# How is integration useful in physics?

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How is integration useful in physics?
3/25/ :34 AM Integration is addition: we just change the notation [Actually, integration is given a different notation because it is a special kind of sum (we will get to that in a bit)] Motivation: Often, we will have an equation defined for a quantity of interest (e.g. magnetic field, moment of inertia, volume) only for a point, or for some shape. If we have more complex shapes, we can find the overall quantity of the system by adding (or integrating) up contributions from our point/shape formulation Addition Integration Itcanbeshown.com Sirajuddin, David © 2007 Microsoft Corporation. All rights reserved. Microsoft, Windows, Windows Vista and other product names are or may be registered trademarks and/or trademarks in the U.S. and/or other countries. The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after the date of this presentation. MICROSOFT MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS PRESENTATION. 1

Example: Calculating Area
Consider a function for 0 ≤ x ≤ 2p What is the area scribed by the curve? Itcanbeshown.com Sirajuddin, David

Example: Calculating Area
We can sum the areas of smaller shapes (e.g. rectangles) to find it Area f(xi) = height, Dx = width We have taken sample values at 4 points here, but this is not a good approximation How do we improve our approximation? Take more sample values, and decrease the width Itcanbeshown.com Sirajuddin, David

Example: Calculating Area
Try 5 values of f(x): Area f(xi) = height, Dx = width This is a better estimate! What happens if we take more samples and continue to decease the width Dx? Itcanbeshown.com Sirajuddin, David

Example: Calculating Area
We find as we sample more points, and continue to decrease the width, we find an exact area Area f(xi) = height, Dx = width, N = number of points sampled Itcanbeshown.com Sirajuddin, David

Example: Calculating Area
Specifically, we note we get an exact area in a special limit: Dx  0 Number of points sampled in f(xi), N  ∞, i.e. we sample every value of x In this limit, notation is changed So that, in all, can write We sometimes say that we “add up” differential elements Here, a differential element is one of our samples: f(xi)Dx. A differential element of length is defined as Dx = dx (this word is what the ‘d’ is for, it does not mean a derivative at all), specifically an element dx is defined to have zero length, Dx  0. The end! That is all integration is Itcanbeshown.com Sirajuddin, David

Generalized Area Calculation
Area of any shape: It is possible to rewrite this if a functon f is given Notice how integrating over dy is not useful here (see right) without some manipulation [f(y) is not one-to-one] In cylindrical coordinates: or (chain rule) Itcanbeshown.com Sirajuddin, David

Example: Calculating Volume
This generalizes to volumes directly, consider a paraboloid: This time, instead of adding up areas, we add up volumes DVi = AiDxi Itcanbeshown.com Sirajuddin, David © 2007 Microsoft Corporation. All rights reserved. Microsoft, Windows, Windows Vista and other product names are or may be registered trademarks and/or trademarks in the U.S. and/or other countries. The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after the date of this presentation. MICROSOFT MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS PRESENTATION. 8

Example: Calculating Volume
This generalizes to volumes directly, consider a paraboloid: This time, instead of adding up areas, we add up volumes DVi = AiDxi As Dx  0 (or DV  0), we set Dx = dx (or DV = dV), and we calculate an exact volume in our sum (integral) when we sample N  ∞ points (for this case, adding up disc volumes is convenient, width dx, area A) or (triple integral) (single integral) Itcanbeshown.com Sirajuddin, David © 2007 Microsoft Corporation. All rights reserved. Microsoft, Windows, Windows Vista and other product names are or may be registered trademarks and/or trademarks in the U.S. and/or other countries. The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after the date of this presentation. MICROSOFT MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS PRESENTATION. 9

Generalized Volume Calculation
Volume of any shape: It is possible to rewrite this if the area A can be found as a function of x, y, or z…for the example on the right we can sum discs in the x direction, or cylindrical shells in the y and z directions In other coordinate systems Discs: or Cyl. Shells: or (Cylindrical) (Spherical) Itcanbeshown.com Sirajuddin, David

Example: Stalactites/Stalagmites
Stalactites/stalagmites are found in limestone caves. They are formed from rainwater percolating through the soil to cave ceilings, where they dissolve limestone and “pull” it downward (Shown here, ~ 500,000 years of formation) What is the mass of a single stalactite? Let us model it as the following: Stalactite is ~ cone of height H, radius R Its mass density r [mass/volume] changes linearly with depth z r(z) = Az + B, where A,B are constants (can be negative) Then, the mass Is this reasonable? Check units: dV = [volume], r(z) = [mass/volume], then Notice that since the density is different at each z, we cannot just multiply some density by the total volume, but instead have to add up the product of each volume element at every z with the density at every z (i.e. integrate) Carlsbad Caverns, NM [Wayfaring Travel Guide] Itcanbeshown.com Sirajuddin, David

Example: Stalactites/Stalagmites
Geometry is convenient to add up discs Disc radius = r (this changes with z!) We can take r to be x, or y (see figure), either is equivalent Disc height/thickness = dz  each disc has a volume dV = dAdz = py2dz Where dA = area of the circular face of a disc of radius r = y We put a ‘d’ in front of dz, dA, and dV just to mean ‘differential’ This is just the language, the letter does not do anything (not a derivative) Radius r is the same as y, we notice y changes linearly with z, If it helps, turn the picture on its side to find this equation So that, Think about this! This gives us the distance in y measured from the center of the cone (a radius). Not only for one value, this function gives us the radius at any value of z we want Itcanbeshown.com Sirajuddin, David