Arc Length and Curvature
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By the end of this session, you will be able to:
- Review core multivariable mathematical concepts and engineering calculus prerequisites.
π Micro-Lecture
Engineering Context: Shaping Motion Paths
In this session, we investigate the mathematical principles of Arc Length and Curvature and explore how engineers apply these concepts to analyze real-world physical and structural systems.
Skill Block 1
Explanations and derivations of the core concepts.
Worked Example
Given the line segment given by r(t)=(3t2β7)i^+(β4t2β10)j^β:
- Find the arc length s for 0β€tβ€4.
- Solve for the arc length function s(t) and evaluate arc length s for 0β€tβ€4.
- Find the arc length parametrization, r(s).
π‘ Reveal Worked Solution
Here is the complete step-by-step solution to the problem presented in the image.
Given
First, let's find the derivative vector rβ²(t) and its magnitude β£β£rβ²(t)β£β£, as they are required for all three parts:
Now, find the magnitude (speed):
Since our domain is 0β€tβ€4, t is non-negative, so:
1.
Find the arc length s for 0β€tβ€4
Using the definite integral formula for arc length:
2.
Solve for the arc length function s(t) and evaluate arc length s for 0β€tβ€4
To find the arc length function s(t) starting from a=0:
Evaluating this function at the upper limit t=4:
Answer: * Arc length function: s(t)=5t2
- Evaluated arc length: 80
3.
Find the arc length parametrization, r(s)
Following the parametrization steps:
- Invert the arc length function to solve for t in terms of s:
Since tβ₯0:
- Substitute t(s) back into the original position vector r(t):
Answer: The arc length parametrization is r(s)=(53βsβ7)i^+(β54βsβ10)j^β for 0β€sβ€80.
Active Practice Problem
Given the position vector for a helix shape r(t)=3sin(t)i^+3cos(t)j^β:
- Find the arc length s for 0β€tβ€4Ο.
- Solve for the arc length function s(t) and evaluate arc length s for 0β€tβ€4Ο.
- Find the arc length parametrization, r(s).
Workspace
Solution
Given
First, let's find the velocity vector rβ²(t) and its magnitude β£β£rβ²(t)β£β£:
Now, compute the magnitude (speed):
Using the fundamental trigonometric identity cos2(t)+sin2(t)=1:
1. Find the arc length s for 0β€tβ€4Ο
Using the definite integral formula for arc length:
Answer: The arc length s is 12Ο (or approximately 37.70).
2. Solve for the arc length function s(t) and evaluate arc length s for 0β€tβ€4Ο
To find the arc length function s(t) starting from a=0:
Evaluating this function at the upper limit t=4Ο:
Answer: * Arc length function: s(t)=3t
- Evaluated arc length: 12Ο
3. Find the arc length parametrization, r(s)
Following the arc length parametrization steps:
- Invert the arc length function to solve for t in terms of s:
- Substitute t(s) back into the original position vector r(t):
Answer: The arc length parametrization is r(s)=3sin(3sβ)i^+3cos(3sβ)j^β for 0β€sβ€12Ο.
Extra Practice & Extensions
- Solve for the arc length from 0β€tβ€Ο.
- Explain whether or not a closed domain such as this can or cannot be used to perform an arc length parametrization of the position vector r(s).
Refer to the 'Core Theory' tab or review the Worked Example demonstration for this skill.
π Regroup 1
- Review common misconceptions and clarify key notations.
Skill Block 2
Explanations and derivations of the core concepts.
Worked Example
Given the position vector r(t)=β¨4t2,(β4tβ3),β2tβ©:
- Solve for the unit tangent vector.
- Solve for the curvature at t=1.
π‘ Reveal Worked Solution
Given
First, let's find the first derivative vector rβ²(t) and its magnitude:
Now, compute the magnitude (speed):
We can factor out a 4 from under the radical:
1. Solve for the unit tangent vector
The formula for the unit tangent vector is T^(t)=β£β£rβ²(t)β£β£rβ²(t)β. Dividing each component of rβ²(t) by its magnitude:
Simplify by dividing each term by 2:
2. Solve for the curvature at t=1
Since we already have the position vector, it is much more efficient to use the cross-product formula for curvature:
- Find the second derivative vector rβ²β²(t):
- Compute the cross product rβ²(t)Γrβ²β²(t):
- Find the magnitude of this cross product:
- Evaluate β£β£rβ²(t)β£β£ at t=1:
- Calculate the curvature ΞΊ(1):
To rationalize the denominator:
- Curvature at t=1: ΞΊ(1)=2121β25ββ (or approximately 0.0464)
Active Practice Problem
Given the position vector r=β¨2cos(t),2sin(t),tβ©:
- Solve for the unit tangent vector.
- Solve for the radius of curvature.
Workspace
Solution
Given
First, let's find the first derivative vector rβ²(t) and its magnitude:
Now, compute the magnitude (speed):
Using the fundamental identity sin2(t)+cos2(t)=1:
1. Solve for the unit tangent vector
The formula for the unit tangent vector is T^(t)=β£β£rβ²(t)β£β£rβ²(t)β. Dividing each component of rβ²(t) by its constant magnitude:
2. Solve for the radius of curvature
To find the radius of curvature Ο=ΞΊ1β, we first need to determine the curvature ΞΊ. Since we have a fully worked out unit tangent vector T^(t), we can use the derivative method:
- Find T^β²(t):
- Compute its magnitude β£β£T^β²(t)β£β£:
- Calculate curvature ΞΊ:
- Calculate the radius of curvature Ο:
Answer: * Unit tangent vector: T^(t)=β¨β5β2βsin(t),5β2βcos(t),5β1ββ©
- Radius of curvature: Ο=25β=2.5
Extra Practice & Extensions
Refer to the 'Core Theory' tab or review the Worked Example demonstration for this skill.
π Regroup 2
- Reflect on the physical modeling applications and mathematical setups.
Skill Block 3
Explanations and derivations of the core concepts.
Worked Example
A particle is moving along the given path:
π‘ Reveal Worked Solution
Here is the step-by-step solution to the Skill 3 Demo problem shown in the image.
Given
To decompose the acceleration vector into its tangential component aTβ and normal component aNβ, we use the following formulas from your learning activity document:
Step 1: Find the Velocity Vector v(t) and evaluate at t=1
Take the first derivative of the position vector r(t):
Evaluating at t=1:
Step 2: Find the Speed β£β£v(1)β£β£
Compute the magnitude of the velocity vector at t=1:
Step 3: Find the Acceleration Vector a(t) and evaluate at t=1
Take the derivative of the velocity vector v(t):
Evaluating at t=1:
Step 4: Calculate the Tangential Component of Acceleration aTβ
Compute the dot product vβ a at t=1:
Now divide by the speed β£β£v(1)β£β£:
Step 5: Calculate the Normal Component of Acceleration aNβ
Since v and a are 2D vectors lying in the xy-plane, we can treat them as 3D vectors with a z-component of 0 to find their cross product:
Find the magnitude of this cross product vector:
Now divide by the speed β£β£v(1)β£β£:
Answer
The decomposition of the acceleration vector at t=1 yields:
Tangential component: aTβ=3577β22834β (approx. 381.82)
Normal component: aNβ=3577β5880β (approx. 98.32)
Active Practice Problem
Given the position vector r(t)=βt5i^β7t2j^β, decompose the acceleration vector into its tangential and normal components at t=1.
Refer to the 'Core Theory' tab or review the Worked Example demonstration for this skill.
Answer: - Normal component: aNβ=221β210β (approx. 14.13)
Step-by-Step Execution
Given
To decompose the acceleration vector into its tangential component aTβ and normal component aNβ, we use the standard formulas:
Step 1: Find the Velocity Vector v(t) and evaluate at t=1
Take the first derivative of the position vector r(t):
Evaluating at t=1:
Step 2: Find the Speed β£β£v(1)β£β£
Compute the magnitude of the velocity vector at t=1:
Step 3: Find the Acceleration Vector a(t) and evaluate at t=1
Take the derivative of the velocity vector v(t):
Evaluating at t=1:
Step 4: Calculate the Tangential Component of Acceleration aTβ
Compute the dot product vβ a at t=1:
Now divide by the speed β£β£v(1)β£β£:
Step 5: Calculate the Normal Component of Acceleration aNβ
Since v and a are 2D vectors lying in the xy-plane, we append a z-component of 0 to compute their cross product:
Find the magnitude of this cross product vector:
Now divide by the speed β£β£v(1)β£β£:
Answer
The acceleration vector components at t=1 are:
Tangential component: aTβ=221β296β (approx. 19.91)
Normal component: aNβ=221β210β (approx. 14.13)
π Regroup 3
- Verify calculations and mathematical reasoning.
π Synthesis Wrap-up
- Core takeaways from Session 06 and overview of homework homework assignment: Arc Length and Curvature Motion in Space.