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Determine the angular momentum of a 75-g particle about the origin of coordinates when the particle is at \(x=4.4 \mathrm{~m}\), \(y=-6.0 \mathrm{~m},\) and it has velocity \(v=(3.2 \hat{\mathbf{i}}-8.0 \hat{\mathbf{k}}) \mathrm{m} / \mathrm{s}\)

Short Answer

Expert verified
The angular momentum is \( 3.6 \hat{\mathbf{i}} + 2.64 \hat{\mathbf{j}} + 1.44 \hat{\mathbf{k}} \) kg m²/s.

Step by step solution

01

Understand the Problem

We are tasked with finding the angular momentum of a particle. Angular momentum is a vector quantity dependent on the particle's position, mass, and velocity, and it is calculated about a specified origin.
02

Gather Known Information

The problem provides the particle's mass, position, and velocity. The mass is 75 g, which is equivalent to 0.075 kg. The position vector is \( \mathbf{r} = 4.4 \hat{\mathbf{i}} - 6.0 \hat{\mathbf{j}} \), and the velocity vector is \( \mathbf{v} = 3.2 \hat{\mathbf{i}} - 8.0 \hat{\mathbf{k}}\).
03

Write the Formula for Angular Momentum

The angular momentum \( \mathbf{L} \) of a particle about a point is given by the cross product of the position vector \( \mathbf{r} \) and the linear momentum \( \mathbf{p} \), \( \mathbf{L} = \mathbf{r} \times \mathbf{p} \). Here, \( \mathbf{p} = m\mathbf{v} \), where \( m \) is the mass.
04

Calculate Linear Momentum

Convert the mass to kilograms and calculate the linear momentum \( \mathbf{p} \):\[ \mathbf{p} = m\mathbf{v} = 0.075(3.2\hat{\mathbf{i}} - 8.0\hat{\mathbf{k}}) = (0.24\hat{\mathbf{i}} - 0.6\hat{\mathbf{k}}) \text{ kg} \cdot \text{m/s} \].
05

Set Up the Cross Product

To calculate \( \mathbf{L} = \mathbf{r} \times \mathbf{p} \), use the determinant of vectors:\[ \mathbf{L} = | \begin{array}{ccc} \hat{\mathbf{i}} & \hat{\mathbf{j}} & \hat{\mathbf{k}} \ 4.4 & -6.0 & 0 \ 0.24 & 0 & -0.6 \end{array} | \].
06

Perform the Cross Product Calculation

Calculate the components of \( \mathbf{L} \) using the determinant:\[ \mathbf{L} = \hat{\mathbf{i}}(0 - (-0.6 (6.0)) + \hat{\mathbf{j}}(0 - (4.4)(-0.6)) + \hat{\mathbf{k}}((4.4)(0) - (-6.0)(0.24)) \] which simplifies to \[ \mathbf{L} = \hat{\mathbf{i}}(3.6) + \hat{\mathbf{j}}(2.64) + \hat{\mathbf{k}}(1.44) \].
07

Conclude with the Result

Therefore, the angular momentum of the particle in vector form is \( 3.6 \hat{\mathbf{i}} + 2.64 \hat{\mathbf{j}} + 1.44 \hat{\mathbf{k}} \) kg m²/s.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Cross Product
In physics, the cross product plays a crucial role, especially when discussing vector quantities such as angular momentum. The cross product is an operation between two vectors that results in another vector perpendicular to the plane established by the original vectors.
This operation is particularly valuable in three-dimensional space, where understanding the direction of resultant vectors is essential.
The formula to calculate the cross product of two vectors \(\mathbf{A}\) and \(\mathbf{B}\) is given by:\[ \mathbf{A} \times \mathbf{B} = | \begin{array}{ccc} \hat{\mathbf{i}} & \hat{\mathbf{j}} & \hat{\mathbf{k}} \A_x & A_y & A_z \B_x & B_y & B_z \end{array} | \]
  • The cross product yields a vector orthogonal to both \(\mathbf{A}\) and \(\mathbf{B}\).
  • The magnitude of the resulting vector can also be found using the formula \( |\mathbf{A}||\mathbf{B}|\sin(\theta)\), where \(\theta\) is the angle between \(\mathbf{A}\) and \(\mathbf{B}\).
In our problem, the angular momentum \(\mathbf{L}\) is determined by the cross product of the position vector and the linear momentum. This allows us to find not only the magnitude but also the direction of the angular momentum.
Linear Momentum
Linear momentum is a core concept in physics that helps us understand how objects move. It is defined as the product of an object's mass and its velocity.
The momentum of an object, denoted as \(\mathbf{p}\), serves as a vector quantity reflecting both the direction and magnitude of motion.
The formula for linear momentum is expressed as:\[ \mathbf{p} = m\mathbf{v} \]
  • Here, \(m\) represents the mass of the object.
  • \(\mathbf{v}\) stands for the velocity vector of the object.
In the given exercise, the mass of the particle makes a direct impact on the linear momentum.
Converting the mass to kilograms and using the velocity provided, we computed the linear momentum as \((0.24\hat{\mathbf{i}} - 0.6\hat{\mathbf{k}})\) kg m/s.
This step is crucial as it feeds into the calculation of the angular momentum, where linear momentum is one of the multiplying vectors in the cross product operation.
Vector Calculus
Vector calculus is an essential tool in physics for understanding relationships between vector fields. It involves the use of mathematical techniques to operate on vector functions and fields.
This concept is indispensable when dealing with multi-dimensional spaces, such as analyzing forces and motion in physics.
In our problem, vector calculus steps in when performing the cross product operation, which is part of the solution process to find angular momentum.
Here are some elements of vector calculus relevant to our task:
  • Vector Representation: Developing an understanding of vectors and how to represent them using unit vectors, such as \((\hat{\mathbf{i}}, \hat{\mathbf{j}}, \hat{\mathbf{k}})\).
  • Determinants: Using determinants to calculate products like the cross product to simplify complex vector operations.
Through vector calculus, we bridge geometry and algebra, providing a framework to solve physical problems.
In this exercise, vector calculus aids in setting up and simplifying the cross product that calculates the angular momentum of the given particle.

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Most popular questions from this chapter

$$ \vec{\mathbf{A}} \text { and } \vec{\mathbf{B}} $$ $$ \vec{\mathbf{A}} \times \vec{\mathbf{B}} $$ $$\vec{\mathbf{B}} \text { points south. }(b) \overline{\mathbf{A}} \text { points cast, } \vec{\mathbf{B}} $$ straight down. (c) \(\overline{\mathbf{A}}\) points straight up, \(\vec{\mathbf{B}}\) points north. (d) \(\vec{\mathbf{A}}\) points straight up, \(\vec{\mathbf{B}}\) points straight down.

(II) Suppose our Sun eventually collapses into a white dwarf, losing about half its mass in the process, and winding up with a radius 1.0\(\%\) of its existing radius. Assuming the lost mass carries away no angular momentum, what would the Sun's new rotation rate be? (Take the Sun's current period to be about 30 days) What would be its final kinetic energy in terms of its initial kinetic energy of today?

Show that the velocity \(\overrightarrow{\mathbf{v}}\) of any point in an object rotating with angular velocity \(\overrightarrow{\boldsymbol{\omega}}\) about a fixed axis can be written $$ \overrightarrow{\mathbf{v}}=\overrightarrow{\boldsymbol{\omega}} \times \overrightarrow{\mathbf{r}} $$ where \(\overrightarrow{\mathbf{r}}\) is the position vector of the point relative to an origin O located on the axis of rotation. Can O be anywhere on the rotation axis? Will \(\overrightarrow{\mathbf{v}}=\overrightarrow{\boldsymbol{\omega}} \times \overrightarrow{\mathbf{r}}\) if \(\mathrm{O}\) is located at a point not on the axis of rotation?

A person stands on a platform, initially at rest, that can rotate freely without friction. The moment of inertia of the person plus the platform is \(I_{P}\). The person holds a spinning bicycle wheel with its axis horizontal. The wheel has moment of inertia \(I_{\mathrm{W}}\) and angular velocity \(\omega_{\mathrm{W}}\). What will be the angular velocity \(\omega_{\mathrm{P}}\) of the platform if the person moves the axis of the wheel so that it points \((a)\) vertically upward, (b) at a \(60^{\circ}\) angle to the vertical, \((c)\) vertically downward? (d) What will \(\omega_{\mathrm{P}}\) be if the person reaches up and stops the wheel in part \((a) ?\)

Let \(\overrightarrow{\mathbf{A}}, \overrightarrow{\mathbf{B}},\) and \(\overrightarrow{\mathbf{C}}\) be three vectors, which for generality we assume do not all lie in the same plane. Show that \(\overrightarrow{\mathbf{A}} \cdot(\overrightarrow{\mathbf{B}} \times \overrightarrow{\mathbf{c}})=\overrightarrow{\mathbf{B}} \cdot(\overrightarrow{\mathbf{C}} \times \overrightarrow{\mathbf{A}})=\overrightarrow{\mathbf{C}} \cdot(\overrightarrow{\mathbf{A}} \times \overrightarrow{\mathbf{B}})\)

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