Hurricanes are considered the powerhouse of weather phenomena. To fuel their need for fury, they use warm ocean air to absorb warm ocean water. Winds, which come together at the surface of the ocean, then force the warm ocean air to rise. The air eventually cools and forms clouds. This process is continuously repeated, resulting in air and clouds rotating around what is known as the eye of the storm. As this occurs at faster and faster rates, the hurricane generates more and more power to unleash on those closest to it. Now, these chilling, yet majestic, phenomena are prime examples of rotational motion. Therefore, let this article introduces the concept of rotational motion.
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Jetzt kostenlos anmeldenHurricanes are considered the powerhouse of weather phenomena. To fuel their need for fury, they use warm ocean air to absorb warm ocean water. Winds, which come together at the surface of the ocean, then force the warm ocean air to rise. The air eventually cools and forms clouds. This process is continuously repeated, resulting in air and clouds rotating around what is known as the eye of the storm. As this occurs at faster and faster rates, the hurricane generates more and more power to unleash on those closest to it. Now, these chilling, yet majestic, phenomena are prime examples of rotational motion. Therefore, let this article introduces the concept of rotational motion.
Below we will define rotational motion and discuss how it is divided into different types.
Rotational Motion is defined as a type of motion associated with objects that travel in a circular path.
Rotational Motion can be divided into three types.
The physics behind rotational motion is described by a concept known as kinematics. Kinematics is a field within physics that focuses on the motion of an object without referencing the forces causing the motion. Kinematics focuses on variables such as acceleration, velocity, displacement, and time which can be written in terms of linear or rotational motion. When studying rotational motion, we use the concept of rotational kinematics. Rotational kinematics refers to rotational motion and discusses the relationship between rotational motion variables.
Note that velocity, acceleration, and displacement are all vector quantities meaning they have magnitude and direction.
The rotational motion variables are:
Angular velocity is the change in the angle with respect to time. Its corresponding formula is $$ \omega = \frac{\theta}{t}$$ where angular velocity is measured in radians per second, \(\mathrm{\frac{rad}{s}}\).
The derivative of this equation yields the equation
$$\omega = \frac{\mathrm{d}\theta}{\mathrm{d}t},$$
which is the definition of instantaneous angular velocity.
Angular acceleration is the change in angular velocity with respect to time. Its corresponding formula is $$ \alpha = \frac{\omega}{t} $$ where angular acceleration is measured in radians per second squared, \(\mathrm{\frac{rad}{s^2}}\).
The derivative of this equation yields the equation
$$\alpha = \frac{\mathrm{d}\omega}{\mathrm{d}t},$$
which is the definition of instantaneous angular acceleration.
Angular displacement is the product of angular velocity and time. Its corresponding formula is $$ \theta = \omega t $$ where angular displacement is measured in radians, \(\mathrm{rad}\).
Time is time. $$ \mathrm{time} = t $$ where time is measured in seconds, \(s\).
Before diving deeper into rotational kinematics, we must be sure to recognize and understand the relationship between kinematic variables. This can be seen when looking at the variables in the table below.
Variable | Linear | Linear SI units | Angular | Angular SI units | Relationship |
acceleration | $$a$$ | $$\frac{m}{s^2}$$ | $$\alpha$$ | $$\mathrm{\frac{rad}{s^2}}$$ | $$\begin{aligned}a &= \alpha r \\\alpha &= \frac{a}{r}\end{aligned}$$ |
velocity | $$v$$ | $$\frac{m}{s}$$ | \(\omega\) | $$\mathrm{\frac{rad}{s}}$$ | $$\begin{aligned}v &= \omega r \\\omega &= \frac{v}{r}\end{aligned}$$ |
displacement | $$x$$ | $$m$$ | \(\theta\) | $$\mathrm{rad}$$ | $$\begin{aligned}x &= \theta r \\\theta &= \frac{x}{r}\end{aligned}$$ |
time | $$t$$ | $$s$$ | \(t\) | $$\mathrm{s}$$ | $$t = t$$ |
Note that \(r\) represents the radius and time is the same in both linear and angular motion.
As a result, kinematic equations of motion can be written in terms of linear and rotational motion. However, it is important to understand that although equations are written in terms of different variables, they are of the same form because rotational motion is the equivalent counterpart of linear motion.
Remember these kinematic equations only apply when acceleration, for linear motion, and angular acceleration, for rotational motion, are constant.
The relationship between rotational motion and rotational motion variables is expressed through three kinematic equations, each of which is missing a kinematic variable.
$$\omega=\omega_{o} + \alpha{t}$$
$$\Delta{\theta} =\omega_o{t}+\frac{1}{2}{\alpha}t$$
$$\omega^2={\omega_{o}}^2 +2{\alpha}\Delta{\theta}$$
where \(\omega\) is final angular acceleration, \(\omega_0\) is the initial angular velocity, \(\alpha\) is angular acceleration, \(t\) is time, and \( \Delta{\theta} \) is angular displacement.
These kinematic equations only apply when angular acceleration is constant.
As we have discussed rotational kinematics, it is also important for us to discuss rotational dynamics. Rotational dynamics deals with the motion of an object and the forces causing the object to rotate. In rotational motion, we know this force is torque.
Below will we define torque and its corresponding mathematical formula.
In order to formulate Newton's second law in terms of rotational motion, we must first define torque.
Torque is represented by \(\tau\) and is defined as the amount of force applied to an object that will cause it to rotate about an axis.
The equation for torque can be written in the same form as Newton's second law, \(F=ma\), and is expressed as $$\tau = I \alpha$$
where \(I\) is the moment of inertia and \(\alpha\) is angular acceleration. Torque can be expressed this way as it is the rotational equivalent of force.
Note that the moment of inertia is the measurement of an object's resistance to angular acceleration. Formulas regarding an object's moment inertia will vary depending on the shape of the object.
However, when the system is at rest, it is said to be in rotational equilibrium. Rotational equilibrium is defined as a state in which neither a system's state of motion nor its internal energy state changes with respect to time. Therefore, for a system to be at equilibrium, the sum of all forces acting on the system must be zero. In rotational motion, this means that the sum of all torques acting on a system must equal zero.
$$ \sum \tau = 0 $$
The sum of all torques acting on a system can be zero if the torques are acting in opposite directions thus canceling out.
The relationship between angular acceleration and torque is expressed when the equation, \( \tau={I}\alpha \) is rearranged to solve for angular acceleration. As a result, the equation becomes\( \alpha=\frac{\tau}{I} \). Thus, we can determine that angular acceleration is proportional to torque and inversely proportional to the moment of inertia.
To solve rotational motion examples, the five rotational kinematic equations can be used. As we have defined rotational motion and discussed its relation to kinematics and linear motion, let us work through some examples to gain a better understanding of rotational motion. Note that before solving a problem, we must always remember these simple steps:
Let us apply the rotational kinematic equations to a spinning top.
A spinning top, initially at rest, is spun and moves with an angular velocity of \(3.5\,\mathrm{\frac{rad}{s}}\). Calculate the top's angular acceleration after \(1.5\,\mathrm{s}\).
Based on the problem, we are given the following:
As a result, we can identify and use the equation, ,\( \omega=\omega_{o} + \alpha{t} \) to solve this problem. Therefore, our calculations are:
$$\begin{aligned}\omega &= \omega_{o} + \alpha{t} \\\omega-\omega_{o} &= \alpha{t} \\\alpha &= \frac{\omega-\omega_{o}}{t} \\\alpha &= \frac{3.5\,\frac{rad}{s}- 0}{1.5\,s} \\\alpha &= 2.33\,\frac{rad}{s}\end{aligned}$$
The angular acceleration of the top is \(2.33\,\mathrm{\frac{rad}{s^2}}\).
Next, we will do the same thing for a tornado.
What is the angular acceleration of a tornado, initially at rest, if its angular velocity is given to be \(95\,\mathrm{\frac{rad}{s}}\) after \(7.5\,\mathrm{s}\)? What is the tornado's angular displacement?
Based on the problem, we are given the following:
As a result, we can identify and use the equation, \( \omega=\omega_{o}+\alpha{t} \), to solve the first part of this problem. Therefore, our calculations are:\begin{align}\omega &= \omega_{o} + \alpha{t} \\\omega-\omega_{o} &= \alpha{t} \\\alpha &= \frac{\omega-\omega_{o}}{t} \\\alpha &= \frac{95\,\mathrm{\frac{rad}{s}} - 0}{7.5\,\mathrm{s}} \\\alpha &= 12.67\,\mathrm{\frac{rad}{s^2}}\end{align}
Now using this calculated angular acceleration value and the equation, \( \Delta{\theta}=\omega_o{t}+\frac{1}{2}{\alpha}t \), we can calculate the tornado's angular displacement as follows:\begin{align}\Delta{\theta} &= \omega_o{t}+\frac{1}{2}{\alpha}t \\\Delta{\theta} &= \left(0\right) \left(7.5\,\mathrm{s}\right) + \frac{1}{2}\left(12.67\,\mathrm{\frac{rad}{s^2}} \right)\left({7.5\,\mathrm{s}}\right)^2 \\\Delta{\theta} &= \frac{1}{2}\left(12.67\,\mathrm{\frac{rad}{s^2}} \right) ({7.5\,\mathrm{s}})^2 \\\Delta{\theta} &= 356.3\,\mathrm{rad}\end{align}
The angular displacement of the tornado is \(356.3\,\mathrm{rad}\).
For our last example, we will apply the torque equation to a rotating object.
An object, whose moment of inertia is \( 32\,\mathrm{\frac{kg}{m^2}} \) rotates with an angular acceleration of \( 6.8\,\mathrm{\frac{rad}{s^2}} \). Calculate the amount of torque needed for this object to rotate about an axis.
After reading the problem, we are given:
Therefore, applying the equation for torque expressed in the form of Newton's second law, our calculations will be as followed:\begin{align}\tau &= {I}\alpha \\\tau &= \left(32\,\mathrm{\frac{kg}{m^2}}\right)\left(6.8\,\mathrm{\frac{rad}{s^2}}\right) \\\tau &= 217.6\,\mathrm{N\,m}\end{align}
The amount of torque needed to rotate the object about an axis is \( 217.6\,\mathrm{N\,m} \).
Rotational Motion is defined as a type of motion associated with objects that travel in a circular path.
Example of rotational motion are hurricanes, fan blades, a wheel of a car, and the earth orbiting the sun.
Motion about a fixed axis, rotation about an axis in rotation, and a combination of rotational and translational motion.
Linear motion is converted to rotational motion by using the formulas which describe how kinematic motion variables are related to one another.
Pure rotation is motion that is about a fixed axis.
What is the difference between the acceleration and the acceleration vector?
When we refer to the acceleration of a moving object, we are referring to the magnitude of its acceleration vector. The acceleration vector gives us the magnitude and direction of the acceleration.
In your own words, define acceleration vector.
The acceleration vector is a vector that gives us the magnitude and direction of the acceleration of an object.
What is the symbol for the acceleration vector?
\(\vec{a}\)
How do you find the average acceleration vector?
You find the average acceleration vector by taking the difference between the final and initial velocity vectors and dividing that by the difference in time.
What is the formula for the average acceleration vector?
\(\vec{a}_{avg}=\frac{\Delta \vec{v}}{\Delta t}=\frac{\vec{v}_2-\vec{v}_1}{t_2-t_1}\).
Finish this sentence: The acceleration vector points in the same direction as the...
vector found when taking the difference between the final and initial velocities.
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