Section 1.3 The second derivative
ΒΆMotivating Questions
How does the derivative of a function tell us whether the function is increasing or decreasing at a point or on an interval?
What can we learn by taking the derivative of the derivative (the second derivative) of a function f?
What does it mean to say that a function is concave up or concave down? How are these characteristics connected to certain properties of the derivative of the function?
What are the units of the second derivative? How do they help us understand the rate of change of the rate of change?
Preview Activity 1.3.1.
The position of a car driving along a straight road at time t in minutes is given by the function y=s(t) that is pictured in Figure 1.3.2. The car's position function has units measured in thousands of feet. For instance, the point (2,4) on the graph indicates that after 2 minutes, the car has traveled 4000 feet.
In everyday language, describe the behavior of the car over the provided time interval. In particular, you should carefully discuss what is happening on each of the time intervals [0,1], [1,2], [2,3], [3,4], and [4,5], plus provide commentary overall on what the car is doing on the interval [0,12].
On the lefthand axes provided in Figure 1.3.3, sketch a careful, accurate graph of y=sβ²(t).
What is the meaning of the function y=sβ²(t) in the context of the given problem? What can we say about the car's behavior when sβ²(t) is positive? when sβ²(t) is zero? when sβ²(t) is negative?
Rename the function you graphed in (b) to be called y=v(t). Describe the behavior of v in words, using phrases like βv is increasing on the interval β¦β and βv is constant on the interval β¦.β
Sketch a graph of the function y=vβ²(t) on the righthand axes provide in Figure 1.3.3. Write at least one sentence to explain how the behavior of vβ²(t) is connected to the graph of y=v(t).
Subsection 1.3.1 Increasing, decreasing, or neither
So far, we have used the words increasing and decreasing intuitively to describe a function's graph. Here we define these terms more formally.Definition 1.3.4.
Given a function f(x) defined on the interval (a,b), we say that f is increasing on (a,b) provided that for all x, y in the interval (a,b), if x<y, then f(x)<f(y). Similarly, we say that f is decreasing on (a,b) provided that for all x, y in the interval (a,b), if x<y, then f(x)>f(y).
Subsection 1.3.2 The Second Derivative
We are now accustomed to investigating the behavior of a function by examining its derivative. The derivative of a function f is a new function given by the ruleDefinition 1.3.6.
The second derivative is defined by the limit definition of the derivative of the first derivative. That is,
Subsection 1.3.3 Concavity
In addition to asking whether a function is increasing or decreasing, it is also natural to inquire how a function is increasing or decreasing. There are three basic behaviors that an increasing function can demonstrate on an interval, as pictured in Figure 1.3.7: the function can increase more and more rapidly, it can increase at the same rate, or it can increase in a way that is slowing down. Fundamentally, we are beginning to think about how a particular curve bends, with the natural comparison being made to lines, which don't bend at all. More than this, we want to understand how the bend in a function's graph is tied to behavior characterized by the first derivative of the function.Definition 1.3.10.
Let f be a differentiable function on an interval (a,b). Then f is concave up on (a,b) if and only if fβ² is increasing on (a,b); f is concave down on (a,b) if and only if fβ² is decreasing on (a,b).
Activity 1.3.2.
The position of a car driving along a straight road at time t in minutes is given by the function y=s(t) that is pictured in Figure 1.3.11. The car's position function has units measured in thousands of feet. Remember that you worked with this function and sketched graphs of y=v(t)=sβ²(t) and y=vβ²(t) in Preview Activity 1.3.1.
On what intervals is the position function y=s(t) increasing? decreasing? Why?
On which intervals is the velocity function y=v(t)=sβ²(t) increasing? decreasing? neither? Why?
Acceleration is defined to be the instantaneous rate of change of velocity, as the acceleration of an object measures the rate at which the velocity of the object is changing. Say that the car's acceleration function is named a(t). How is a(t) computed from v(t)? How is a(t) computed from s(t)? Explain.
What can you say about sβ²β² whenever sβ² is increasing? Why?
-
Using only the words increasing, decreasing, constant, concave up, concave down, and linear, complete the following sentences. For the position function s with velocity v and acceleration a,
on an interval where v is positive, s is .
on an interval where v is negative, s is .
on an interval where v is zero, s is .
on an interval where a is positive, v is .
on an interval where a is negative, v is .
on an interval where a is zero, v is .
on an interval where a is positive, s is .
on an interval where a is negative, s is .
on an interval where a is zero, s is .
Remember that a function is increasing on an interval if and only if its first derivative is positive on the interval.
See (a).
Remember that the first derivative of a function measures its instantaneous rate of change.
Think about how \(s''(t) = [s'(t)]'\text{.}\)
Be very careful with your letters: \(s\text{,}\) \(v\text{,}\) and \(a\text{.}\)
Increasing: \(0\lt t\lt 2\text{,}\) \(3\lt t\lt 5\text{,}\) \(7\lt t\lt 9\text{,}\) and \(10\lt t\lt 12\text{.}\) Decreasing: never.
Velocity is increasing on \(0\lt t\lt 1\text{,}\) \(3\lt t\lt 4\text{,}\) \(7\lt t\lt 8\text{,}\) and \(10\lt t\lt 11\text{;}\) \(y = v(t)\) is decreasing on \(1\lt t\lt 2\text{,}\) \(4\lt t\lt 5\text{,}\) \(8\lt t\lt 9\text{,}\) and \(11\lt t\lt 12\text{.}\) Velocity is neither constant on \(2\lt t\lt 3\text{,}\) \(5\lt t\lt 7\text{,}\) and \(9\lt t\lt 10\text{.}\)
\(a(t) = v'(t)\) and \(a(t) = s''(t)\text{.}\)
\(s''(t)\) is positive since \(s'(t)\) is increasing.
increasing.
decreasing.
constant.
increasing.
decreasing.
constant.
concave up.
concave down.
linear.
The position function \(y = s(t)\) increasing on the intervals \(0\lt t\lt 2\text{,}\) \(3\lt t\lt 5\text{,}\) \(7\lt t\lt 9\text{,}\) and \(10\lt t\lt 12\text{,}\) because at every point in such intervals, \(s'(t)\) is positive. For the provided function, \(s(t)\) is never decreasing because its derivative is never negative.
The velocity function \(y = v(t)\) appears to be increasing on the intervals \(0\lt t\lt 1\text{,}\) \(3\lt t\lt 4\text{,}\) \(7\lt t\lt 8\text{,}\) and \(10\lt t\lt 11\) because the curve \(y = s(t)\) is concave up which corresponds to an increasing first derivative \(y =s'(t)\text{.}\) Similarly, \(y = v(t)\) appears to be decreasing on the intervals \(1\lt t\lt 2\text{,}\) \(4\lt t\lt 5\text{,}\) \(8\lt t\lt 9\text{,}\) and \(11\lt t\lt 12\) because the curve \(y = s(t)\) is concave down which corresponds to a decreasing first derivative \(y =s'(t)\text{.}\) On the intervals \(2\lt t\lt 3\text{,}\) \(5\lt t\lt 7\text{,}\) and \(9\lt t\lt 10\text{,}\) the curve \(y = s(t)\) is constant, and thus linear, so neither concave up nor concave down.
Since \(a(t)\) is the instantaneous rate of change of \(v(t)\text{,}\) \(a(t) = v'(t)\text{.}\) And because \(v(t) = s'(t)\text{,}\) it follows that \(a(t) = v'(t) = [s'(t)]' = s''(t)\text{,}\) so acceleration is the second derivative of position.
Because \(s''(t)\) is the first derivative of \(s'(t)\text{,}\) when \(s'(t)\) is increasing, \(s''(t)\) must be positive.
-
For the position function \(s(t)\) with velocity \(v(t)\) and acceleration \(a(t)\text{,}\)
on an interval where \(v(t)\) is positive, \(s(t)\) is increasing.
on an interval where \(v(t)\) is negative, \(s(t)\) is decreasing.
on an interval where \(v(t)\) is zero, \(s(t)\) is constant.
on an interval where \(a(t)\) is positive, \(v(t)\) is increasing.
on an interval where \(a(t)\) is negative, \(v(t)\) is decreasing.
on an interval where \(a(t)\) is zero, \(v(t)\) is constant.
on an interval where \(a(t)\) is positive, \(s(t)\) is concave up.
on an interval where \(a(t)\) is negative, \(s(t)\) is concave down.
on an interval where \(a(t)\) is zero, \(s(t)\) is linear.
Activity 1.3.3.
A potato is placed in an oven, and the potato's temperature F (in degrees Fahrenheit) at various points in time is taken and recorded in the following table. Time t is measured in minutes. In (((Unresolved xref, reference "act-1-5-1"; check spelling or use "provisional" attribute)))Activity , we computed approximations to Fβ²(30) and Fβ²(60) using central differences. Those values and more are provided in the second table below, along with several others computed in the same way.
t | F(t) |
0 | 70 |
15 | 180.5 |
30 | 251 |
45 | 296 |
60 | 324.5 |
75 | 342.8 |
90 | 354.5 |
t | Fβ²(t) |
0 | NA |
15 | 6.03 |
30 | 3.85 |
45 | 2.45 |
60 | 1.56 |
75 | 1.00 |
90 | NA |
What are the units on the values of Fβ²(t)?
Use a central difference to estimate the value of Fβ²β²(30).
What is the meaning of the value of Fβ²β²(30) that you have computed in (b) in terms of the potato's temperature? Write several careful sentences that discuss, with appropriate units, the values of F(30), Fβ²(30), and Fβ²β²(30), and explain the overall behavior of the potato's temperature at this point in time.
Overall, is the potato's temperature increasing at an increasing rate, increasing at a constant rate, or increasing at a decreasing rate? Why?
Remember that the derivative's units are βunits of output per unit of input.β
-
To estimate \(g'(a)\text{,}\) we can use
\begin{equation*} g'(a) \approx \frac{g(a+h)-g(a-h)}{2h} \end{equation*}for an appropriate choice of \(h\text{.}\)
For each of the values \(F'(30)\) and \(F''(30)\text{,}\) think about what they tell you about expected upcoming behavior in \(F(t)\) and \(F'(t)\text{,}\) respectively.
Think concavity.
Degrees Fahrenheit per minute.
\(F''(30) \approx -0.119\text{.}\)
At the moment \(t = 30\text{,}\) the temperature of the potato is 251 degrees; its temperature is rising at a rate of 3.85 degrees per minute; and the rate at which the temperature is rising is falling at a rate of -0.119 degrees per minute per minute.
Iincreasing at a decreasing rate.
\(F'(t)\) has units measured in degrees Fahrenheit per minute.
-
Using a central difference,
\begin{equation*} F''(30) \approx \frac{F'(45)-F'(15)}{30} = \frac{2.45-6.03}{30} \approx -0.119\text{.} \end{equation*} The value \(F''(30) \approx -0.119\text{,}\) which is measured in degrees per minute per minute tells us, along with the other data, that at the moment \(t = 30\text{,}\) the temperature of the potato is 251 degrees, that its temperature is rising at a rate of 3.85 degrees per minute, and that the rate at which the temperature is rising is falling at a rate of -0.119 degrees per minute per minute. That is, while the temperature is rising, it is rising at a slower and slower rate. At \(t = 31\text{,}\) we'd expect that the rate of increase of the potato's temperature would have dropped to about 3.73 degrees per minute.
The potato's temperature increasing at a decreasing rate because the values of the first derivative of \(F\) are falling. Equivalently, this is because the value of \(F''(t)\) is negative throughout the given time interval.
Activity 1.3.4.
This activity builds on our experience and understanding of how to sketch the graph of fβ² given the graph of f.
In Figure 1.3.14, given the respective graphs of two different functions f, sketch the corresponding graph of fβ² on the first axes below, and then sketch fβ²β² on the second set of axes. In addition, for each, write several careful sentences in the spirit of those in Activity 1.3.2 that connect the behaviors of f, fβ², and fβ²β². For instance, write something such as
fβ² is on the interval , which is connected to the fact that f is on the same interval , and fβ²β² is on the interval.
but of course with the blanks filled in. Throughout, view the scale of the grid for the graph of f as being 1Γ1, and assume the horizontal scale of the grid for the graph of fβ² is identical to that for f. If you need to adjust the vertical scale on the axes for the graph of fβ² or fβ²β², you should label that accordingly.
Subsection 1.3.4 Summary
A differentiable function f is increasing at a point or on an interval whenever its first derivative is positive, and decreasing whenever its first derivative is negative.
By taking the derivative of the derivative of a function f, we arrive at the second derivative, fβ²β². The second derivative measures the instantaneous rate of change of the first derivative. The sign of the second derivative tells us whether the slope of the tangent line to f is increasing or decreasing.
A differentiable function is concave up whenever its first derivative is increasing (or equivalently whenever its second derivative is positive), and concave down whenever its first derivative is decreasing (or equivalently whenever its second derivative is negative). Examples of functions that are everywhere concave up are y=x2 and y=ex; examples of functions that are everywhere concave down are y=βx2 and y=βex.
The units on the second derivative are βunits of output per unit of input per unit of input.β They tell us how the value of the derivative function is changing in response to changes in the input. In other words, the second derivative tells us the rate of change of the rate of change of the original function.
Exercises 1.3.5 Exercises
ΒΆ1. Comparing f,fβ²,fβ²β² values.
2. Signs of f,fβ²,fβ²β² values.
3. Acceleration from velocity.
4. Rates of change of stock values.
5. Interpreting a graph of fβ².
6.
Suppose that y=f(x) is a differentiable function for which the following information is known: f(2)=β3, fβ²(2)=1.5, fβ²β²(2)=β0.25.
Is f increasing or decreasing at x=2? Is f concave up or concave down at x=2?
Do you expect f(2.1) to be greater than β3, equal to β3, or less than β3? Why?
Do you expect fβ²(2.1) to be greater than 1.5, equal to 1.5, or less than 1.5? Why?
Sketch a graph of y=f(x) near (2,f(2)) and include a graph of the tangent line.
\(f\) is increasing and concave down at \(x=2\text{.}\)
Greater.
Less.
Since \(f'(2)\) is positive, \(f\) is increasing at \(x=2\text{,}\) and since \(f''(2)\) is negative, \(f\) is concave down at \(x=2\text{.}\)
We are given that \(f(2) = -3\text{.}\) Since \(f\) is increasing at \(x=2\text{,}\) we expect \(f(2.1)\) to be greater than \(f(2) = -3\text{.}\)
Since \(f''(2)\) is negative, we also know that \(f'\) is decreasing at \(2\text{.}\) Thus, we expect that \(f'(2.1)\) will be less than \(f'(2) = 1.5\text{.}\)
-
In the following figure, we see a possible graph of a function \(f\) that passes through the point \((2,-3)\) at an instantaneous rate of \(f'(2) = 1.5\) and in such a way that its second derivative at \(x = 2\) is negative.
7.
For a certain function y=g(x), its derivative is given by the function pictured in Figure 1.3.15.
What is the approximate slope of the tangent line to y=g(x) at the point (2,g(2))?
How many real number solutions can there be to the equation g(x)=0? Justify your conclusion fully and carefully by explaining what you know about how the graph of g must behave based on the given graph of gβ².
On the interval β3<x<3, how many times does the concavity of g change? Why?
Use the provided graph to estimate the value of gβ²β²(2).
\(g'(2) \approx 1.4\text{.}\)
At most one.
\(9\text{.}\)
\(g''(2) \approx 5.5 \text{.}\)
From the given graph of \(g'\text{,}\) we may estimate that \(g'(2) \approx 1.4\text{.}\)
Observe that \(g'\) is always positive. This tells us that \(g\) must be always increasing. Therefore, it follows that \(g\) may cross the \(x\)-axis at most one time, and hence there can be at most one solution to \(g(x) = 0\text{.}\)
On \(-3 \lt x \lt 3\text{,}\) \(g\) changes concavity \(g\) times, since \(g'\) changes from increasing to decreasing or from decreasing to increasing \(9\) times. Whenever \(g'\) is increasing, \(g''\) is positive, and whenever \(g'\) is decreasing, \(g''\) is negative. Hence, whenever \(g'\) changes from increasing to decreasing or vice versa, it follows that \(g''\) changes sign, and this causes a change in the concavity of \(g\text{.}\) We therefore see that while \(g\) is always increasing, \(g\) changes concavity many times.
-
From the given graph of \(g'\text{,}\) we can see that \(g'(1.9) \approx 0.9\) and \(g'(2.1) \approx 2\text{.}\) Using a central difference, it follows that
\begin{equation*} g''(2) \approx \frac{g'(2.1) - g(1.9)}{0.2} \approx \frac{2-0.9}{0.2} = 5.5\text{.} \end{equation*}
8.
A bungee jumper's height h (in feet ) at time t (in seconds) is given in part by the table:
t | 0.0 | 0.5 | 1.0 | 1.5 | 2.0 | 2.5 | 3.0 | 3.5 | 4.0 | 4.5 | 5.0 |
h(t) | 200 | 184.2 | 159.9 | 131.9 | 104.7 | 81.8 | 65.5 | 56.8 | 55.5 | 60.4 | 69.8 |
t | 5.5 | 6.0 | 6.5 | 7.0 | 7.5 | 8.0 | 8.5 | 9.0 | 9.5 | 10.0 |
h(t) | 81.6 | 93.7 | 104.4 | 112.6 | 117.7 | 119.4 | 118.2 | 114.8 | 110.0 | 104.7 |
Use the given data to estimate hβ²(4.5), hβ²(5), and hβ²(5.5). At which of these times is the bungee jumper rising most rapidly?
Use the given data and your work in (a) to estimate hβ²β²(5).
What physical property of the bungee jumper does the value of hβ²β²(5) measure? What are its units?
Based on the data, on what approximate time intervals is the function y=h(t) concave down? What is happening to the velocity of the bungee jumper on these time intervals?
\(h'(4.5) \approx 14.3\text{;}\) \(h'(5) \approx 21.2\text{;}\) \(h'(5.5) \approx = 23.9\text{;}\) rising most rapidly at \(t = 5.5\text{.}\)
\(h'(5) \approx 9.6 \text{.}\)
Acceleration of the bungee jumper in feet per second per second.
\(0 \lt t \lt 2\text{,}\) \(6 \lt t \lt 10\text{.}\)
-
We will use central differences to estimate the requested derivatives. Thus,
\begin{align*} h'(4.5) &\approx \frac{h(5)-h(4)}{5-4} = \frac{69.8 - 55.5}{1} = 14.3\\ h'(5) &\approx \frac{h(5.5)-h(4.5)}{5.5-4.4} = \frac{81.6 - 60.4}{1} = 21.2\\ h'(5.5) &\approx \frac{h(6)-h(5)}{6-5} = \frac{93.7 - 69.8}{1} = 23.9 \end{align*}The bungee jumper is rising most rapidly at \(t = 5.5\text{,}\) at a rate of about \(23.9\) feet per second.
-
We again use a central difference, this time to estimate \(h''(5)\text{.}\) Observe that
\begin{equation*} h''(5) \approx \frac{h'(5.5) - h'(4.5)}{5.5-4.5} = \frac{23.9 - 14.3}{1} = 9.6 \end{equation*} \(h''(5)\) measures the acceleration of the bungee jumper at \(t = 5\text{.}\) Thus we know that the person's acceleration at \(t = 5\) is about \(9.6\) feet per second per second. This measures the instantaneous rate of change of the jumper's velocity.
The bungee jumper's position function is concave down on approximately the intervals \(0 \lt t \lt 2\text{,}\) \(6 \lt t \lt 10\text{.}\) This is easiest to see if we plot the data using a program like Excel. On these time intervals, the velocity of the jumper is decreasing: either the velocity is positive and getting less positive, or negative and getting more negative.
9.
For each prompt that follows, sketch a possible graph of a function on the interval β3<x<3 that satisfies the stated properties.
y=f(x) such that f is increasing on β3<x<3, concave up on β3<x<0, and concave down on 0<x<3.
y=g(x) such that g is increasing on β3<x<3, concave down on β3<x<0, and concave up on 0<x<3.
y=h(x) such that h is decreasing on β3<x<3, concave up on β3<x<β1, neither concave up nor concave down on β1<x<1, and concave down on 1<x<3.
y=p(x) such that p is decreasing and concave down on β3<x<0 and is increasing and concave down on 0<x<3.
-
When a function is increasing, its graph rises as \(x\) increases, and when a function is concave up, that means the graph of the function looks bowl-shaped. A graph of an increasing, concave up function \(f\) on the interval \((-3,0)\) is shown in the following figure. When a function is concave down, its graph looks like an upside down bowl. The function \(f\) in the figure is increasing and concave down on the interval \((0,3)\) Note that \(f\) is also increasing on the entire interval \((-3,3)\text{.}\)
-
When a function is increasing, its graph rises as \(x\)increases, and when a function is concave down, that means the graph of the function looks like an upside down bowl. A graph of an increasing, concave down function \(g\) on the interval \((-3,0)\) is shown in the figure below. When a function is concave up, its graph looks like a bowl. The function \(g\) in the figure is increasing and concave up on the interval \((0,3)\text{.}\) Note that \(g\) is also increasing on the entire interval \((-3,3)\text{.}\)
-
When a function is decreasing, its graph falls as \(x\) increases, and when a function is concave up, that means the graph of the function looks bowl-shaped. A graph of a decreasing, concave up function \(h\) on the interval \((-3,-1)\)is shown in the figure below. To be neither concave up or concave down, a graph has to be linear, so the graph of \(h\) in the following figure is a straight line on \((-1,1)\text{.}\) When a function is concave down, its graph looks like an upside down bowl. The function \(h\) in the figure is decreasing and concave down on the interval \((1,3)\text{.}\) Note that \(h\) is decreasing on the entire interval \((-3,3)\text{.}\)
-
When a function is decreasing, its graph falls as \(x\) increases, and when a function is concave down, that means the graph of the function looks like an upside down bowl. A graph of a decreasing, concave down function \(p\) on the interval \((-3,0)\) is shown in the figure below. When a function is increasing, its graph rises as \(x\) increases. The function \(p\) in the figure is increasing and concave down on the interval \((0,3)\text{.}\)