Problem 79 - University Physics with Modern Physics

Question

Vectors \(\overrightarrow{A}\) and \(\overrightarrow{B}\) have scalar product \(-\)6.00, and their vector product has magnitude \(+\)9.00. What is the angle between these two vectors?

Step-by-Step Solution

Answer

The angle between the vectors is approximately 123.69 degrees.

Step 1: Understanding Given Data

We are provided with two pieces of information about vectors \( \overrightarrow{A} \) and \( \overrightarrow{B} \). First, their dot product is \( \overrightarrow{A} \cdot \overrightarrow{B} = -6.00 \). Secondly, their cross product magnitude is \( |\overrightarrow{A} \times \overrightarrow{B}| = 9.00 \). We seek the angle \( \theta \) between them.

Step 2: Relation of Dot Product with Cosine

The dot product of two vectors \( \overrightarrow{A} \) and \( \overrightarrow{B} \) is given by the formula: \[ \overrightarrow{A} \cdot \overrightarrow{B} = |\overrightarrow{A}| |\overrightarrow{B}| \cos(\theta) \] Using this, \( \cos(\theta) = \frac{-6.00}{|\overrightarrow{A}| |\overrightarrow{B}|} \).

Step 3: Relation of Cross Product with Sine

The magnitude of the cross product is given by: \[ |\overrightarrow{A} \times \overrightarrow{B}| = |\overrightarrow{A}| |\overrightarrow{B}| \sin(\theta) \] From this, we find \( \sin(\theta) = \frac{9.00}{|\overrightarrow{A}| |\overrightarrow{B}|} \).

Step 4: Solving for Angle \( \theta \)

To find \( \theta \), we need the quotient \( \frac{\sin(\theta)}{\cos(\theta)} = \tan(\theta) \). Thus, \[ \tan(\theta) = \frac{9.00}{-6.00} = -\frac{3}{2} \] Therefore, \( \theta = \tan^{-1}\left(-\frac{3}{2}\right) \).

Step 5: Finding Specific Angle

Using a calculator, solve \( \theta = \tan^{-1}(-\frac{3}{2}) \). We find the angle \( \theta \) to be approximately \( -56.31 \text{ degrees} \). However, the standard positive angle that is within \( 0 \) to \( 180 \text{ degrees} \) is \( 180 - 56.31 \approx 123.69 \text{ degrees} \), since the tangent function is periodic.

Key Concepts

Scalar Product

The scalar product, often known as the dot product, is one of the fundamental operations you can perform on two vectors. It combines two vectors to produce a single scalar (number), which indicates some level of projection of one vector onto another.

For vectors \( \overrightarrow{A} \) and \( \overrightarrow{B} \), their dot product is calculated as:

• \( \overrightarrow{A} \cdot \overrightarrow{B} = |\overrightarrow{A}| |\overrightarrow{B}| \cos(\theta) \)

Here, \( \theta \) is the angle between the vectors, and \(|\overrightarrow{A}|\) and \(|\overrightarrow{B}|\) are their magnitudes.

This product is commutative, meaning \( \overrightarrow{A} \cdot \overrightarrow{B} = \overrightarrow{B} \cdot \overrightarrow{A} \). It's particularly useful in applications such as determining if vectors are perpendicular, as the dot product will be zero when they are at a right angle.

Vector Product

The vector product, or cross product, provides a powerful way to combine two vectors in three-dimensional space. Unlike the dot product, which results in a scalar, the cross product results in another vector.

For vectors \( \overrightarrow{A} \) and \( \overrightarrow{B} \), the vector product is identified by a new vector whose magnitude is given by:

• \( |\overrightarrow{A} \times \overrightarrow{B}| = |\overrightarrow{A}| |\overrightarrow{B}| \sin(\theta) \)

The direction of this new vector is perpendicular to both \( \overrightarrow{A} \) and \( \overrightarrow{B} \), following the right-hand rule.

This product is non-commutative, meaning \( \overrightarrow{A} \times \overrightarrow{B} = - (\overrightarrow{B} \times \overrightarrow{A}) \). It's often used in physics and engineering to determine torque or rotational force.

Dot Product

The dot product is a type of scalar product, but it bears its own special emphasis, especially in geometry and physics. It's defined as:

• \( \overrightarrow{A} \cdot \overrightarrow{B} = ax_b + by_b + cz_b \)

Where \(a, b, c\) are the respective components of \( \overrightarrow{A}\), and \(x, y, z\) are those of \( \overrightarrow{B} \).

One application of the dot product is finding the angle between two vectors. If the product yields zero, the vectors are perpendicular. For non-zero results, it can provide insight into how much two vectors "align" with each other.

This product is crucial in simplifying calculations in machine learning, guides collision detection in video games, and assists in projections in 3D graphics.

Cross Product

The cross product, being a specific form of vector product, serves a unique purpose in vector mathematics. It's represented as a vector whose magnitude and direction are derived from its operands.

The magnitude is calculated as:

• \( |\overrightarrow{A} \times \overrightarrow{B}| = |\overrightarrow{A}| |\overrightarrow{B}| \sin(\theta) \)

This results in a new vector, essentially pointing out of the plane formed by \( \overrightarrow{A} \) and \( \overrightarrow{B} \).

In practice, this product is useful for determining the area of parallelograms defined by vectors, calculating moments and forces in physics, and aiding in operations like finding normals to surfaces in computer graphics. Its application is indispensable for tasks involving rotation and orientation.

Angle Between Vectors

Determining the angle between two vectors is crucial in various applications ranging from physics to computer graphics.

This angle, denoted as \(\theta\), can be computed using the relationships established by both the dot and cross product:

• \( \cos(\theta) = \frac{\overrightarrow{A} \cdot \overrightarrow{B}}{|\overrightarrow{A}||\overrightarrow{B}|} \)
• \( \sin(\theta) = \frac{|\overrightarrow{A} \times \overrightarrow{B}|}{|\overrightarrow{A}||\overrightarrow{B}|} \)

To find the angle, you calculate the inverse trigonometric functions based on these relationships.

If tasked with finding the angle: work out both sine and cosine using the formulas above and then derive \( \theta \) using \( \tan(\theta) = \frac{\sin(\theta)}{\cos(\theta)} \). This method gives a clear, process-oriented way to find the specific angle between two vectors in any scenario.