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Ch 10: Dynamics of Rotational Motion
Chapter 10, Problem 10

One force acting on a machine part is F = (-5.00 N)i + (4.00 N)j. The vector from the origin to the point where the force is applied is r = (-0.450 m)i +(0.150 m)j. (a) In a sketch, show r, F, and the origin.

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Identify the vectors given in the problem: the force vector \( \mathbf{F} = (-5.00 \, \text{N})\mathbf{i} + (4.00 \, \text{N})\mathbf{j} \) and the position vector \( \mathbf{r} = (-0.450 \, \text{m})\mathbf{i} + (0.150 \, \text{m})\mathbf{j} \).
Draw a coordinate system with x and y axes. The origin (0,0) is your starting point for plotting vectors.
Plot the position vector \( \mathbf{r} \) starting from the origin. Move -0.450 m along the x-axis (leftward) and then 0.150 m along the y-axis (upward). Mark the endpoint as the point where the force is applied.
From the endpoint of the position vector \( \mathbf{r} \), draw the force vector \( \mathbf{F} \). Move -5.00 N along the x-axis (leftward) and 4.00 N along the y-axis (upward) from the endpoint of \( \mathbf{r} \).
Label the vectors appropriately and ensure the sketch clearly shows the origin, the vectors \( \mathbf{r} \) and \( \mathbf{F} \), and the directions they are pointing.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Vector Representation

Vectors are mathematical entities that have both magnitude and direction. In this context, the force vector F and the position vector r are represented in a Cartesian coordinate system, where 'i' and 'j' denote the unit vectors in the x and y directions, respectively. Understanding how to graphically represent these vectors is essential for visualizing their relationships and effects.
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Force and Position Vectors

The force vector F describes the influence acting on an object, while the position vector r indicates the location where this force is applied relative to a reference point, typically the origin. The components of these vectors can be used to analyze the system's behavior, such as calculating torque or determining the resultant force acting on the object.
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Sketching Vectors

Creating a sketch of vectors involves plotting them on a coordinate plane, starting from the origin. The position vector r is drawn from the origin to the point (-0.450 m, 0.150 m), while the force vector F is represented starting from the same point, illustrating its direction and magnitude. This visual representation aids in understanding the spatial relationship between the force and the point of application.
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Related Practice
Textbook Question
Under some circumstances, a star can collapse into an extremely dense object made mostly of neutrons and called a neutron star. The density of a neutron star is roughly 10^14 times as great as that of ordinary solid matter. Suppose we represent the star as a uniform, solid, rigid sphere, both before and after the collapse. The star's initial radius was 7.0 * 10^5 km (comparable to our sun); its final radius is 16 km. If the original star rotated once in 30 days, find the angular speed of the neutron star.
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Textbook Question
Calculate the torque (magnitude and direction) about point O due to the force F in each of the cases sketched in Fig. E10.1. In each case, both the force F and the rod lie in the plane of the page, the rod has length 4.00 m, and the force has magnitude F = 10.0 N. (a)

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Textbook Question
A machinist is using a wrench to loosen a nut. The wrench is 25.0 cm long, and he exerts a 17.0-N force at the end of the handle at 37° with the handle (Fig. E10.7).

(a) What torque does the machinist exert about the center of the nut?
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Textbook Question
Three forces are applied to a wheel of radius 0.350 m, as shown in Fig. E10.4. One force is perpendicular to the rim, one is tangent to it, and the other one makes a 40.0° angle with the radius. What is the net torque on the wheel due to these three forces for an axis perpendicular to the wheel and passing through its center?

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Textbook Question
A 12.0-kg box resting on a horizontal, frictionless surface is attached to a 5.00-kg weight by a thin, light wire that passes over a frictionless pulley (Fig. E10.16). The pulley has the shape of a uniform solid disk of mass 2.00 kg and diameter 0.500 m. After the system is released, find

(b) the acceleration of the box, and
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Textbook Question
A cord is wrapped around the rim of a solid uniform wheel 0.250 m in radius and of mass 9.20 kg. A steady horizontal pull of 40.0 N to the right is exerted on the cord, pulling it off tangentially from the wheel. The wheel is mounted on frictionless bearings on a horizontal axle through its center. (b) Find the magnitude and direction of the force that the axle exerts on the wheel.

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