The Magnitudes Of The Two Displacement Vectors Are

Let's delve into the fascinating world of displacement vectors, exploring what happens when we consider their magnitudes – their lengths, essentially – and how these lengths relate to each other. Displacement vectors are fundamental tools for describing motion and position changes, not just in physics but across various scientific and engineering fields. Understanding the interplay of their magnitudes offers valuable insights into the geometry of movement and its overall effect.

Understanding Displacement Vectors

A displacement vector represents the change in position of an object. It's a vector quantity, meaning it possesses both magnitude (length) and direction. Imagine walking from point A to point B; the displacement vector is the arrow pointing directly from A to B, regardless of the path you took. The length of this arrow is the magnitude of the displacement, and the direction is, well, the direction of the arrow.

Magnitude: The magnitude of a displacement vector is a scalar quantity, representing the distance between the initial and final positions. It's always a non-negative value.
Direction: The direction specifies the orientation of the displacement, usually expressed as an angle relative to a reference axis (e.g., North, East, or the positive x-axis).

The Significance of Magnitudes

Why focus on the magnitudes of displacement vectors? Because they provide a crucial piece of information: the net distance traveled in a specific direction. This is particularly important when dealing with multiple displacements or complex motions. Consider these scenarios:

Navigation: If you're charting a course, the magnitudes of displacement vectors tell you how far you need to travel in each leg of the journey.
Robotics: In robotics, controlling the magnitude of displacement vectors is essential for precise movements and task completion.
Engineering: Civil engineers use displacement vectors to analyze the deformation of structures under stress, and the magnitudes reveal the extent of this deformation.
Physics: Analyzing projectile motion involves breaking down the displacement into horizontal and vertical components, and understanding their magnitudes is crucial for calculating range and time of flight.

Comparing the Magnitudes of Two Displacement Vectors

Now, let's focus on situations where we have two displacement vectors, often labeled A and B, and we want to understand the relationship between their magnitudes, denoted as |A| and |B|, respectively. There are several ways to compare them:

Equality: |A| = |B|. This means both displacements cover the same net distance. They may be in different directions, but the distance covered is identical.
Inequality: |A| > |B| or |A| < |B|. This indicates that one displacement covers a greater net distance than the other.
Ratio: |A| / |B|. This gives you a direct comparison of how much larger or smaller one displacement is compared to the other. For example, if |A| / |B| = 2, then displacement A covers twice the distance of displacement B.

Adding Displacement Vectors: Resultant Displacement

A common and important operation is adding displacement vectors. If an object undergoes two successive displacements, A and B, the resultant displacement R is the vector sum of A and B:

R = A + B

The magnitude of the resultant displacement, |R|, is not necessarily equal to the sum of the magnitudes of A and B (|A| + |B|). This is a crucial point! The magnitudes only add directly if A and B are in the same direction.

The Triangle Inequality

The relationship between the magnitudes of A, B, and R is governed by the triangle inequality:

| |A| - |B| | ≤ |R| ≤ |A| + |B|

Let's break this down:

|R| ≤ |A| + |B|: The magnitude of the resultant displacement is always less than or equal to the sum of the magnitudes of the individual displacements. The maximum possible magnitude of R occurs when A and B are in the same direction.
| |A| - |B| | ≤ |R|: The magnitude of the resultant displacement is always greater than or equal to the absolute difference between the magnitudes of the individual displacements. The minimum possible magnitude of R occurs when A and B are in opposite directions.

Visualizing Vector Addition

The triangle inequality becomes clear when visualizing vector addition. Imagine drawing vectors A and B head-to-tail. The resultant vector R is the vector that closes the triangle, going from the tail of A to the head of B.

Maximum |R|: When A and B are aligned, they form a straight line, and |R| = |A| + |B|.
Minimum |R|: When A and B are anti-aligned (opposite directions), they form a straight line, and |R| = | |A| - |B| |.
Other Angles: For any other angle between A and B, the magnitude of R will fall somewhere between these two extremes.

Calculating the Magnitude of the Resultant Vector

To find the exact magnitude of the resultant vector |R|, we can use the Law of Cosines:

|R|^2 = |A|^2 + |B|^2 + 2 |A| |B| cos(θ)

Where θ is the angle between vectors A and B. Therefore:

|R| = √(|A|^2 + |B|^2 + 2 |A| |B| cos(θ))

θ = 0° (Same Direction): cos(0°) = 1, so |R| = √(|A|^2 + |B|^2 + 2 |A| |B|) = √((|A| + |B|)^2) = |A| + |B|.
θ = 180° (Opposite Direction): cos(180°) = -1, so |R| = √(|A|^2 + |B|^2 - 2 |A| |B|) = √((|A| - |B|)^2) = | |A| - |B| |.
θ = 90° (Perpendicular): cos(90°) = 0, so |R| = √(|A|^2 + |B|^2). This is the Pythagorean theorem.

Examples and Applications

Let's solidify our understanding with some examples:

Example 1: Walking a Path

A person walks 10 meters East (A) and then 5 meters North (B).

|A| = 10 m
|B| = 5 m
θ = 90°

The magnitude of the resultant displacement |R| is:

|R| = √(10^2 + 5^2) = √(100 + 25) = √125 ≈ 11.18 m

The person's net displacement is approximately 11.18 meters from their starting point.

Example 2: A Boat Crossing a River

A boat travels across a river with a velocity of 8 m/s perpendicular to the current (A). The river current has a velocity of 3 m/s (B).

|A| = 8 m/s
|B| = 3 m/s
θ = 90°

The magnitude of the boat's resultant velocity |R| is:

|R| = √(8^2 + 3^2) = √(64 + 9) = √73 ≈ 8.54 m/s

The boat's actual speed relative to the riverbank is approximately 8.54 m/s.

Example 3: Tug-of-War

Two people are playing tug-of-war. One person pulls with a force of 150 N to the right (A), and the other pulls with a force of 120 N to the left (B).

|A| = 150 N
|B| = 120 N
θ = 180°

The magnitude of the net force |R| is:

|R| = |150 - 120| = 30 N

The net force on the rope is 30 N to the right.

Applications in Real Life:

Aircraft Navigation: Pilots use vector addition to account for wind velocity when calculating the aircraft's ground speed and heading. The plane's velocity relative to the air is one vector, and the wind velocity is another. The resultant vector determines the plane's actual movement over the ground.
GPS Systems: GPS devices rely on displacement vectors to determine your location and track your movement. By analyzing the signals from multiple satellites, the device can calculate your displacement vector and display your position on a map.
Sports Analysis: In sports like golf or baseball, understanding displacement vectors is crucial for analyzing trajectories and optimizing performance. The initial velocity of a ball can be broken down into horizontal and vertical components, and the magnitudes of these components determine the ball's range and height.
Computer Graphics: Displacement vectors are used extensively in computer graphics to manipulate objects in 3D space. By applying transformations such as rotations and translations, artists can create realistic animations and visual effects.

Common Mistakes to Avoid

Adding Magnitudes Directly: As emphasized earlier, avoid simply adding the magnitudes of displacement vectors unless they are in the same direction. Always consider the angle between the vectors.
Ignoring Direction: Displacement is a vector quantity. Don't forget to account for the direction of the displacements.
Confusing Distance and Displacement: Distance is the total length of the path traveled, while displacement is the net change in position. They are only equal if the motion is in a straight line without changing direction.
Incorrectly Applying the Law of Cosines: Double-check that you are using the correct angle (θ) between the vectors when applying the Law of Cosines.

Scalar vs. Vector Quantities

A scalar quantity has only magnitude, while a vector quantity has both magnitude and direction. Examples of scalar quantities include:

Temperature
Speed (not velocity)
Mass
Energy
Time

Examples of vector quantities include:

Displacement
Velocity
Acceleration
Force
Momentum

It's crucial to distinguish between scalar and vector quantities when solving physics problems. You cannot simply add scalar and vector quantities together. For example, you cannot add temperature to velocity.

Beyond Two Vectors: Multiple Displacements

The principles we've discussed extend to situations with more than two displacement vectors. To find the resultant displacement of multiple vectors, you can:

Resolve Each Vector into Components: Break down each vector into its x, y, and z components (depending on the dimensionality of the problem).
Add the Components: Add all the x-components together, all the y-components together, and all the z-components together.
Reconstruct the Resultant Vector: Use the Pythagorean theorem (in 2D or 3D) to find the magnitude of the resultant vector, and use trigonometry to find its direction.

This process, known as component-wise addition, is a powerful technique for handling complex vector addition problems.

Conclusion

Understanding the magnitudes of displacement vectors is fundamental to analyzing motion and position changes. While the magnitude represents the net distance covered, its relationship with other displacement vectors becomes intricate when considering direction. Remember the triangle inequality and the Law of Cosines to accurately calculate resultant displacements. By avoiding common mistakes and practicing with examples, you can master the concepts and apply them to a wide range of scientific and engineering problems. The key takeaway is that magnitudes provide crucial information, but direction is equally important when working with displacement vectors. Ignoring direction leads to incorrect results. So, keep both magnitude and direction in mind as you continue exploring the fascinating world of vectors!