Problem 22 - Algebra and Trigonometry Real Mathematics, Real People

Question

Use the dot product to find the magnitude of \(\mathbf{u}.\) $$\mathbf{u}=9 \mathbf{i}$$

Step-by-Step Solution

Answer

The magnitude of the vector \(\mathbf{u} = 9 \mathbf{i}\) is 9.

Step 1: Identify the vector

The given vector is \(\mathbf{u}=9 \mathbf{i}\)

Step 2: Apply the formula for magnitude using the dot product

The formula for finding the magnitude of a vector using the dot product, when the components of the vector are \(u_{1}, u_{2}, ..., u_{n}\), is \(\|u\| = \sqrt{u_{1}^2 + u_{2}^2 + \ldots + u_{n}^2}\) . In this case, the vector only has the i component, so we only have \(u_1\), which is 9.

Step 3: Calculate

Substitute \(u_1 = 9\) into the formula to get \|u\| = \(\sqrt{9^2}\)

Key Concepts

Dot Product

The dot product is a fundamental operation in vector algebra. It involves multiplying the corresponding components of two vectors and summing the results. This operation can be symbolized as \( \mathbf{a} \cdot \mathbf{b} = a_1b_1 + a_2b_2 + \ldots + a_nb_n \). The dot product is especially useful for calculating the angle between two vectors and determining the magnitude of a vector itself.

When dealing with a vector such as \( \mathbf{u} = 9 \mathbf{i} \), the dot product can help us calculate the magnitude straightforwardly, as shown in the given exercise. For a single component vector like this one (with only an \( \mathbf{i} \) component), the dot product simply becomes \( 9 \times 9 \), resulting in 81.

It's interesting how the dot product relates to magnitudes and angles:

• For angles: \( \mathbf{a} \cdot \mathbf{b} = \|\mathbf{a}\| \|\mathbf{b}\| \cos(\theta) \), where \( \theta \) is the angle between vectors.
• A dot product of zero indicates perpendicular vectors.

This powerful operation simplifies many vector calculations and provides insights into the geometric relationships between vectors.

Unit Vectors

Unit vectors are crucial in vector algebra as they are the building blocks for any vector directions. Essentially, a unit vector has a magnitude of 1 and is used to denote direction without regard to magnitude. They are typically symbolized using a small bold letter with a hat, like \( \mathbf{\hat{i}}, \mathbf{\hat{j}}, \) and \( \mathbf{\hat{k}} \), representing the direction along the x, y, and z axes respectively.

In our exercise, the vector \( \mathbf{u} = 9 \mathbf{i} \) signifies a vector with a direction parallel to the x-axis. The \( \mathbf{i} \) in this vector is the unit vector along the x-axis.

• To convert a vector into its unit vector form, divide the vector by its magnitude.
• Unit vectors help standardize calculations, especially in physics, where direction needs to be isolated.

Thus, if we needed a unit vector in the direction of \( \mathbf{u} \), it would simply be \( \mathbf{\hat{i}} \). Understanding unit vectors allows for flexibility in navigation through any vector space, helping in breaking down vectors into more manageable parts.

Vector Components

Vector components divide a vector into parts based on different axes, which helps in simplifying many engineering and physics problems. Each component impacts the vector's behavior along a specified direction. For example, in a 2D plane, a vector \( \mathbf{v} \) could have components along the x and y axes: \( \mathbf{v} = v_x \mathbf{i} + v_y \mathbf{j} \). These components describe how much of the vector's magnitude affects each axis.

Understanding vector components is essential because:

• They allow representation of any vector in multi-dimensional space.
• Help in breaking down forces, velocities, and other quantities in physics.

For the vector \( \mathbf{u} = 9 \mathbf{i} \), the component is solely along the x-axis, simplifying calculations like finding the magnitude, as you only need to consider the x-component (9 in this case). This simplicity highlights the elegance of using components to represent more complex scenarios. Working with vector components facilitates the combination and decomposition of vectors, essential in both theoretical and applied sciences.