Chapter 11: Problem 82
A particle has mass 0.5 kg and is traveling along the line \(x=5.0 \mathrm{m}\) at \(2.0 \mathrm{m} / \mathrm{s}\) in the positive \(y\) -direction. What is the particle's angular momentum about the origin?
Short Answer
Expert verified
The particle's angular momentum about the origin is \(\boldsymbol{L} = 5.0 \,\mathrm{kg} \cdot \mathrm{m}^2/\mathrm{s}\, \boldsymbol{\hat{k}}\).
Step by step solution
01
Identify Given Values
Before starting, let's list the given values:
1. Mass of the particle (m) = 0.5 kg
2. Position of the particle in the x-direction (x) = 5.0 m
3. Velocity of the particle in the y-direction (v_y) = 2.0 m/s
02
Calculate Position and Momentum Vectors
First, we will calculate the position vector \(\boldsymbol{r}\) and linear momentum \(\boldsymbol{p}\). The position vector is given as \(\boldsymbol{r} = x\boldsymbol{\hat{i}} + y\boldsymbol{\hat{j}}\). Since the particle's position only has an x-component, the position vector is \(\boldsymbol{r} = (5.0 \,\mathrm{m}) \boldsymbol{\hat{i}}\).
Next, we calculate the linear momentum as \(\boldsymbol{p} = m \boldsymbol{v}\). Since the particle is only moving in the positive y-direction, the momentum vector is \(\boldsymbol{p} = (0.5 \,\mathrm{kg})(2.0 \,\mathrm{m}/\mathrm{s}) \boldsymbol{\hat{j}} = 1.0 \,\mathrm{kg} \cdot \mathrm{m}/\mathrm{s} \boldsymbol{\hat{j}}\).
03
Calculate Angular Momentum with Cross Product
Now, we will compute the angular momentum by finding the cross product of the position and momentum vectors: \[\boldsymbol{L} = \boldsymbol{r} \times \boldsymbol{p}\]
The cross product of two vectors \(\boldsymbol{A} = A_x\boldsymbol{\hat{i}} + A_y\boldsymbol{\hat{j}} + A_z\boldsymbol{\hat{k}}\) and \(\boldsymbol{B} = B_x\boldsymbol{\hat{i}} + B_y\boldsymbol{\hat{j}} + B_z\boldsymbol{\hat{k}}\) is
\[\boldsymbol{A} \times \boldsymbol{B} =
\begin{vmatrix}
\boldsymbol{\hat{i}} & \boldsymbol{\hat{j}} & \boldsymbol{\hat{k}}\\
A_x & A_y & A_z \\
B_x & B_y & B_z
\end{vmatrix}\]
Applying the formula for cross product to the position and momentum vectors, we get
\[\boldsymbol{L} = \begin{vmatrix}
\boldsymbol{\hat{i}} & \boldsymbol{\hat{j}} & \boldsymbol{\hat{k}}\\
5.0 & 0 & 0 \\
0 & 1.0 & 0
\end{vmatrix} = (0) \boldsymbol{\hat{i}} - (0) \boldsymbol{\hat{j}} + (5.0)(1.0) \boldsymbol{\hat{k}} = 5.0 \,\mathrm{kg} \cdot \mathrm{m}^2/\mathrm{s}\, \boldsymbol{\hat{k}}\]
04
Write the Final Answer
The particle's angular momentum about the origin is \(\boldsymbol{L} = 5.0 \,\mathrm{kg} \cdot \mathrm{m}^2/\mathrm{s}\, \boldsymbol{\hat{k}}\).
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Particle Momentum
Particle momentum is a fundamental concept in physics that describes the quantity of motion an object possesses. It depends on two main factors: the mass of the particle and its velocity.
- The momentum (\( \boldsymbol{p} \)) is mathematically expressed as the product of mass (\( m \)) and velocity (\( \boldsymbol{v} \)):\[ \boldsymbol{p} = m \boldsymbol{v} \]
- In our original exercise, the particle has a mass of 0.5 kg and moves with a velocity of 2.0 m/s.Thus, its momentum is:\[ \boldsymbol{p} = (0.5 \,\mathrm{kg})(2.0 \, \mathrm{m}/\mathrm{s}) = 1.0 \, \mathrm{kg} \, \cdot \, \mathrm{m}/\mathrm{s} \]
- This momentum acts in the direction of the particle’s velocity, which in this case is the positive y-direction.
Cross Product
The cross product is a mathematical operation used to find a vector that is perpendicular to two given vectors.It's crucial when calculating quantities like torque and angular momentum.
- For two vectors \( \boldsymbol{A} \) and \( \boldsymbol{B} \), the cross product \( \boldsymbol{A} \times \boldsymbol{B} \) is given by:\[\boldsymbol{A} \times \boldsymbol{B} = \begin{vmatrix} \boldsymbol{\hat{i}} & \boldsymbol{\hat{j}} & \boldsymbol{\hat{k}}\ A_x & A_y & A_z \ B_x & B_y & B_z \end{vmatrix} \]
- The resulting vector is orthogonal to both \( \boldsymbol{A} \) and \( \boldsymbol{B} \).
- In the solved exercise, the cross product helps determine the angular momentum of the particle.
Angular Momentum Calculation
Angular momentum is a property of any object that is rotating or moving along a path about a point.It's calculated by the cross product of the position and momentum vectors.
- The formula for angular momentum (\( \boldsymbol{L} \)) is:\[\boldsymbol{L} = \boldsymbol{r} \times \boldsymbol{p} \]
- In our example, the position vector \( \boldsymbol{r} \) is \( (5.0 \,\mathrm{m}) \boldsymbol{\hat{i}} \), and the momentum vector is \( (1.0 \,\mathrm{kg} \cdot \mathrm{m}/\mathrm{s}) \boldsymbol{\hat{j}} \).
- Performing the cross product yields:\[\boldsymbol{L} = \begin{vmatrix} \boldsymbol{\hat{i}} & \boldsymbol{\hat{j}} & \boldsymbol{\hat{k}}\ 5.0 & 0 & 0 \ 0 & 1.0 & 0 \end{vmatrix} = 5.0 \,\mathrm{kg} \cdot \mathrm{m}^2/\mathrm{s} \, \boldsymbol{\hat{k}}\]
- This indicates that the angular momentum is directed along the z-axis.
Position Vector
The position vector is a geometric representation indicating a particle's position in space relative to an origin.It plays an essential role in determining angular momentum.
- The position vector \( \boldsymbol{r} \) is expressed as:\[\boldsymbol{r} = x \boldsymbol{\hat{i}} + y \boldsymbol{\hat{j}}\]
- In the given exercise, because the particle is moving along the line \( x = 5.0 \, \mathrm{m} \), the position vector is simply:\( \boldsymbol{r} = (5.0 \, \mathrm{m}) \boldsymbol{\hat{i}} \).
- This visualization clarifies the particle's position successfully for further calculations.
Linear Momentum
Linear momentum refers to the momentum of an object moving in a straight line.It is directly proportional to both the mass and velocity of the object.
- The linear momentum formula is straightforward:\[\boldsymbol{p} = m \boldsymbol{v} \]
- It follows the direction of velocity, meaning if the object moves right, the linear momentum points right as well.
- For the particle discussed, its linear momentum is \( 1.0 \, \mathrm{kg} \cdot \mathrm{m}/\mathrm{s} \) in the positive y-direction, emphasizing its straightforward path.