Problem 1 - Advanced Engineering Mathematics

Question

Graph the curve traced by the given vector function. $$ \mathbf{r}(t)=2 \sin t \mathbf{i}+4 \cos t \mathbf{j}+t \mathbf{k} ; t \geq 0 $$

Step-by-Step Solution

Answer

The vector function traces a helix in 3D space.

Step 1: Understand the Vector Function

The vector function is given as \( \mathbf{r}(t) = 2 \sin t \mathbf{i} + 4 \cos t \mathbf{j} + t \mathbf{k} \). It represents a position in 3D space for any given value of \( t \geq 0 \). The components are parameterized by \( t \) for the \( x \), \( y \), and \( z \) axes respectively: \( x(t) = 2 \sin t \), \( y(t) = 4 \cos t \), \( z(t) = t \).

Step 2: Identify Projection in the XY-plane

Looking at the \( x(t) = 2 \sin t \) and \( y(t) = 4 \cos t \) components, recognize these form an ellipse. This is an ellipse centered at the origin, with a semi-major axis of 4 (along the \( y \)-axis) and a semi-minor axis of 2 (along the \( x \)-axis).

Step 3: Identify Behavior in the Z-direction

The \( z \)-component is \( t \), indicating a linear increase in the \( z \) direction as \( t \) increases, essentially adding a twist that takes the form of a helix.

Step 4: Sketch the Helical Curve

Understanding the projections, the function forms a helical structure. As \( t \) increases, the curve traces an ellipse in the \( xy \)-plane while moving linearly upward, giving the shape of a three-dimensional helix. The curve moves counter-clockwise when viewed from above and elevates linearly along the \( z \)-axis.

Key Concepts

Parametric Equations

Parametric equations are a way of describing a curve in space by expressing the coordinates of points on the curve as functions of a variable, often denoted as \( t \). Instead of using a single equation involving \( x \), \( y \), and \( z \), we use separate equations for each coordinate. This allows us to express complex shapes that cannot be easily described by standard equations. For a vector function like \( \mathbf{r}(t) = 2 \sin t \mathbf{i} + 4 \cos t \mathbf{j} + t \mathbf{k} \), each component corresponds to one of the coordinates:

• \( x(t) = 2 \sin t \)
• \( y(t) = 4 \cos t \)
• \( z(t) = t \)

These parametric equations help in describing the path traced by the point as \( t \) changes. In this case, they help illustrate how each component of the motion unfolds in three-dimensional space. Using parametric equations is key for modeling motions and paths in physics and engineering.

Helical Curves

A helical curve is a three-dimensional spiral, similar to the shape of a spring or a twirling ribbon. The vector function \( \mathbf{r}(t) \) in the exercise exhibits this helical nature. In the given example, the helical shape arises because the curve maintains a circular motion in one plane while advancing steadily in a perpendicular direction. For our vector function:

• The motion in the \( xy \)-plane forms an ellipse, rotating around the origin.
• The \( z \)-component \( z(t) = t \) adds a linear increase in height.

As \( t \) increases, the combination of these movements creates a helical structure. Visualizing helical curves is important in contexts such as DNA structures and mechanical components like screws and coiled springs.

3D Graphing

3D graphing allows us to visualize curves and surfaces in three dimensions. When dealing with parametric vector functions, such as \( \mathbf{r}(t) = 2 \sin t \mathbf{i} + 4 \cos t \mathbf{j} + t \mathbf{k} \), 3D graphing helps us comprehend how the curve behaves over different dimensions.To graph a helical curve:

• Focus on the projections in each plane: the ellipse in the \( xy \)-plane, and the line in the \( z \)-direction.
• Combine these perspectives to illustrate the complete spiral path.
• Use software tools or sketching methods to draw the 3D path, seeing how it rises as it revolves.

The ability to visualise such curves is useful across various fields including architecture, physics, and computer graphics.

Ellipse in xy-plane

An ellipse in the \( xy \)-plane is a closed curve that resembles an elongated circle. In the given vector function, the terms \( x(t) = 2\sin t \) and \( y(t) = 4\cos t \) define this elliptical shape. The properties of this ellipse are:

• Semi-major axis along the y-axis of length 4
• Semi-minor axis along the x-axis of length 2

The ellipse's center is the origin \((0,0)\). Movement around this ellipse for different values of \( t \) results in a full circular pattern. The ellipse lies flat within the \( xy \)-plane, meaning while it undergoes motion due to changes in \( t \), it doesn't deviate from this plane. Recognizing ellipse structures within parametric equations can help us comprehend sections of motion in real-world applications such as planetary orbits and optical lenses.