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Path independence and how to calculate the line integral of a conservative vector field.

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If C is a smooth curve given by the vector function r(t)=x(t)i+y(t)j,atb is a conservative vector field with continuous components, and f is a differentiable function such that F=f; i.e., f is a potential function of F. Then
CF·dr=Cf·dr=f(r(b))-f(r(a))=f(x(b),y(b))-f(x(a),y(a)) .

The Fundamental Theorem for Line Integrals has many important uses, including the following.

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A cell phone manufacturer can use the Fundamental Theorem for Line Integrals to determine the amount of kinetic energy your cell phone will have if you drop it from a certain height.
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A NASA physicist planning to launch a satellite from the surface of the Earth into low Earth orbit can use the Fundamental Theorem for Line Integrals to find the work done by gravity on this satellite in order to determine how much fuel will be necessary for the rocket carrying the satellite to complete its mission.
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A hydroelectric engineer can use the Fundamental Theorem for Line Integrals to find the amount of kinetic energy a waterfall will have at the bottom of the fall in order to determine how much electricity can be produced from this waterfall.

LINE INTEGRALS

Without using the FTC for Line Integrals


C F d r =
C P d x + Q d y
=
0 0.01
( 4 t 4 e 4 t + 6 t 3 e 3 t + 4 t 3 e 4 t + 6 t 2 e 3 t ) d t
=
0.00
=
e4 +2e3

Using the FTC for Line Integrals


C F d r
= f ( x ( 0.01 ) , y ( 0.01 ) ) f ( x ( 0 ) , y ( 0 ) )
=0.01 4e4×0.01 +2×e3×
= e4 +2e3
Time
0.01
Without using FTC for Line Integrals
Using FTC for Line Integrals





Path
C 1 F d r = 0.000000

INSTRUCTIONS

Consider the vector field
F
= ( 3 x 2 + 2 x y 2 ) i
+ ( 2 x 2 y + 3 y 2 ) j .

The function
f ( x , y ) = x 3 + x 2 y 2 + y 3
is a potential function of
F . Here we will consider two ways of evaluating
C F d r , where
C = { ( x , y ) :
x ( t ) = t e t ,
y ( t ) = t e t ,
0 t 1 } .

i. C F d r
= C P d x
+ Q d y ,


where P = 3 x 2 + 2 x y 2
and Q = 2 x 2 y + 3 y 2 .

ii. Using the Fundamental Theorem for Line Integrals.

PRACTICE QUESTIONS

1. Without using the Fundamental Theorem for Line Integrals, what technique(s) of integration will be used? Select all that apply.
2. For what values of t do the two methods of evaluating the integral differ ?
3. Which of the following are advantages of evaluating line integrals by using the Fundamental Theorem for Line Integrals?

INSTRUCTIONS

Select a path, then move the “Time” slider to answer the following questions.

PRACTICE QUESTIONS

1. How do the line integrals along all four paths from t = 0 to t = 0.5  compare?
2. How do the line integrals along all four paths from t = 0 to t = 1 compare?
3. Is this vector field a conservative vector field on [ 0 , 1 ] × [ 0 , 1 ] ?

INSTRUCTIONS

Consider the vector field
F = y x 2 + y 2 i + x x 2 + y 2 j .
Move the sliders to change the location and shape of the closed curve to answer the following questions.

PRACTICE QUESTIONS

1. Move the sliders so the center is located at ( 5 , 5 ) . Based only on the values obtained by moving the sliders, for what values of the “Horizontal width from center” and “Vertical width from center” is
C F d r 0 ?
2. Move the sliders so the “Horizontal width from center” is 2 and the “Vertical width from center” is 3. For what locations of the center is C F d r 0 ? Select all that apply.
3. Is this line integral independent of path on D = [ - 10 , 10 ] × [ - 10 , 10 ]
? Select all that apply.
Result

x can be any number from to