< Previous Section Home Next Section >

# Section 5.3Exact Net Accumulation Functions from Exact Rate of Change Functions

## 5.3.1 The Issue of "Good Enough" Approximations

The goal of this section is to define an exact net accumulation function given its exact rate of change function. To this point we developed a method for approximating an exact rate of change function’s exact net accumulation function (see Equation 5.2.6).

Values of the function A that we constructed are approximately equal to the exact net variation in values of the function f for every value in f’s domain at which f has a rate of change at a moment. We do not know the function f, but by knowing its exact rate of change function $r_f$ and the initial value of f, we are able to create an approximation of f by re-creating how values of f accumulate.

### When is $\Delta x$ "small enough" to produce approximations that are "close enough"?

A reasonable question is, "How well do values of A approximate the net variation in values of f given any value of $\Delta x$?"

In the highway elevation example of Section 5.2 it would be reasonable to say that approximations that are within 1 meter of the actual elevation are close enough. That would mean that if our approximations stop changing in the ones place once $\Delta x$ becomes smaller than some value, then that value is small enough to give approximations that are close enough to the exact value of f at that value of x.

Figure 5.3.1 uses the function round defined as $$\mathrm{round}(x,n)=\frac{\lfloor 10^n x+0.5 \rfloor}{10^n}$$ to round numbers to n decimal places. For example, $\mathrm{round}(152.3974, 2) = 152.40$, and $\mathrm{round}(152.3974, -1) = 150$.

Figure 5.3.1 shows that the value of A(1.5) rounded to 0 decimal places (to the nearest 1) stops changing when $\Delta x$ becomes less than about 0.008. So it appears that $\Delta x=0.007$ km is small enough for $1243+A(1.5)$ to be within 1 meter of $f(1.5)$. Given that $f(1.5)$ is within 1 meter of $1243+A(1.5)$, or 1308 meters, this means that we are within $\frac{1}{10}$ of 1% of the actual value of f. Notice that we can make this claim without knowing the actual value of f.

Figure 5.3.1. Searching for a value of $\Delta x$ that gives values of A(1.5) within 1 meter of f(1.5).

One issue that you will address in the exercises is whether a value of $\Delta x$ that produces a "close enough" value for A(1.5) also produces "close enough" values of A for other values of x.

While we will not delve into this issue in detail, the "goodness" of our approximation for a given value of $\Delta x$ depends on how rapidly the rate of change function varies within completed $\Delta x$-intervals. The more that $r_f$ varies, the smaller $\Delta x$ must be so that it is unproblematic to assume that $r_f$ is constant over $\Delta x$-intervals.

Put slightly differently, the more that $r_f$ varies over an interval, the more problematic it is to assume that $r_f$ is constant over that interval and hence the more problematic it is to assume that f is essentially linear over that interval.

Another perspective on "close enough" is when we seek a graph of an approximate accumulation function that is close enough to the graph of its exact accumulation function for us to understand the exact accumulation function’s behavior. To understand this graphical notion of "close enough" we’ll examine the case of an object falling under the influence of gravity.

### A Free-Falling Object

Free-falling bodies near the earth’s surface accelerate at -9.8 (m/sec)/sec. The value -9.8 ignores air resistance. The unit (m/sec)/sec means that we are tracking the object’s velocity in m/sec with respect to the number of seconds it has accelerated.

A positive acceleration means the object's velocity is increasing. It does not mean the object is going up. A negative acceleration means the object's velocity is decreasing. It does not mean the ball is going down.

Suppose the object is a metal ball. The fact that the ball accelerates at -9.8 (m/sec)/sec every moment of free-fall means that its velocity after t seconds has varied $-9.8t \text{ m/sec}$.

Let h be the function that relates the ball’s upward displacement from a given reference point with the number of seconds elapsed since the ball had a velocity of 0. The rate of change of h with respect to t after t seconds have elapsed is $r_h(t) = -9.8t$. That is, the ball's rate of change of displacement from its reference point with respect to the number of seconds accelerated is $r_h(t) = -9.8t$.

A velocity of 0 at a moment does not mean the ball had a displacement of 0 at that moment. The ball could have any displacement from its reference point at a moment its velocity is 0.

The function $r_h$ is the exact rate of change function for h. The fact that $r_h(2.3)=-22.54$ means that after 2.3 seconds, the ball’s displacement at that moment varies at the rate of $-22.54 \text{m/sec}$. "At that moment" means there is some interval of width $\Delta t$ that includes 2.3, and the ball’s height varies at an essentially constant rate of $-22.54\,\text{m/sec}$ over that interval of time.

Put another way, $r_h(2.3)=-22.54$ means the variation in the ball’s displacement from its reference point is $-22.54\,dt$ meters for values of $dt$ between 0 and $\Delta t$.

We can use the method developed in Section 5.2 to approximate h, the function that gives the ball’s exact displacement from its reference height at each value of t.

The statements for this approximation of h are in Equation 5.3.1. Download the GC file containing them by clicking here.

Equation 5.3.1. Approximation of h.

Figure 5.3.1 shows GC’s display of $y=A(x), x≥a$, with various values of $\Delta x$ and various values of a. Two things change within the animation in Figure 5.3.1: (1) The value of $\Delta t$ varies from 1, to 0.5, to 0.1, to 0.01 for a given value of a, and (2) the value of a varies from 0, to 2, to -1, to -3. We will discuss these two aspects of Figure 5.3.1 separately.

Figure 5.3.2 starts with $a=0$ and $\Delta t=1$. The piecewise linearity of A is evident. The animation then transits to $a=0$ and $\Delta t=0.5$, $a=0$ and $\Delta t=0.1$, and then $a=0$ and $\Delta t=0.01$.

The displayed graphs of $y=A(x)$ become smoother, and successive displayed graphs change less and less. The shape of the graph of $y=h(x)$, the graph of the exact accumulation of displacement with respect to time that $y=A(x)$ approximates, is clear. It starts at 0, decreases, and decreases faster with passing time. This makes perfect sense. As the metal ball falls, its speed (the absolute value of velocity) increases, so its distance fallen increases in successive $\Delta t$-intervals.

Figure 5.3.2 then displays the graph of $y=A(x)$ when $a=2$. Putting aside for the moment what $a=2$ might mean, the behavior of $y=A(x)$ again shows a definite a tendency to a smooth graph as $\Delta t$ becomes smaller. Similarly for $a=-1$ and for $a=-3$. The graphs of $y=A(x)$ appear smoother and the graphs appear to change less as $\Delta t$ becomes smaller.

Figure 5.3.2. Approximate displacement functions $y=A(x),\,a\le x$ for $r_h(t)=-9.8t$
with various values of $\Delta t$ and various values of a.

Figure 5.3.3 displays the graphs of $y=A(x)$ grouped by value of a. The first row shows $a=0$ with various values of $\Delta t$, the second row shows $a=2$ with various values of $\Delta t$, the third shows $a=-1$, and the fourth row shows $a=-3$.

 a) b) c) d)

Figure 5.3.3. Approximate displacement functions $y=A(x),\,a\le x$ for $r_h(t)=-9.8t$
with various values of $\Delta t$ and various values of a.

Each row of Figure 5.3.3 illustrates the same tendency as we saw in Figure 5.3.2, that the displayed graphs of $y=A(x)$ became smoother as $\Delta t$ became smaller.

This illustrates the second aspect of "close enough". Letting $\Delta t=0.01$ is sufficient for us to have a clear idea of the behavior of h, the exact net displacement function generated from $r_h(x)=-9.8x$ and $a=a_0$.

## 5.3.2 The meaning of values of a

We must understand what it means that $a=0$ to understand the meaning of $a=a_0$ for $a_0\neq 0$.

In the metal ball example, the value $t=0$ is the moment the ball's velocity is 0. It is not the moment we began recording variations in the ball's displacement. Keep in mind that the ball's velocity varies at the rate of $-9.8\text{ (m/sec)/sec}$ regardless of when we start recording variations in its displacement.

• $a=0$ means we started recording accumulated variations in the ball's displacement at the moment the ball's velocity was 0.
• $a=1$ means we started recording accumulated variations in the ball's displacement 1 second after the ball's velocity was 0.
• $a=-3$ means we started recording accumulated variations in the ball's displacement 3 seconds before the ball's velocity was 0.

The value of a in the ball example is the number of seconds before or after the ball's velocity is 0 that we began recording accumulated variations in the ball's displacement.

We need not start recording accumulated variations in h when t is 0. The meaning of $t=0$ is that we establish a reference time, and the meaning of a is the number of seconds before or after this reference time that we begin recording accumulated variations.

But we must also keep in mind that $r_h(a)$ is the ball’s velocity at $t=a$. That is, at $t=0$ the ball is momentarily at rest: $r_h(0)=0$ no matter the value of a.

In Row b of Figure 5.3.3, $a=2$ means only that we started recording accumulated variations in displacement 2 seconds after the ball had a velocity of 0. At the moment $t=2$, we begin recording accumulated displacements, so $h(2)=0$ because no displacements have accumulated. Zero seconds have passed since recording began (at $t=2$ seconds).

But the ball had a velocity of $-19.6 \text{ m/sec}$ at $t=2$. It is as if we started a video recording of the falling ball 2 seconds after the moment it had a velocity of $0 \text{ m/sec}$. The video would record how the ball's displacement varied during filming, but we would have no record of what the ball did before the video started.

In Row c of Figure 5.3.3 we see that $a=-1$. This value of a means that we started recording the ball’s displacement 1 second before the ball's velocity was 0. Given the ball's velocity varied at $-9.8 \text{ m/sec}$ during the one second preceding $t=0$, the ball must have had a positive velocity in the second prior to $t=0$.

Row d of Figure 5.3.3 illustrates the discussion of Row c more vividly. We leave it for you to explain Row d in the same way we explained Row c.

In short, the meaning of a value of a is that it is the moment in h’s domain where we start recording accumulated variations in the value of h or in the value of any function that approximates h.

When we graph the net accumulation function and decrease the value of a, we see more of the accumulation function's graph, taking into consideration that $A(a)=0$. When we graph the net accumulation function and increase the value of a, we see less of the accumulation function's graph, taking into consideration that $A(a)=0$.

### Initial Value of Accumulation

$A(x)$ approximates the accumulation that accrues starting from $x=a$. This means that $A(a)$ is always 0. This makes sense since at $x=a$ there has been no accumulation from a.

We must add the accumulation in f that occurred up to $x=a$ if we are to end with an approximate accumulation function for the exact accumulation function f at a value of x. But the accumulation in f up to $x=a$ is simply $f(a)$. Hence, for any value of x, $f(x)\approx f(a)+A(x)$.

We saw this in the Elevation from Rate example. The elevation function E was approximated by (elevation at the border) + (accumulated variations in elevation from the border).

The number 1243 was the value of $E(0)$, when $x=0$ km from the border. So the approximate elevation was $1243+A(x)$ meters at x meters from the border.

It is often the case that we do not know the initial value of accumulation. In those cases all we can do is approximate accumulated variations from the initial value of x, and therefore we can at best say that $f(x)\approx f(a)+A(x)$, where $f(a)$ is the accumulation function’s (often unknown) value at $x=a$.

## Exercise Set 5.3.2

1. Download this file and use it for parts (a)-(d) of this question. Adjust the scale of either set of axes as necessary.

a) Define $r_f$ as $r_f(x)=x^2+0.5$. Vary the value of a. Explain the behavior of $y=A(x)$ in terms of the value of a and the values of $r_f$.

b) Define $r_f$ as $r_f(x)=x^2-3x+1$. Vary the value of a. Explain the behavior of $y=A(x)$ in terms of the value of a and the values of $r_f$.

c) Define $r_f$ as $r_f(x)=x(x-1)(x-2)$. Vary the value of a. Explain the behavior of $y=A(x)$ in terms of the value of a and the values of $r_f$.

d) Define $r_f$ as $r_f(x)=2xe^{\dfrac{-x}{2}}$. Vary the value of a. Explain the behavior of $y=A(x)$ in terms of the value of a and the values of $r_f$.

2. Download this file and use it for parts (a)-(d) of this question. In each part, move the $\Delta x$ slider and observe GC’s displayed graph. Explain the effect of values of $\Delta x$ on the behavior of $y=r(x)$ and the behavior of $y’=A(x’)$.

a) $r_f(x) = x^2 + 0.5$

b) $r_f(x) = x^2 -3x+1$

c) $r_f(x) = x(x-1)(x-2)$

d) $r_f(x) = 2xe^{\dfrac{-x}{2}}$

## 5.3.3 Representing Exact Accumulation

### Accumulation versus Net Accumulation

It is worth revisiting the distinction between accumulation and net accumulation.

• If f is an accumulation function, then f(x) for any value of x gives the measure of the accumulating quantity at that value of x. All of it.
• In the situations we have dealt with, we developed a method for approximating the accumulation of quantities from some starting point. These approximations are the net accumulation in the quantity -- the accumulation from the reference point to a value of x, not the entire accumulation up to the value of x.

### Exact accumulation and exact net accumulation

We explored the idea of "close enough" approximations to an exact net accumulation function in Section 5.3.2. We settled the question of "close enough" to understand that "close enough" depends on the precision you need for your present purpose.

Given that today's computers have 64-bit operating systems, computers can represent numbers only to 19 significant base-ten digits.

Round-off errors accumulate every time a computer add two numbers, so at some point "close enough" becomes the practical matter that there is a limit to the computations that our computers can perform accurately.

Although we cannot compute accumulation functions exactly, we can nevertheless represent exact net accumulation functions for exact rate of change functions.

When we start with an exact rate of change function we can, in theory if not in practice, make $\Delta x$ so small that making it any smaller has no discernable effect on the value of $A(x)$, our approximation of the exact net accumulation from a to x.

We shall use the notation $A_f(x)$ to represent f's exact net accumulation from a to x. Using the accumulation function's name as a subscript follows the convention we use for differentiating between approximate and exact rate of change functions.

When $r_f$ is an exact rate of change function, any value $r_f(x)$ is an exact rate of change of f at the moment x.

The function f having an exact rate of change of $r_f(x)$ at a value of x means that f varies at essentially a constant rate of change over a small interval containing that value of x.

This means that we can, in theory, approximate the variation in f around that value of x to any degree of precision. We just need to make $\Delta x$ small enough so that $r_f(x)dx$ is essentially equal to the actual variation in f as $dx$ varies from 0 to $\Delta x$ over that $\Delta x$-interval.

The standard mathematical representation for the exact net accumulation over an interval from a to x of the function f that comes from the exact rate of change function $r_f$ is $$\int_a^x r_f(t)dt.$$

This representation of exact net accumulation from exact rate of change entails all the meanings in our representations of approximate accumulation functions.

• The symbol "$\int$" derives from Gottfried Leibniz' use of the German "s" for "summe" ("sum" in English).
• The letters a and x have the same meaning as in our approximate net accumulation function A. The value of a is the reference value where net accumulation begins; the value of x varies.
• The value of t varies from a to x for each value of x. Use any letter other than "a" or "x" in place of "t". The value of this letter varies from the value of a to the value of x.
• The expression $r_f(t)dt$ represents the accumulation that accrues at the rate of $r_f(t)$ as $dt$ varies infinitesimally.

$A_f$ represents the exact accumulation function that comes from $r_f$ over the interval from the value of a to the value of x. So, $$A_f(x)=\int_a^x r_f(t)dt.$$

The function $A_f$ is not necessarily the same as f, because the integral is the net accumulation in f over the interval [a, x].

Put another way, $A_f(x)$ is the accumulation due to f varying at the rate $r_f(t)$ over every moment t from a to x. It is not the entire accumulation up to x.  $A_f(x)$ omits the accumulation in f up to the value of a. Therefore, $$f(x) = \int_a^x r_f(t)dt + f(a).$$

Read the statement $A_f(x)=\int_a^x r_f(t)dt$ out loud as, "$A_f(x)$ is the net accumulation of variations in f due to f varying at the rate of $r_f(t)$ at every moment t from a to x".

## Thinking about how x and t vary in $\displaystyle{A_f(x)=\int_a^x r_f(t)dt}$

It is important to have an image of how x and t vary in $A_f(x)=\int_a^x r_f(t)dt$. In one sense, we take t as a variable and turn x into a parameter whose value varies.

For each value of x in the domain of f, t varies from a to x, and the accumulated variations in f as t varies produce the value $A_f(x)$. Then x takes on another value and t varies from a to x, to produce another value of $A_f(x)$. And so on as x varies smoothly from a.

The power of integral notation is that it embodies all the features that of our method for constructing the approximate accumulation function A.

• The initial reference point (a) is given at the bottom of  of the integral sign.
• The value to which we are accumulating variations in f is given at the top of the integral sign.
• The rate of change the accumulation is built from is part of the argument to the integral sign, as is the differential by which the accumulation varies.
• The product $r_f(t)dt$ represents the bit of variation in f that comes from f varying at a rate of $r_f(t)$ as $dt$ varies slightly.

## Integrals in GC

Figure 5.3.4 shows integral solutions to the problems in Sections 5.2.3 (Elevation from Rate) and 5.3.1 (Falling Metal Ball). Enter these equations into GC to practice using integral notation. Type ctrl-shift-I to get the integral sign in GC, or select Math/Integral. Press the tab key to move among the parts of the integral.

Study the definitions of E and h in Figure 5.3.4. Compare them to the approximate solutions in Sections 5.2.3 (Highway Elevation) and 5.3.1 (Falling Metal Ball) above. You should see all the details of the approximate accumulation function within the integral that represents the exact accumulation function.

Figure 5.3.4. GC understands the integral sign. This page from GC shows the displacement function h that we developed in Section 5.2.3

It takes awhile to appreciate the economy of integral notation. As we stated, all the details of our method of approximation are entailed in integral notation.

Put another way, the integral notation is self-contained. You don’t need to define parameters and auxiliary functions prior to the definition of the approximation function.

Integral notation has an added feature. It represents an exact accumulation function, in open form.

An open form definition of a function is one that gives a conceptual outline of the function’s meaning, but it does not give specific instructions for computing the function’s value in a definite number of steps.

A function is defined in closed form if its definition tells us how to compute values of the function in terms of a finite number of operations on values of variables or on values of familiar functions. Chapter 6 addresses the issue of obtaining accumulation functions in closed form from rate of change functions in closed form.

## Exercise Set 5.3.3

1. Let $r_f(x)=xe^{-x}$.
1. Explain, in your own words, the meaning of "$\int$" in $\int_a^x r_f(t)dt$.
2. Explain, in your own words, the roles of x and t in $\int_a^x r_f(t)dt$.
3. Explain, in your own words, the meaning of $r_f(t)dt$ in $\int_a^x r_f(t)dt$.
4. Explain, in your own words, the meaning of $\int_a^x r_f(t)dt$.
2. Answer Exercise 5.2.4, part (d), using integral notation.

3. Density of a material at a point is defined as the rate of change of mass with respect to volume. The density of a concrete block at a point in its interior is 67 kg/$\mathrm{cm^3}$. This means that if a small cube around that point were to increase in volume by 0.001 $\mathrm{cm}^3$, the mass within that volume would increase by (67)(0.001) kg.

A concrete beam is painted on one end. It has uniform density in any cross section, but the density of the cross sections varies with distance from its painted end. The beam is 10m long and has a cross-section of 12cm x 5 cm. The rate of change function for the beam’s mass with respect to its distance from its painted end is $r_m(d) = 67+12\cos(3d)\sin(4d) \text{ g/cm$^3$},\, 0\le d\le 10$ meters.

1. Define a function that gives the mass of the concrete beam as a function of distance from its painted end.
Graph your function in GC.
2. Click on a point on the graph. Record its coordinates. Interpret the coordinates in terms of the beam’s mass and in terms of $r_m$.
3. What is the beam’s mass that is made by the segment from 2 to 5 feet from its painted end?
4. We need to mark the place on the beam where 50% of its mass is on either side of the mark. Where should we place the mark?
4. (Adapted from Briggs & Cochran, Ch. 6.1, Problem 57).

• A Newton is the force required to accelerate a mass of 1 kg at the rate of 1 (m/sec)/sec.
• Energy is commonly measured in units of Joules (J), which is the work done by a force of one Newton when its point of application moves one meter in the direction of the force.
• The rate at which energy is used is known as power. Power is measured in J/sec. One J/sec is called a Watt.
• It is also common to measure an amount of energy in kilowatt-hours (kWh). One kWh is the amount of energy used when it is used at the rate of 1000 J/sec (1000 Watts) for one hour.
• A megawatt-hour is the amount of energy used when it is used at the rate of one million Watts (one million J/sec) for one hour.

5. A city consumes electrical energy on a given day at an approximate rate of $r_E(t)$, where $E(t)$ is the energy, in megawatt-hours, that the city consumes in t hours since midnight. The rate of change function $r_E$ is defined, in megawatts, as $$r_E(t) = 400 - 300\cos\left(\frac{\pi}{12}(t-5)\right)\text{ if }0 \le t.$$

a) Fill in the blanks in 5 different ways to make each statement true:

3 Megawatt-hours = The energy consumed at a rate of __________ Joules/sec for __________ hours.

3 Megawatt-hours = The energy consumed at a rate of __________ Joules/sec for __________ hours.

3 Megawatt-hours = The energy consumed at a rate of __________ Joules/sec for __________ hours.

3 Megawatt-hours = The energy consumed at a rate of __________ Joules/sec for __________ hours.

3 Megawatt-hours = The energy consumed at a rate of __________ Joules/sec for __________ hours.

b) Why is it sensible that $\dfrac{\pi}{12}(t-5)$ is the argument to cosine in this model of the city’s rate of electrical energy consumption? (Examine the graph of $r_E$.)

c) Define the function E that gives the electrical energy this city will have consumed x hours after midnight on a given day. Explain how your function produces a value that is in the desired units. Does the value of x have a necessary upper bound?

d) Approximately how much electrical energy does this city use in a typical day? In a typical week? In a typical month? In a typical year? Be sure to state quantity’s units.

e) This city’s electric utility company charges for electricity at the rate of $0.13 per kilowatt-hour for electricity used between 7:00a and 5:00p, and at the rate of$0.07 per kilowatt-hour otherwise. What is this city’s electrical bill for one day?

f) Burning 1 kg of coal produces about 450 kWh of energy. How many kg of coal are required to meet the energy needs of the city for one day? One year?

g) A wind turbine normally generates electricity at a rate of 200 kW. Approximately how many wind turbines would be required to meet the needs of this city for one day? One year?

h) A medium-sized household typically uses about 300-500 kilowatt-hours of electrical energy in a month during the fall. Residential consumption is typically about 30% of a major city’s electrical energy consumption in that same period. Approximately how large is this city’s population?

6. The discussion in Section 5.3.2 regarding the meaning of a in an approximate accumulation function applies to integrals, too. Explain why $$f(x) = f(a) + \int_a^x r_f(t)dt$$ for all values of x given any value of a in the domain of f.

 < Previous Section Home Next Section >