?

Answer: The volume calculation using cylindrical shells involves summing the volumes of multiple shells, while the volume calculation using disks/washers involves summing the volumes of multiple disks/washers.

### Volume by Cylindrical Shells

Volume by cylindrical shells is a technique used in calculus to find the volume of a solid of revolution. The technique involves dividing the solid into infinitely thin cylindrical shells and finding the volume of each shell. The sum of the volumes of all the shells is then taken to find the total volume of the solid. The cylindrical shells method is particularly useful when the solid of revolution is formed by rotating a curve around a vertical or horizontal axis, and the integration can be set up using the cylindrical coordinates.

### To find the volume of the solid of revolution using cylindrical shells, the following steps are followed:

Step 1: Identify the axis of revolution, and draw a cross-section of the solid of revolution. Step 2: Divide the cross-section into infinitesimally thin cylindrical shells, each of width dx. Step 3: Find the radius of each cylindrical shell. This can be done by expressing the radius in terms of x, the distance from the axis of revolution. Step 4: Find the height of each cylindrical shell. This can be done by finding the difference between the two functions that define the solid of revolution. Step 5: Find the volume of each cylindrical shell. This can be done by multiplying the height, the circumference (2?r), and the thickness of each shell (dx). Step 6: Integrate the volume of all the cylindrical shells from x=a to x=b to find the total volume of the solid of revolution.

### Let's take an example to illustrate this method.

Example: Find the volume of the solid generated by rotating the curve y = x^2 + 2 about the x-axis between x = 0 and x = 3. Solution: Step 1: The axis of revolution is the x-axis. The cross-section of the solid is a parabola. Step 2: Divide the cross-section into infinitesimally thin cylindrical shells, each of width dx. Step 3: The radius of each cylindrical shell is x, the distance from the x-axis. Step 4: The height of each cylindrical shell is given by the difference between the functions that define the solid of revolution. In this case, the height is given by h = y = x^2 + 2. Step 5: The volume of each cylindrical shell is given by V = 2?rhdx = 2?x(x^2+2)dx. Step 6: The total volume of the solid of revolution is given by integrating the volume of all the cylindrical shells from x = 0 to x = 3: V = ?0^3 2?x(x^2+2)dx = 2?/4(x^4+4x^2)|0^3 = 2?/4(81+36) = 29.25?

### Therefore, the volume of the solid generated by rotating the curve y = x^2 + 2 about the x-axis between x = 0 and x = 3 is 29.25?.

In conclusion, the cylindrical shells method is a powerful technique used to find the volume of a solid of revolution. It involves dividing the solid into thin cylindrical shells, finding the radius and height of each shell, and then integrating the volume of all the shells to find the total volume of the solid. The method is particularly useful when the solid of revolution is formed by rotating a curve around a vertical or horizontal axis, and the integration can be set up using the cylindrical coordinates.