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

A metal bar is in the xy-plane with one end of the bar at the origin. A force F = 97.00 N)i + (-3.00 N)j is applied to the bar at the point x = 3.00 m, y = 4.00 m. (a) In terms of unit vectors i and j, what is the position vector r for the point where the force is applied?

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Identify the coordinates where the force is applied. In this problem, the force is applied at the point (x = 3.00 m, y = 4.00 m).
Recall the definition of a position vector. The position vector \( \vec{r} \) points from the origin to a specific point in space.
Write the position vector \( \vec{r} \) using the coordinates of the point. Since the point is at (3.00 m, 4.00 m), the position vector is \( \vec{r} = 3.00 \, \text{m} \, \hat{i} + 4.00 \, \text{m} \, \hat{j} \).
Express the position vector in terms of unit vectors i and j. The unit vector \( \hat{i} \) corresponds to the x-direction and \( \hat{j} \) corresponds to the y-direction.
Combine the components and unit vectors to form the complete position vector: \( \vec{r} = 3.00 \, \text{m} \, \hat{i} + 4.00 \, \text{m} \, \hat{j} \).

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Key Concepts

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

Position Vector

A position vector represents the location of a point in space relative to a reference point, typically the origin. In a two-dimensional Cartesian coordinate system, it is expressed in terms of unit vectors i and j, where 'i' denotes the x-direction and 'j' denotes the y-direction. For a point (x, y), the position vector is given by r = xi + yj.
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Unit Vectors

Unit vectors are vectors with a magnitude of one that indicate direction. In the Cartesian coordinate system, the unit vector 'i' points in the positive x-direction, while 'j' points in the positive y-direction. They are essential for expressing vectors in a clear and standardized form, allowing for easy manipulation and understanding of vector quantities.
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Force Vector

A force vector is a representation of a force acting on an object, characterized by both magnitude and direction. It is typically expressed in terms of its components along the coordinate axes, such as F = Fxi + Fyj, where Fx and Fy are the force components in the x and y directions, respectively. Understanding force vectors is crucial for analyzing the effects of forces on objects in physics.
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Related Practice
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). (b) What is the maximum torque he could exert with this force, and how should the force be oriented?

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Textbook Question
A metal bar is in the xy-plane with one end of the bar at the origin. A force F = 97.00 N)i + (-3.00 N)j is applied to the bar at the point x = 3.00 m, y = 4.00 m. (b) What are the magnitude and direction of the torque with respect to the origin produced by F?
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Textbook Question
A large wooden turntable in the shape of a flat uniform disk has a radius of 2.00 m and a total mass of 120 kg. The turntable is initially rotating at 3.00 rad/s about a vertical axis through its center. Suddenly, a 70.0-kg parachutist makes a soft landing on the turntable at a point near the outer edge. (a) Find the angular speed of the turntable after the parachutist lands. (Assume that you can treat the parachutist as a particle.)
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Textbook Question
The Spinning Figure Skater. The outstretched hands and arms of a figure skater preparing for a spin can be considered a slender rod pivoting about an axis through its center (Fig. E10.43). When the skater's hands and arms are brought in and wrapped around his body to execute the spin, the hands and arms can be considered a thinwalled, hollow cylinder. His hands and arms have a combined mass of 8.0 kg. When outstretched, they span 1.8 m; when wrapped, they form a cylinder of radius 25 cm. The moment of inertia about the rotation axis of the remainder of his body is constant and equal to 0.40 kg•m2 . If his original angular speed is 0.40 rev/s, what is his final angular speed?

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