Volume 3: The Calculus of Life

Workbook 28.2: Parametric Equations

Directives for the Chronometer:

1. Evaluate: To find a point, plug $t$ into both $x(t)$ and $y(t)$.
2. Derivative: Velocity Vector is $\mathbf{v}(t) = \langle x'(t), y'(t) \rangle$.
3. Slope: $\frac{dy}{dx} = \frac{y'(t)}{x'(t)}$. (Y on Top!).
4. Magnitude: Speed is the length of the Velocity Vector.

Part I: Finding the Path

A path is given by: $x(t) = 3t + 1$ and $y(t) = t^2$.

The Early Moment: Find the $(x, y)$ coordinate at time $t = 0$ and $t = 2$.

At $t=0$: $x=1, y=0$. Point: $(1, 0)$.
At $t=2$: $x=... y=...$ Point: ...

The Velocity: Find the Velocity Vector $\mathbf{v}(t)$ for any time $t$.

$x'(t) = 3$.
$y'(t) = 2t$.
$\mathbf{v}(t) = \langle 3, 2t \rangle$.

The Slope: What is the slope of the path ($ rac{dy}{dx}$) at $t = 5$?

$ rac{dy}{dx} = 2t / 3$.
At $t=5$: $10/3$.

Part II: Circular Timing

A point moves in a circle: $x(t) = \cos(t)$ and $y(t) = \sin(t)$.

The Quarter-Turn: Find the position at $t = \pi/2$.

$x = \cos(\pi/2) = ...$
$y = \sin(\pi/2) = ...$

The Speed: Calculate the Magnitude of the velocity vector at any time $t$.

$x' = -\sin(t)$. $y' = \cos(t)$.
$|\mathbf{v}| = \sqrt{(-\sin t)^2 + (\cos t)^2} = ...$
(Hint: Use the Trig Identity $\sin^2 + \cos^2 = 1$).
The Logic Check:

In the circle problem, your "Speed" ($|\mathbf{v}|$) should be exactly 1. Does this mean the object is moving at a constant speed even though its $x$ and $y$ values are changing? How does this reflect a heart that is "Stable" even in "Seasons of Change"?

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Part III: Eliminating the Parameter

Convert the path $x(t) = t - 5$ and $y(t) = 2t + 1$ into a rectangular equation ($y = f(x)$).

Step 1: $t = x + 5$.
Step 2: $y = 2(x + 5) + 1$.
Final: ...

Part IV: The Challenge (The Arc Length)

The Distance of the Spiral

A path is defined by $x(t) = \cos(t)$ and $y(t) = \sin(t)$ from $t = 0$ to $t = \pi$.
Task: Set up the integral for the Arc Length.
(Hint: We found the magnitude was 1 in Part II).

$L = \int_{0}^{\pi} 1 dt = ...$
Does your answer match half the circumference of a unit circle?

Part V: Transmission (The Echad Extension)

Teacher Log: The Toy Clock

Objective: Explain "Parameter" to a younger sibling using a toy car and a counting game.

The Activity:
1. Say "One-two-three-GO!"
2. On "One," move the car to spot 1.
3. On "Two," move the car to spot 2.

The Lesson: "The car moves because I am counting. The 'Count' is the Parameter. It's the secret number that makes the car go."


Response: __________________________________________________________

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