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What are the magnitude and direction of the torque about the origin on a particle located at coordinates \((0.0,-4.0,3.0) \mathrm{m}\) due to (a) force \(\vec{F}_{A}\) with components \(F_{A x}=2.0 \mathrm{~N}\) and \(F_{A y}=F_{A z}=0\), and \((\mathrm{b})\) force \(\vec{F}_{B}\) with components \(F_{B x}=0\), \(F_{B v}=2.0 \mathrm{~N}\), and \(F_{B z}=4.0 \mathrm{~N} ?\)

Short Answer

Expert verified
The magnitude and direction of torque due to \(\vec{F}_{A}\) are 10 N·m in the \(6\hat{j} + 8\hat{k}\) direction. The magnitude and direction of torque due to \(\vec{F}_{B}\) are 22 N·m in the \(-22\hat{i}\) direction.

Step by step solution

01

Understand the Problem

Determine the torque about the origin due to given forces acting on a particle located at specified coordinates.
02

Identify the coordinates of the particle

The particle is at position \(0.0, -4.0, 3.0\) in meters.
03

Identify force \(\vec{F}_{A}\) components

The force components for \(\vec{F}_{A}\) are \(F_{A x}=2.0 \, N\), \(F_{A y}=0 \, N\), and \(F_{A z}=0 \, N\).
04

Compute torque \(\vec{\tau}_{A}\) due to \(\vec{F}_{A}\)

Calculate the torque using the cross product of the position vector \((\vec{r})\) and \(\vec{F}_{A}\): \(\vec{\tau}_{A} = \vec{r} \times \vec{F}_{A} = |\begin{vmatrix}\hat{i} & \hat{j} & \hat{k} \0 & -4 & 3 \2 & 0 & 0 \end{vmatrix}|= (-4 \cdot 0 - 3 \cdot 0) \hat{i} - (0 \cdot 0 - 3 \cdot 2) \hat{j} + (0 \cdot 0 - (-4) \cdot 2) \hat{k} = 0 \hat{i} + 6 \hat{j} + 8 \hat{k} = 6 \hat{j} + 8 \hat{k} \)
05

Identify force \(\vec{F}_{B}\) components

The force components for \(\vec{F}_{B}\) are \(F_{B x}=0 \, N\), \(F_{B y}=2.0 \, N\), and \(F_{B z}=4.0 \, N\).
06

Compute torque \(\vec{\tau}_{B}\) due to \(\vec{F}_{B}\)

Calculate the torque using the cross product of the position vector \((\vec{r})\) and \(\vec{F}_{B}\): \( \vec{\tau}_{B} = \vec{r} \times \vec{F}_{B} = | \begin{vmatrix}\hat{i} & \hat{j} & \hat{k} \0 & -4 & 3 \0 & 2 & 4 \end{vmatrix}| = (-4 \cdot 4 - 3 \cdot 2) \hat{i} - (0 \cdot 4 - 3 \cdot 0) \hat{j} + (0 \cdot 2 - (-4) \cdot 0) \hat{k} = (-16 - 6) \hat{i} + 0 \hat{j} + 0 \hat{k} = -22 \hat{i}\)
07

Determine the magnitudes of the torques

The magnitude of torque \(\vec{\tau}_{A}\) can be found using: \(|\vec{\tau}_{A}| = \sqrt{6^{2} + 8^{2}} = 10 \text{ N·m}\). The magnitude of torque \(\vec{\tau}_{B}\) can be found using: \(|\vec{\tau}_{B}| = \sqrt{(-22)^{2}} = 22 \text{ N·m}\).
08

Determine the directions of the torques

Torque \(\vec{\tau}_{A}\) has components in the \(\hat{j}\) and \(\hat{k}\) directions. Torque \(\vec{\tau}_{B}\) is in the \(\hat{i}\) direction.

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Key Concepts

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

cross product
Understanding the torque calculation requires knowledge of the cross product operation. In simple terms, the cross product helps to find a vector that is perpendicular to two given vectors. It's essential in torque calculation because torque is the cross product of the position vector \(\vec{r}\) (from the origin to the particle) and the force vector \(\vec{F}\). The cross product is found using the determinant method with a 3x3 matrix setup of unit vectors and the components of \(\vec{r}\) and \(_F\). It's like calculating the area of a parallelogram with the vectors forming two sides. This operation gives us both the magnitude and direction of the torque vector.
vector components
Vectors, including those used in torque problems, are described by their components along the x, y, and z-axes. For torque, you need to understand both the position vector \(\vec{r}\) and the force vector \(\vec{F}\). The position vector indicates where the particle is located in three-dimensional space. Force vectors, on the other hand, indicate the magnitude and direction of the force applied. When calculating the torque, you break down these vectors into their components, like how \(\vec{F}_A\) is broken down into \(F_{Ax} = 2.0 \mathrm{N}\), \(F_{Ay} = 0 \mathrm{N}\), and \(F_{Az} = 0 \mathrm{N}\). By understanding the components, you can systematically approach finding the resulting torque.
magnitude and direction
After calculating the torque using the cross product, it's crucial to determine both its magnitude and direction. The magnitude is the 'size' of the torque vector and can be found using the Pythagorean theorem. For example, \(\vec{\tau}_A\) had components 6 \hat{j} + 8 \hat{k}\, and its magnitude is calculated as \(|\vec{\tau}_A| = \sqrt{6^2 + 8^2} = 10 \text{ N·m}\). The direction tells us where the torque vector points in space. In the case of \(\vec{\tau}_A\), the direction is along the y and z-axes, given by its components. Knowing both the magnitude and direction helps fully describe the vector and its impact on the rotation of the particle.

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Most popular questions from this chapter

Force \(\vec{F}=(-8.0 \mathrm{~N}) \hat{\mathrm{i}}+(6.0 \mathrm{~N}) \hat{\mathrm{j}}\) acts on a particle with position vector \(\vec{r}=(3.0 \mathrm{~m}) \hat{\mathrm{i}}+(4.0 \mathrm{~m}) \mathrm{j} .\) What are (a) the torque on the particle about the origin and (b) the angle between the directions of \(\vec{r}\) and \(\vec{F} ?\)

The rotor of an electric motor has rotational inertia \(I_{m}=2.0 \times 10^{-3} \mathrm{~kg} \cdot \mathrm{m}^{2}\) about its central axis. The motor is used to change the orientation of the space probe in which it is mounted. The motor axis is mounted parallel to the axis of the probe, which has rotational inertia \(I_{p}=12 \mathrm{~kg} \cdot \mathrm{m}^{2}\) about its axis. Calculate the number of revolutions of the rotor required to turn the probe through \(30^{\circ}\) about its axis.

What is the torque about the origin on a jar of jalapeño peppers located at coordinates \((3.0 \mathrm{~m},-2.0 \mathrm{~m}, 4.0 \mathrm{~m})\) due to (a) force \(\vec{F}_{A}=(3.0 \mathrm{~N}) \hat{\mathrm{i}}-(4.0 \mathrm{~N}) \hat{\mathrm{j}}+(5.0 \mathrm{~N}) \hat{\mathrm{k}},(\mathrm{b})\) force \(\vec{F}_{B}=\) \((-3.0 \mathrm{~N}) \hat{\mathrm{i}}-(4.0 \mathrm{~N}) \hat{\mathrm{j}}-(5.0 \mathrm{~N}) \hat{\mathrm{k}}\), and \((\mathrm{c})\) the vector sum of \(\vec{F}_{A}\) and \(\vec{F}_{B} ?\) (d) Repeat part (c) about a point with coordinates \((3.0 \mathrm{~m},\), \(2.0 \mathrm{~m}, 4.0 \mathrm{~m}\) ) instead of about the origin.

A \(2.0 \mathrm{~kg}\) particle-like object moves in a plane with velocity components \(v_{x}=30 \mathrm{~m} / \mathrm{s}\) and \(v_{y}=60 \mathrm{~m} / \mathrm{s}\) as it passes through the point with \((x, y)\) coordinates of \((3.0,-4.0) \mathrm{m}\). Just then, what is its rotational momentum relative to (a) the origin and (b) the point \((-2.0,-2.0) \mathrm{m}\) ?

(A) Show that \(\vec{a} \cdot(\vec{b} \times \vec{a})\) is zero for all vectors \(\vec{a}\) and \(\vec{b}\). (b) What is the magnitude of \(\vec{a} \times(\vec{b} \times \vec{a})\) if there is an angle \(\phi\) between the directions of \(\vec{a}\) and \(\vec{b}\) ?

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