Math Problem Statement

Evaluate the line integral ∫ C F ⋅ d

r where F

⟨ 4 sin ⁡ x , − cos ⁡ y , 10 x z ⟩ and C is the path given by r ( t )

( − 3 t 3 , 3 t 2 , 2 t ) for 0 ≤ t ≤ 1

∫ C F ⋅ d

r

Solution

To evaluate the line integral CFdr\int_C \mathbf{F} \cdot d\mathbf{r}, we proceed as follows:

Given:

F=4sinx,cosy,10xz,\mathbf{F} = \langle 4\sin x, -\cos y, 10xz \rangle, and the path r(t)=(3t3,3t2,2t)\mathbf{r}(t) = (-3t^3, 3t^2, 2t) for 0t10 \leq t \leq 1.

Step 1: Parametrize F\mathbf{F}

Substitute r(t)\mathbf{r}(t) into F\mathbf{F}: x=3t3,y=3t2,z=2t.x = -3t^3, \quad y = 3t^2, \quad z = 2t.

Thus, F(r(t))=4sin(3t3),cos(3t2),10(3t3)(2t).\mathbf{F}(\mathbf{r}(t)) = \langle 4\sin(-3t^3), -\cos(3t^2), 10(-3t^3)(2t) \rangle.

Simplify: F(r(t))=4sin(3t3),cos(3t2),60t4.\mathbf{F}(\mathbf{r}(t)) = \langle 4\sin(-3t^3), -\cos(3t^2), -60t^4 \rangle.

Step 2: Compute drd\mathbf{r}

The derivative of r(t)\mathbf{r}(t) with respect to tt is: r(t)=ddt(3t3,3t2,2t)=9t2,6t,2.\mathbf{r}'(t) = \frac{d}{dt}(-3t^3, 3t^2, 2t) = \langle -9t^2, 6t, 2 \rangle.

Thus, dr=9t2,6t,2dt.d\mathbf{r} = \langle -9t^2, 6t, 2 \rangle \, dt.

Step 3: Compute Fdr\mathbf{F} \cdot d\mathbf{r}

The dot product F(r(t))r(t)\mathbf{F}(\mathbf{r}(t)) \cdot \mathbf{r}'(t) is: F(r(t))r(t)=4sin(3t3),cos(3t2),60t49t2,6t,2.\mathbf{F}(\mathbf{r}(t)) \cdot \mathbf{r}'(t) = \langle 4\sin(-3t^3), -\cos(3t^2), -60t^4 \rangle \cdot \langle -9t^2, 6t, 2 \rangle.

Perform the dot product: F(r(t))r(t)=(4sin(3t3))(9t2)+(cos(3t2))(6t)+(60t4)(2).\mathbf{F}(\mathbf{r}(t)) \cdot \mathbf{r}'(t) = (4\sin(-3t^3))(-9t^2) + (-\cos(3t^2))(6t) + (-60t^4)(2).

Simplify each term: 36t2sin(3t3)6tcos(3t2)120t4.-36t^2 \sin(-3t^3) - 6t\cos(3t^2) - 120t^4.

Step 4: Integrate over t[0,1]t \in [0, 1]

The integral becomes: CFdr=01(36t2sin(3t3)6tcos(3t2)120t4)dt.\int_C \mathbf{F} \cdot d\mathbf{r} = \int_0^1 \big( -36t^2 \sin(-3t^3) - 6t\cos(3t^2) - 120t^4 \big) \, dt.

Split the integral:

CFdr=0136t2sin(3t3)dt016tcos(3t2)dt01120t4dt.\int_C \mathbf{F} \cdot d\mathbf{r} = \int_0^1 -36t^2 \sin(-3t^3) \, dt - \int_0^1 6t\cos(3t^2) \, dt - \int_0^1 120t^4 \, dt.

Solve each term:

  1. First term: Let u=3t3u = -3t^3, so du=9t2dtdu = -9t^2 \, dt: 0136t2sin(3t3)dt=0336sin(u)du9=034sin(u)du.\int_0^1 -36t^2 \sin(-3t^3) \, dt = \int_0^{-3} -36 \sin(u) \frac{du}{-9} = \int_0^{-3} 4 \sin(u) \, du.

The integral of sin(u)\sin(u) is: sin(u)du=cos(u).\int \sin(u) \, du = -\cos(u).

Evaluate: [ \int_0^{-3} 4 \sin(u) , du = -4\cos(u) \Big|_0^{-3} = -4[\cos(-3) - \cos(0)] = -4[\cos(-3) - 1]. ]

  1. Second term: Let v=3t2v = 3t^2, so dv=6tdtdv = 6t \, dt: 016tcos(3t2)dt=03cos(v)dv.\int_0^1 6t \cos(3t^2) \, dt = \int_0^3 \cos(v) \, dv.

The integral of cos(v)\cos(v) is: cos(v)dv=sin(v).\int \cos(v) \, dv = \sin(v).

Evaluate: 03cos(v)dv=sin(v)03=sin(3)sin(0)=sin(3).\int_0^3 \cos(v) \, dv = \sin(v) \Big|_0^3 = \sin(3) - \sin(0) = \sin(3).

  1. Third term: 01120t4dt=12001t4dt.\int_0^1 120t^4 \, dt = 120 \int_0^1 t^4 \, dt.

The integral of t4t^4 is: t4dt=t55.\int t^4 \, dt = \frac{t^5}{5}.

Evaluate: 12001t4dt=120[t55]01=120155=24.120 \int_0^1 t^4 \, dt = 120 \left[\frac{t^5}{5}\right]_0^1 = 120 \cdot \frac{1^5}{5} = 24.

Step 5: Combine the results

CFdr=4[cos(3)1]sin(3)24.\int_C \mathbf{F} \cdot d\mathbf{r} = -4[\cos(-3) - 1] - \sin(3) - 24.

Simplify further if needed.


Do you want detailed explanations for any specific step or further simplification? Here are some related questions for further exploration:

  1. What is the physical interpretation of a line integral in vector fields?
  2. How can we verify line integral results using computational tools?
  3. What are the advantages of parameter substitution in line integrals?
  4. How does the orientation of the curve affect the line integral?
  5. What happens if the vector field is conservative?

Tip: Always check if the vector field is conservative before solving; it can simplify the computation significantly.

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Math Problem Analysis

Mathematical Concepts

Vector Calculus
Line Integrals
Parametric Equations
Dot Product

Formulas

F ⋅ dr = F(r(t)) ⋅ r'(t) dt
r'(t) = d/dt r(t)
Line integral: ∫C F ⋅ dr = ∫a^b F(r(t)) ⋅ r'(t) dt

Theorems

Fundamental Theorem of Line Integrals (if F is conservative)
Change of Variables (u-substitution)

Suitable Grade Level

Undergraduate - Calculus III