/*! This file is auto-generated */ .wp-block-button__link{color:#fff;background-color:#32373c;border-radius:9999px;box-shadow:none;text-decoration:none;padding:calc(.667em + 2px) calc(1.333em + 2px);font-size:1.125em}.wp-block-file__button{background:#32373c;color:#fff;text-decoration:none} Problem 30 At the instant the displacement ... [FREE SOLUTION] | 91Ó°ÊÓ

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At the instant the displacement of a \(2.00 \mathrm{~kg}\) object relative to the origin is \(\vec{d}=(2.00 \mathrm{~m}) \hat{i}+(4.00 \mathrm{~m}) \mathrm{j}-(3.00 \mathrm{~m}) \mathrm{k},\) its velocity is \(\vec{v}=-(6.00 \mathrm{~m} / \mathrm{s}) \mathrm{i}+(3.00 \mathrm{~m} / \mathrm{s}) \mathrm{j}+(3.00 \mathrm{~m} / \mathrm{s}) \mathrm{k}\) and it is subject to a force \(\vec{F}=(6.00 \mathrm{~N}) \hat{\mathrm{i}}-(8.00 \mathrm{~N}) \mathrm{j}+(4.00 \mathrm{~N}) \mathrm{k} .\) Find (a) the acceleration of the object, (b) the angular momentum of the object about the origin, (c) the torque about the origin acting on the object, and (d) the angle between the velocity of the object and the force acting on the object.

Short Answer

Expert verified
(a) \(3.00\hat{i} - 4.00\hat{j} + 2.00\hat{k}\) m/s², (b) \(66.00\hat{i} + 60.00\hat{j} + 60.00\hat{k}\) kg·m²/s, (c) \(-8.00\hat{i} + 26.00\hat{j} - 40.00\hat{k}\) N·m, (d) 128.74°.

Step by step solution

01

Calculate Acceleration

To find the acceleration of the object, use Newton's second law of motion, which states that \( \vec{F} = m \cdot \vec{a} \). Rearranging for acceleration gives \( \vec{a} = \frac{\vec{F}}{m} \). Here, \( \vec{F} = (6.00 \text{ N}) \hat{i} - (8.00 \text{ N}) \hat{j} + (4.00 \text{ N}) \hat{k} \) and mass \( m = 2.00 \text{ kg} \). Thus, \[ \vec{a} = \frac{1}{2.00} \left( 6.00 \hat{i} - 8.00 \hat{j} + 4.00 \hat{k} \right) \]Calculating each component, we get\[ \vec{a} = (3.00 \text{ m/s}^2) \hat{i} - (4.00 \text{ m/s}^2) \hat{j} + (2.00 \text{ m/s}^2) \hat{k} \].
02

Calculate Angular Momentum

The angular momentum \( \vec{L} \) of an object about the origin is given by \( \vec{L} = \vec{r} \times \vec{p} \), where \( \vec{r} \) is the position vector and \( \vec{p} = m \cdot \vec{v} \) is the momentum. Here, \( \vec{r} = (2.00 \text{ m}) \hat{i} + (4.00 \text{ m}) \hat{j} - (3.00 \text{ m}) \hat{k} \). Momentum \( \vec{p} = m \cdot \vec{v} = 2.00 \cdot \left( -6.00 \hat{i} + 3.00 \hat{j} + 3.00 \hat{k} \right) = (-12.00 \hat{i} + 6.00 \hat{j} + 6.00 \hat{k}) \text{ kg m/s} \).To calculate the cross product, use the determinant:\[ \vec{L} = \left| \begin{array}{ccc} \hat{i} & \hat{j} & \hat{k} \ 2.00 & 4.00 & -3.00 \ -12.00 & 6.00 & 6.00 \end{array} \right| \] This simplifies to \[ \vec{L} = \hat{i}(4.00 \times 6.00 - (-3.00) \times 6.00) - \hat{j}(2.00 \times 6.00 - (-3.00) \times (-12.00))+ \hat{k}(2.00 \times 6.00 - 4.00 \times (-12.00)) \]\[ = (48.00 + 18.00)\hat{i} + (24.00 + 36.00)\hat{j} + (12.00 + 48.00)\hat{k} \]\[ = (66.00 \text{ kg m}^2/\text{s}) \hat{i} + (60.00 \text{ kg m}^2/\text{s}) \hat{j} + (60.00 \text{ kg m}^2/\text{s}) \hat{k} \].
03

Calculate Torque

The torque \( \vec{\tau} \) about the origin is given by \( \vec{\tau} = \vec{r} \times \vec{F} \). Using the same position vector \( \vec{r} = (2.00 \text{ m}) \hat{i} + (4.00 \text{ m}) \hat{j} - (3.00 \text{ m}) \hat{k} \) and force \( \vec{F} = (6.00 \text{ N}) \hat{i} - (8.00 \text{ N}) \hat{j} + (4.00 \text{ N}) \hat{k} \), we calculate:\[ \vec{\tau} = \left| \begin{array}{ccc} \hat{i} & \hat{j} & \hat{k} \ 2.00 & 4.00 & -3.00 \ 6.00 & -8.00 & 4.00 \end{array} \right| \]This simplifies to:\[ \vec{\tau} = \hat{i}(4.00 \times 4.00 - (-3.00)\times(-8.00)) - \hat{j}(2.00 \times 4.00 - (-3.00)\times6.00) + \hat{k}(2.00\times(-8.00) - 4.00\times6.00) \]\[ = (16.00 - 24.00)\hat{i} + (8.00 + 18.00)\hat{j} + (-16.00 - 24.00)\hat{k} \]\[ = (-8.00 \text{ N m}) \hat{i} + (26.00 \text{ N m}) \hat{j} - (40.00 \text{ N m}) \hat{k} \].
04

Calculate Angle Between Velocity and Force

To find the angle \( \theta \) between two vectors \( \vec{v} \) and \( \vec{F} \), use the formula \( \cos\theta = \frac{\vec{v} \cdot \vec{F}}{||\vec{v}|| ||\vec{F}||} \). First, find the dot product: \( \vec{v} \cdot \vec{F} = (-6.00)(6.00) + (3.00)(-8.00) + (3.00)(4.00) = -36.00 - 24.00 + 12.00 = -48.00 \).Next, calculate the magnitudes:\( ||\vec{v}|| = \sqrt{(-6.00)^2 + (3.00)^2 + (3.00)^2} = \sqrt{36.00 + 9.00 + 9.00} = \sqrt{54.00} \approx 7.35 \).\( ||\vec{F}|| = \sqrt{(6.00)^2 + (-8.00)^2 + (4.00)^2} = \sqrt{36.00 + 64.00 + 16.00} = \sqrt{116.00} \approx 10.77 \).Now, substitute these into the formula:\( \cos\theta = \frac{-48.00}{7.35 \times 10.77} \approx \frac{-48.00}{79.11} \approx -0.61 \).Finally, calculate \( \theta \) using \( \cos^{-1}(-0.61) \approx 128.74^\circ \).

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Key Concepts

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

Newton's Laws of Motion
Newton's laws of motion are fundamental in understanding how objects behave under various forces. The second of these laws, which states \( \vec{F} = m \cdot \vec{a} \), is particularly important in mechanics. This law tells us that the force \( \vec{F} \) acting on an object is equal to the mass \( m \) of the object multiplied by its acceleration \( \vec{a} \). Simply put, if you know the force acting on an object and its mass, you can determine its acceleration.

In the problem provided, we used this law to find the acceleration of the object. We took the force vector \( (6.00 \text{ N}) \hat{i} - (8.00 \text{ N}) \hat{j} + (4.00 \text{ N}) \hat{k} \) and divided each of its components by the mass, 2.00 kg. This gives us an acceleration of \( 3.00 \text{ m/s}^2 \) in the \( \hat{i} \) direction, \( -4.00 \text{ m/s}^2 \) in the \( \hat{j} \) direction, and \( 2.00 \text{ m/s}^2 \) in the \( \hat{k} \) direction.
This demonstrates how the force applied to an object influences its rate of change of motion based on its mass.
Angular Momentum
Angular momentum is a key concept that explains the rotational motion of objects. It is the product of an object's momentum and its distance from a point, typically calculated using the cross product: \( \vec{L} = \vec{r} \times \vec{p} \). Here, \( \vec{r} \) is the position vector and \( \vec{p} \) is the linear momentum, calculated as the object's mass times its velocity \( m \cdot \vec{v} \).

In our problem, we computed the angular momentum using the provided position and velocity vectors. The position vector is \( (2.00 \text{ m}) \hat{i} + (4.00 \text{ m}) \hat{j} - (3.00 \text{ m}) \hat{k} \), and the momentum is \( ( -12.00 \hat{i} + 6.00 \hat{j} + 6.00 \hat{k} ) \text{ kg m/s} \), after multiplying by the mass, 2.00 kg. We found the angular momentum through determinant calculations of the cross product. The result is \( (66.00 \text{ kg m}^2/ ext{s}) \hat{i} + (60.00 \text{ kg m}^2/ ext{s}) \hat{j} + (60.00 \text{ kg m}^2/ ext{s}) \hat{k} \).
Understanding angular momentum is crucial for analyzing phenomena where objects rotate or revolve around a point.
Torque
Torque is like rotational force that causes objects to rotate. It can be thought of as the rotational equivalent of linear force, determined by the vector cross product \( \vec{\tau} = \vec{r} \times \vec{F} \). \( \vec{r} \) is the position vector and \( \vec{F} \) is the force applied. Torque is vital for understanding how effectively a force causes an angular acceleration in an object.

In the exercise provided, we calculated torque using the given force and position vectors. The position vector is \( (2.00 \text{ m}) \hat{i} + (4.00 \text{ m}) \hat{j} - (3.00 \text{ m}) \hat{k} \) and the applied force is \( (6.00 \text{ N}) \hat{i} - (8.00 \text{ N}) \hat{j} + (4.00 \text{ N}) \hat{k} \). By calculating the determinant of the cross product, the torque results in \( (-8.00 \text{ N m}) \hat{i} + (26.00 \text{ N m}) \hat{j} - (40.00 \text{ N m}) \hat{k} \).
This demonstrates the torque's effectiveness in causing the object to rotate, with each component reflecting the influence in its respective axis.
Vector Calculations
Vectors are fundamental in physics, representing quantities that have both magnitude and direction. Operations with vectors, such as addition, dot products, and cross products, are crucial in mechanics to describe real-world phenomena.

The dot product gives a scalar and represents how much one vector goes in the direction of another. It's used to find angles between vectors using the formula \( \cos\theta = \frac{\vec{v} \cdot \vec{F}}{||\vec{v}|| ||\vec{F}||} \). In our problem, the angle \( \theta \) between velocity and force vectors is found using this calculation. The dot product of the velocity \( (-6.00 \hat{i} + 3.00 \hat{j} + 3.00 \hat{k}) \) and force \( (6.00 \hat{i} - 8.00 \hat{j} + 4.00 \hat{k}) \) is \(-48.00\), leading to \( \cos^{-1}(-0.61) \approx 128.74^\circ \).

Cross products give a vector perpendicular to the plane formed by two vectors and are used for torque and angular momentum. Grasping these vector operations helps solve complex physics problems and gives insights into the relationships between dynamic quantities.

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Most popular questions from this chapter

A yo-yo has a rotational inertia of \(950 \mathrm{~g} \cdot \mathrm{cm}^{2}\) and a mass of \(120 \mathrm{~g}\). Its axle radius is \(3.2 \mathrm{~mm},\) and its string is \(120 \mathrm{~cm}\) long. The yo-yo rolls from rest down to the end of the string. (a) What is the magnitude of its linear acceleration? (b) How long does it take to reach the end of the string? As it reaches the end of the string, what are its (c) linear speed, (d) translational kinetic energy, (e) rotational kinetic energy, and (f) angular speed?

A Cylindrical object of mass \(M\) and radius \(R\) rolls smoothly from rest down a ramp and onto a horizontal section. From there it rolls off the ramp and onto the floor, landing a horizontal distance \(d=0.506 \mathrm{~m}\) from the end of the ramp. The initial height of the object is \(H=0.90 \mathrm{~m}\) the end of the ramp is at height \(h=0.10 \mathrm{~m}\). The object consists of an outer cylindrical shell (of a certain uniform density) that is glued to a central cylinder (of a different uniform density). The rotational inertia of the object can be expressed in the general form \(I=\beta M R^{2},\) but \(\beta\) is not 0.5 as it is for a cylinder of uniform density. Determine \(\beta\)

A body of radius \(R\) and mass \(m\) is rolling smoothly with speed \(v\) on a horizontal surface. It then rolls up a hill to a maximum height \(h\). (a) If \(h=3 v^{2} / 4 g\), what is the body's rotational inertia about the rotational axis through its center of mass? (b) What might the body be?

In unit-vector notation, what is the torque about the origin on a particle located at coordinates \((0,-4.0 \mathrm{~m}, 3.0 \mathrm{~m})\) if that torque is due to (a) force \(\vec{F}_{1}\) with components \(F_{1 x}=2.0 \mathrm{~N}, F_{1 y}=F_{1 z}=0,\) and (b) force \(\vec{F}_{2}\) with components \(F_{2 x}=0, F_{2 y}=2.0 \mathrm{~N}, F_{2 t}=4.0 \mathrm{~N} ?\)

Three Particles of mass \(m=23 \mathrm{~g}\) are fastened to three rods of length \(d=12 \mathrm{~cm}\) and negligible mass. The rigid assembly rotates around point \(O\) at the angular speed \(\omega=0.85 \mathrm{rad} / \mathrm{s} .\) About \(O\) what are (a) the rotational inertia of the assembly, (b) the magnitude of the angular momentum of the middle particle, and (c) the mag. nitude of the angular momentum of the asssembly?

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