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A disk rotates at constant angular acceleration, from angular position \(\theta_{1}=10.0\) rad to angular position \(\theta_{2}=70.0\) rad in \(6.00 \mathrm{~s}\). Its angular velocity at \(\theta_{2}\) is \(15.0 \mathrm{rad} / \mathrm{s}\). (a) What was its angular velocity at \(\theta_{1} ?\) (b) What is the angular acceleration? (c) At what angular position was the disk initially at rest? (d) Graph \(\theta\) versus time \(t\) and angular speed \(\omega\) versus \(t\) for the disk, from the beginning of the motion (let \(t=0\) then \()\).

Short Answer

Expert verified
Find \(\omega_1\) and \(\alpha\): \(\alpha = 1.25\, \text{rad/s}^2\), \(\omega_1 = 7.5\, \text{rad/s}\). Initial rest position: \(\theta_0 = 7.5\, \text{rad}\).

Step by step solution

01

Understanding the Problem

The exercise involves a disk with constant angular acceleration that moves from an initial angular position of \(\theta_1 = 10.0\, \text{rad}\) to a final position \(\theta_2 = 70.0\, \text{rad}\) over a period of \(6.00\, \text{s}\). The final velocity is given as \(\omega_2 = 15.0\, \text{rad/s}\). We need to find the initial angular velocity, the angular acceleration, and where the disk started from rest.
02

Use Kinematic Equation for Angular Velocity at \(\theta_1\)

To find the initial angular velocity \(\omega_1\), use the angular kinematic equation: \[ \omega_2 = \omega_1 + \alpha t \]where \(\omega_2 = 15.0\, \text{rad/s}\), \(t = 6.0\, \text{s}\), and \(\alpha\) is unknown. Rearrange to find \(\omega_1\): \[ \omega_1 = \omega_2 - \alpha t \].However, \(\alpha\) is still unknown, so defer this calculation until \(\alpha\) is found.
03

Use Angular Kinematic Equation for Angular Acceleration

The angular displacement \(\Delta \theta\) is 60 rad \((\theta_2 - \theta_1)\). Use the formula:\[ \Delta \theta = \omega_1 t + \frac{1}{2} \alpha t^2 \]Substitute \(\Delta \theta = 60\, \text{rad}\) and \(t = 6.0\, \text{s}\):\[ 60 = \omega_1 (6) + \frac{1}{2} \alpha (6)^2 \]. Solve this system along with equations from the previous step.
04

Solve for \(\omega_1\) and \(\alpha\)

Rearranging Step 3's equation:\[ \omega_1 = \frac{60 - 18\alpha}{6} \]Substitute in the rearranged expression from Step 2:\[ 15 = \left( \frac{60 - 18\alpha}{6} \right) + 6\alpha \].Solve this to find \(\alpha\) first and then substitute \(\alpha\) back in to find \(\omega_1\).
05

Calculate Initial Rest Position

Once \(\omega_1\) and \(\alpha\) are known, determine the initial rest position \(\theta_0\) using:\[ \omega_0^2 = 0 = \omega_1^2 + 2\alpha (\theta_0 - \theta_1) \].Rearrange and solve for \(\theta_0\):\[ \theta_0 = \theta_1 - \frac{\omega_1^2}{2\alpha} \].
06

Graph the Motion

Plot \(\theta(t)\) using the calculated values in the kinematic equation:\[ \theta(t) = \theta_1 + \omega_1 t + \frac{1}{2} \alpha t^2 \],from \(t = 0\) to \(t = 6\, \text{s}\). For \(\omega(t)\), use:\[ \omega(t) = \omega_1 + \alpha t \].Graph these based on the previous calculations.

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Key Concepts

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

Angular Displacement
Angular displacement is the measure of the change in the angular position of an object as it moves along a circular path. It is measured in radians, which is the standard unit of angular measure used in many areas of mathematics. In the given problem, the angular displacement is from \(\theta_1 = 10.0\, \text{rad}\) to \(\theta_2 = 70.0\, \text{rad}\). This means the disk has rotated a total of \(70.0 - 10.0 = 60.0\) radians in the given time period of 6 seconds. Understanding angular displacement is crucial for analyzing rotational motion, as it provides the basis to calculate other important quantities, like angular velocity and angular acceleration. Unlike linear displacement, which measures distance directly along a path, angular displacement measures how much an object has rotated relative to its starting position, which helps to track its rotational journey over time.
Angular Velocity
Angular velocity is a vector quantity that describes the rate of change of the angular position of an object over time. It tells us how fast something is spinning, and in which direction. In our problem, the final angular velocity of the disk is given as \(\omega_2 = 15.0\, \text{rad/s}\) at \(\theta_2\). Angular velocity can be calculated using the equation: \[ \omega = \frac{\Delta \theta}{\Delta t} \]where \(\Delta \theta\) is the angular displacement, and \(\Delta t\) is the time interval.Angular velocity is crucial for determining how quickly an object is rotating at any given point. By understanding how angular velocity changes, we can gain insights into the object's rotational dynamics. In the problem, we are tasked to find the initial angular velocity, \(\omega_1\), at \(\theta_1\) using the relationship between initial and final angular velocity and angular acceleration over time.
Angular Acceleration
Angular acceleration refers to the rate of change of angular velocity over time. It indicates how quickly an object is speeding up or slowing down its rotation. For a disk or any rotating object, angular acceleration can be constant or it can vary with time. In our exercise, it was mentioned that the disk rotates with a constant angular acceleration.The formula to calculate angular acceleration, \(\alpha\), is:\[ \alpha = \frac{\Delta \omega}{\Delta t} \]where \(\Delta \omega\) represents the change in angular velocity, and \(\Delta t\) is the time period during which the change occurs. In the problem, using the kinematic equations, we solve for \(\alpha\). This quantity plays a crucial role in determining the dynamics of rotational motion, as it helps pinpoint how the angular velocity is changing, which in turn affects how fast the object spins at any point in time.
Rotational Motion
Rotational motion is the movement of an object around a central axis or point. This type of motion is characterized by angular displacement, angular velocity, and angular acceleration. It differs from linear motion, where objects move along a straight path; in rotational motion, the path is circular. Rotational motion can be described using several equations similar to those used for linear motion but adapted for circular paths. These kinematic equations help to understand how factors like torque and moment of inertia influence the object's rotational behavior. In our problem, the rotational motion of the disk is analyzed over a specified duration. By applying kinematic equations, we can determine various aspects of its motion, such as initial and final angular velocities, angular displacement over time, and the constant angular acceleration that affects its motion. Understanding these concepts is essential in fields ranging from mechanical engineering to physics-based simulations.

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