7 3 Volumes Disk and Washer Methods Suppose

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7. 3 Volumes Disk and Washer Methods

7. 3 Volumes Disk and Washer Methods

Suppose I start with this curve. My boss at the ACME Rocket Company has

Suppose I start with this curve. My boss at the ACME Rocket Company has assigned me to build a nose cone in this shape. So I put a piece of wood in a lathe and turn it to a shape to match the curve.

How could we find the volume of the cone? One way would be to

How could we find the volume of the cone? One way would be to cut it into a series of thin slices (flat cylinders) and add their volumes. The volume of each flat cylinder (disk) is: In this case: r= the y value of the function thickness = a small change in x = dx

The volume of each flat cylinder (disk) is: If we add the volumes, we

The volume of each flat cylinder (disk) is: If we add the volumes, we get:

This application of the method of slicing is called the disk method. The shape

This application of the method of slicing is called the disk method. The shape of the slice is a disk, so we use the formula for the area of a circle to find the volume of the disk. If the shape is rotated about the x-axis, then the formula is: A shape rotated about the y-axis would be:

The region between the curve , and the y-axis is revolved about the y-axis.

The region between the curve , and the y-axis is revolved about the y-axis. Find the volume. y x We use a horizontal disk. The thickness is dy. The radius is the x value of the function. volume of disk

The natural draft cooling tower shown at left is about 500 feet high and

The natural draft cooling tower shown at left is about 500 feet high and its shape can be approximated by the graph of this equation revolved about the y-axis: The volume can be calculated using the disk method with a horizontal disk.

Example: Using calculus, derive the formula for finding the volume of a sphere of

Example: Using calculus, derive the formula for finding the volume of a sphere of radius r. The region bounded by a semicircle and its diameter shown below is revolved about the x-axis, which gives us a sphere of radius r. Area of each cross section? (circle) What is y? –r dx r Equation of semicircle:

Example: Using calculus, derive the formula for finding the volume of a sphere of

Example: Using calculus, derive the formula for finding the volume of a sphere of radius r. Area of each cross section? (circle) Volume: –r r (Remember r is just a number!!!)

Example: Using calculus, derive the formula for finding the volume of a sphere of

Example: Using calculus, derive the formula for finding the volume of a sphere of radius r. Volume: –r r

Example: Find the volume of the solid generated when the region bounded by y

Example: Find the volume of the solid generated when the region bounded by y = x 2, x = 2, and y = 0 is rotated about the line x = 2. Area of each cross section? (circle) What is r? dy r=2–x r 2 Remember, we are using a dy here!!! So,

Example: Find the volume of the solid generated when the region bounded by y

Example: Find the volume of the solid generated when the region bounded by y = x 2, x = 2, and y = 0 is rotated about the line x = 2. Area of each cross section? (circle) Volume: r 2 Bounds? From 0 to intersection (y-value!!!)

The region bounded by and is revolved about the y-axis. Find the volume. If

The region bounded by and is revolved about the y-axis. Find the volume. If we use a horizontal slice: The “disk” now has a hole in it, making it a “washer”. The volume of the washer is: outer radius inner radius

This application of the method of slicing is called the washer method. The shape

This application of the method of slicing is called the washer method. The shape of the slice is a circle with a hole in it, so we subtract the area of the inner circle from the area of the outer circle. The washer method formula is:

If the same region is rotated about the line x=2: The outer radius is:

If the same region is rotated about the line x=2: The outer radius is: r R The inner radius is: