Upload
sam
View
235
Download
0
Embed Size (px)
Citation preview
7/23/2019 Chap12_Sec2 [Compatibility Mode]
1/45
VECTOR FUNCTIONS
7/23/2019 Chap12_Sec2 [Compatibility Mode]
2/45
VECTOR FUNCTIONS
Later in this chapter, we are going to use
vector functions to describe the motion of
planets and other objects through space.
Here, we prepare the way by developing
the calculus of vector functions.
7/23/2019 Chap12_Sec2 [Compatibility Mode]
3/45
12.2
Derivatives and Integrals
of Vector Functions
In this section, we will learn how to:
Develop the calculus of vector functions.
VECTOR FUNCTIONS
7/23/2019 Chap12_Sec2 [Compatibility Mode]
4/45
The derivative r of a vector function
is defined in much the same way as for
real-valued functions.
DERIVATIVES
7/23/2019 Chap12_Sec2 [Compatibility Mode]
5/45
if this limit exists.
DERIVATIVE
0
( ) ( )'( ) lim
h
d t h t t
dt h
+ = =
r r rr
Equation 1
7/23/2019 Chap12_Sec2 [Compatibility Mode]
6/45
The geometric significance
of this definition is shown asfollows.
DERIVATIVE
7/23/2019 Chap12_Sec2 [Compatibility Mode]
7/45
If the points Pand Q have position
vectors r(t) and r(t + h), then represents
the vector r(t + h) r(t).
This can thereforebe regarded as
a secant vector.
SECANT VECTOR
PQuuur
7/23/2019 Chap12_Sec2 [Compatibility Mode]
8/45
If h > 0, the scalar multiple (1/h)(r(t + h) r(t))
has the same direction as r(t + h) r(t).
As h 0, it appears
that this vector
approaches a vector
that lies on the
tangent line.
DERIVATIVES
7/23/2019 Chap12_Sec2 [Compatibility Mode]
9/45
For this reason, the vector r(t) is called
the tangent vector to the curve defined by r
at the point P,
provided:
r(t) exists
r(t) 0
TANGENT VECTOR
7/23/2019 Chap12_Sec2 [Compatibility Mode]
10/45
The tangent line to Cat Pis defined to be
the line through Pparallel to the tangent
vector r(t).
TANGENT LINE
7/23/2019 Chap12_Sec2 [Compatibility Mode]
11/45
We will also have occasion to consider
the unit tangent vector:
UNIT TANGENT VECTOR
'( )( )| '( ) |
tT tt
= rr
7/23/2019 Chap12_Sec2 [Compatibility Mode]
12/45
The following theorem gives us
a convenient method for computing
the derivative of a vector function r:
Just differentiate each component of r.
DERIVATIVES
7/23/2019 Chap12_Sec2 [Compatibility Mode]
13/45
If r(t) = f(t), g(t), h(t) = f(t) i + g(t)j + h(t) k,
where f, g, and h are differentiable functions,
then:
r(t) = f(t), g(t), h(t)
= f(t) i + g(t)j + h(t) k
DERIVATIVES Theorem 2
7/23/2019 Chap12_Sec2 [Compatibility Mode]
14/45
DERIVATIVES Proof
0
0
0
0 0 0
'( )
1lim [ ( ) ( )]
1lim ( ), ( ), ( ), ( ), ( ), ( )
( ) ( ) ( ) ( ) ( ) ( )lim , ,
( ) ( ) ( ) ( ) ( ) ( )lim , lim , lim
t
t
t
t t t
t
t t t
t
f t t g t t h t t f t g t h tt
f t t f t g t t g t h t t h tt t t
f t t f t g t t g t h t t h t
t t
= +
= + + +
+ + + =
+ + + =
r
r r
'( ), '( ), '( )t
f t g t h t
=
7/23/2019 Chap12_Sec2 [Compatibility Mode]
15/45
a. Find the derivative of
r(t) = (1 + t3) i + tetj + sin 2t k
b. Find the unit tangent vector at the point
where t= 0.
DERIVATIVES Example 1
7/23/2019 Chap12_Sec2 [Compatibility Mode]
16/45
According to Theorem 2, we differentiate
each component of r:
r(t) = 3t2 i + (1 t)etj + 2 cos 2t k
DERIVATIVES Example 1 a
7/23/2019 Chap12_Sec2 [Compatibility Mode]
17/45
As r(0) = i and r(0) =j + 2k, the unit tangent
vector at the point (1, 0, 0) is:
DERIVATIVES Example 1 b
'(0) 2
(0) | '(0) | 1 4
1 2
5 5
+
= = +
= +
r j k
T r
k
7/23/2019 Chap12_Sec2 [Compatibility Mode]
18/45
For the curve ,
find r(t) and sketch the position vector r(1)
and the tangent vector r(1).
DERIVATIVES Example 2
( ) (2 )t t t= + r i j
7/23/2019 Chap12_Sec2 [Compatibility Mode]
19/45
We have:
and
DERIVATIVES Example 2
1'( )2
1'(1)
2
tt
=
=
r i j
r i j
7/23/2019 Chap12_Sec2 [Compatibility Mode]
20/45
The curve is a plane curve.
Elimination of the parameter from
the equations , y = 2 t gives:
y = 2 x2, x 0
DERIVATIVES Example 2
x t=
7/23/2019 Chap12_Sec2 [Compatibility Mode]
21/45
The position vector r(1) = i +j starts
at the origin.
The tangent vector r(1)
starts at the
corresponding point
(1, 1).
DERIVATIVES Example 2
7/23/2019 Chap12_Sec2 [Compatibility Mode]
22/45
Find parametric equations for the tangent line
to the helix with parametric equations
x = 2 cos t y = sin t z = t
at the point (0, 1, /2).
DERIVATIVES Example 3
7/23/2019 Chap12_Sec2 [Compatibility Mode]
23/45
The vector equation of the helix is:
r(t) = 2 cos t, sin t, t
Thus,
r
(t) =
2 sin t, cos t, 1
DERIVATIVES Example 3
7/23/2019 Chap12_Sec2 [Compatibility Mode]
24/45
The parameter value corresponding to
the point (0, 1, /2) is t= /2.
So, the tangent vector there is:
r(/2) = 2, 0, 1
DERIVATIVES Example 3
7/23/2019 Chap12_Sec2 [Compatibility Mode]
25/45
The tangent line is the line through
(0, 1, /2) parallel to the vector 2, 0, 1.
So, by Equations 2 in Section 12.5,
its parametric equations are:
DERIVATIVES
2 12
x t y z t
= = = +
Example 3
7/23/2019 Chap12_Sec2 [Compatibility Mode]
26/45
The helix and the tangent line in
the Example 3 are shown.
DERIVATIVES
7/23/2019 Chap12_Sec2 [Compatibility Mode]
27/45
Just as for real-valued functions,
the second derivative of a vector function r
is the derivative of r, that is, r = (r).
For instance, the second derivative
of the function in Example 3 is:
r(t) =2 cos t, sin t, 0
SECOND DERIVATIVE
7/23/2019 Chap12_Sec2 [Compatibility Mode]
28/45
The next theorem shows that
the differentiation formulas for real-valued
functions have their counterparts for
vector-valued functions.
DIFFERENTIATION RULES
7/23/2019 Chap12_Sec2 [Compatibility Mode]
29/45
Suppose:
u and v are differentiable vector functions
c is a scalar
fis a real-valued function
DIFFERENTIATION RULES Theorem 3
7/23/2019 Chap12_Sec2 [Compatibility Mode]
30/45
Then,
DIFFERENTIATION RULES
1. [ ( ) ( )] ( ) ( )
2. [ ( )] ( )
3. [ ( ) ( )] '( ) ( ) ( ) ( )
dt t t t
dt
d c t c t dt
df t t f t t f t t
dt
+ = +
=
= +
u v u' v'
u u'
u u u'
Theorem 3
7/23/2019 Chap12_Sec2 [Compatibility Mode]
31/45
DIFFERENTIATION RULES Theorem 3
( )
4. [ ( ) ( )] ( ) ( ) ( ) ( )
5. [ ( ) ( )] ( ) ( ) ( ) ( )
6. [ ( ( ))] '( ) ( ) (Chain Rule)
dt t t t t t
dt
dt t t t t t
dt
d f t f t f tdt
= +
= +
=
u v u' v u v'
u v u' v u v'
u u'
7/23/2019 Chap12_Sec2 [Compatibility Mode]
32/45
This theorem can be proved either:
Directly from Definition 1
By using Theorem 2 and the corresponding
differentiation formulas for real-valued functions
DIFFERENTIATION RULES
7/23/2019 Chap12_Sec2 [Compatibility Mode]
33/45
The proof of Formula 4 follows.
The remaining are left as exercises.
DIFFERENTIATION RULES
7/23/2019 Chap12_Sec2 [Compatibility Mode]
34/45
Let
u(t) = f1(t), f2(t), f3(t)
v(t) = g1(t), g2(t), g3(t)
Then,
FORMULA 4 Proof
1 1 2 2 3 3
3
1
( ) ? ( )
( ) ( ) ( ) ( ) ( ) ( )
( ) ( )i i
i
t t
f t g t f t g t f t g t
f t g t=
= + +
=
u v
7/23/2019 Chap12_Sec2 [Compatibility Mode]
35/45
So, the ordinary Product Rule gives:
FORMULA 4 Proof
3
1
3
1
3
1
3 3
1 1
[ ( ) ( )] ( ) ( )
[ ( ) ( )]
[ '( ) ( ) ( ) '( )]
'( ) ( ) ( ) '( )
( ) ( ) ( ) ( )
i i
i
i i
i
i i i i
i
i i i i
i i
d dt t f t g t
dt dt d
f t g tdt
f t g t f t g t
f t g t f t g t
t t t t
=
=
=
= =
=
=
= +
= +
= +
u v
u' v u v'
7/23/2019 Chap12_Sec2 [Compatibility Mode]
36/45
Show that, if |r(t)| = c(a constant),
then r(t) is orthogonal to r(t) for all t.
DIFFERENTIATION RULES Example 4
7/23/2019 Chap12_Sec2 [Compatibility Mode]
37/45
Since
r(t) r(t) = |r(t)|2 = c2
and c2 is a constant,
Formula 4 of Theorem 3 gives:
DIFFERENTIATION RULES Example 4
0 [ ( ) ( )]
'( ) ( ) ( ) '( )2 '( ) ( )
dt t
dt
t t t t
t t
=
= + =
r r
r r r r
r r
7/23/2019 Chap12_Sec2 [Compatibility Mode]
38/45
Thus,
r(t) r(t) = 0
This says that r(t) is orthogonal to r(t).
DIFFERENTIATION RULES
7/23/2019 Chap12_Sec2 [Compatibility Mode]
39/45
Geometrically, this result says:
If a curve lies on a sphere with center
the origin, then the tangent vector r(t) is
always perpendicular to the position vector r(t).
DIFFERENTIATION RULES
7/23/2019 Chap12_Sec2 [Compatibility Mode]
40/45
The definite integral of a continuous vector
function r(t) can be defined in much the same
way as for real-valued functionsexcept that
the integral is a vector.
INTEGRALS
7/23/2019 Chap12_Sec2 [Compatibility Mode]
41/45
However, then, we can express
the integral of rin terms of the integrals
of its component functions f, g, and h
as follows.
We use the notation of Chapter 5.
INTEGRALS
7/23/2019 Chap12_Sec2 [Compatibility Mode]
42/45
INTEGRALS
*
1
* *
1 1
*
1
( )
lim ( )
lim ( ) ( )
( )
b
a
n
in
i
n n
i in
i i
n
i
i
t dt
t t
f t t g t t
h t t
=
= =
=
=
= +
+
r
r
i j
k
7/23/2019 Chap12_Sec2 [Compatibility Mode]
43/45
Thus,
This means that we can evaluate an integral of a vector
function by integrating each component function.
INTEGRALS
r(t)dta
b
= f(t)dta
b
i + g(t)dta
b
j + h(t)dta
b
k
7/23/2019 Chap12_Sec2 [Compatibility Mode]
44/45
We can extend the Fundamental Theorem
of Calculus to continuous vector functions:
Here, R is an antiderivative of r, that is, R(t) = r(t).
We use the notation r(t) dtfor indefinite integrals(antiderivatives).
INTEGRALS
]b
a(t) ( ) ( ) ( )
b
adt t b a= = r R R R
7/23/2019 Chap12_Sec2 [Compatibility Mode]
45/45
If r(t) = 2 cos t i + sin tj + 2t k, then
where:
C is a vector constant of integration
INTEGRALS Example 5
r(t)dt = 2cos t dt( )i + sint dt( )j + 2t dt( )k= 2sin ti cos tj + t2k + C
2
2 200
2
( ) [2sin cos ]
2
4
t dt t t t
= +
= + +
r i j k
i j k