• Understanding Rotational Motion in Physics

  • Relationship Between Rotational Kinematics and Linear Kinematics

  • Rotational Motion Formulas

  • Rotational Kinematics and Rotational Dynamics

  • Newton's Second Law for Rotational Motion

  • Torque and Angular Acceleration

  • Rotational Motion Examples

  • Rotational Motion Summary

Home > Blog > What is Rotational Motion: An In-Depth Guide to Circular Dynamics

What is Rotational Motion: An In-Depth Guide to Circular Dynamics

Rotational motion is a fascinating concept prevalent throughout our natural world. From the smallest subatomic particles to the vast cosmic dance of planets and stars, revolving circular motion is everywhere we look. A classic example can be seen in the graceful pirouettes of ice skaters.

As they spin across the ice, figure skaters maintain an incredible rotational velocity through a delicate balance of precisely coordinated limb and core movements. By pulling in their arms and tilting their bodies at just the right angles, skaters are able to spin faster and faster while retaining impeccable form. 

This rotational versatility stems from an application of our understanding of fundamental physics principles like angular momentum and centrifugal force. Their mastery of these concepts allows skaters to turn and twirl with fluid beauty on the ice.

Understanding Rotational Motion in Physics

Rotational motion describes how objects rotate. It is governed by the principles of kinematics. Kinematics is a branch of physics that deals with the motion of objects without considering the forces involved. It focuses on variables such as change in position, velocity, acceleration, and time, which can be applied to both linear and rotational motion. When studying rotational motion, we delve into the realm of rotational kinematics. Rotational kinematics explores the relationships between various rotational motion variables.

It's important to note that velocity, acceleration, and displacement are vector quantities, meaning they have both magnitude and direction.

Key Rotational Motion Variables

The essential rotational motion variables include:

  1. Angular velocity
  2. Angular acceleration
  3. Angular displacement
  4. Time

Understanding Angular Velocity, \(\omega\)

Angular velocity represents the rate of change of angle with respect to time. It is calculated using the formula $$ \omega = \frac{\theta}{t} $$ where angular velocity is measured in radians per second (\(\mathrm{\frac{rad}{s}}\)).

The derivative of this equation gives us

$$\omega = \frac{\mathrm{d}\theta}{\mathrm{d}t},$$

which defines instantaneous angular velocity.

Understanding Angular Acceleration, \(\alpha\)

Angular acceleration is the rate of change of angular velocity over time. It is determined by the formula $$ \alpha = \frac{\omega}{t} $$ where angular acceleration is measured in radians per second squared (\(\mathrm{\frac{rad}{s^2}}\)).

The derivative of this equation gives us

$$\alpha = \frac{\mathrm{d}\omega}{\mathrm{d}t},$$

which defines instantaneous angular acceleration.

Understanding Angular Displacement, \(\theta\)

Angular displacement is the result of angular velocity multiplied by time. It is calculated using the formula $$ \theta = \omega t $$ where angular displacement is measured in radians (\(\mathrm{rad}\)).

Understanding Time, \(t\)

Time is simply time. It is represented as $$ \mathrm{time} = t $$ and is measured in seconds (\(s\)).

Relationship Between Rotational Kinematics and Linear Kinematics

Before diving deeper into rotational kinematics, it is important to understand the relationship between kinematic variables. This relationship can be seen when comparing 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, 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.

Rotational Motion Formulas

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.

Rotational Kinematics and Rotational Dynamics

When discussing rotational kinematics, it is essential to also delve into rotational . Rotational focuses on the motion of an object and the forces that induce rotation. In rotational motion, torque is the force responsible for this rotation.

Newton's Second Law for Rotational Motion

To understand Newton's second law in the context of rotational motion, we must first define torque.


Torque, denoted by \(\tau\), is the force applied to an object that causes it to rotate about an axis.

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, similar to Newton's second law \(F=ma\), is expressed as $$\tau = I \alpha$$

Here, \(I\) represents the moment of inertia and \(\alpha\) denotes angular acceleration. Torque is the rotational equivalent of force.

It is important to note that the moment of inertia quantifies an object's resistance to angular acceleration, with formulas varying based on the object's shape.

When a system is at rest, it is in rotational equilibrium. Rotational equilibrium occurs when a system's motion and internal energy remain constant over time. For equilibrium, the sum of all forces must be zero, and in rotational motion, the sum of all torques must equal zero: $$ \sum \tau = 0 $$

If torques act in opposite directions, they can cancel out, resulting in a zero sum.

Torque and Angular Acceleration

The relationship between angular acceleration and torque is captured in the equation \( \tau={I}\alpha \), which can be rearranged to solve for angular acceleration as \( \alpha=\frac{\tau}{I} \). This shows that angular acceleration is directly proportional to torque and inversely proportional to moment of inertia.

Rotational Motion Examples

When tackling rotational motion problems, the five rotational kinematic equations come in handy. By understanding the connection between rotational motion, kinematics, and linear motion, we can work through examples to enhance our comprehension. Before solving a problem, remember these steps:

  1. Read and identify given variables.
  2. Determine the problem's requirements and needed formulas.
  3. Apply formulas and solve.
  4. Use visuals if necessary.

Example 1

Let's apply rotational kinematic equations to a bicycle wheel.

A bicycle wheel, initially at rest, is set into motion reaching an angular velocity of \(4.0\,\mathrm{\frac{rad}{s}}\) after \(2.0\,\mathrm{s}\). Calculate the wheel's angular acceleration.

Based on the problem, we are given the following:

  • Initial velocity
  • Final velocity
  • Time

As a result, we can identify and use the equation, \( \omega=\omega_{o} + \alpha{t} \) to solve this problem. Therefore, our calculations are:

$$\begin{aligned}\omega &= \omega_{o} + \alpha{t} \\\omega-\omega_{o} &= \alpha{t} \\\alpha &= \frac{\omega-\omega_{o}}{t} \\\alpha &= \frac{4.0\,\frac{rad}{s}- 0}{2.0\,s} \\\alpha &= 2.0\,\frac{rad}{s^2}\end{aligned}$$

The angular acceleration of the wheel is \(2.0\,\mathrm{\frac{rad}{s^2}}\).

Example 2

Next, let's calculate the rotational dynamics of a ceiling fan.

Given a ceiling fan that starts from rest and achieves an angular velocity of \(20\,\mathrm{\frac{rad}{s}}\) after \(4\,\mathrm{s}\), calculate its angular acceleration and displacement.

We use the equation:

\( \alpha = \frac{\omega - \omega_{o}}{t} \)

Substitute the values:

\( \alpha = \frac{20\,\mathrm{\frac{rad}{s}} - 0}{4\,\mathrm{s}} = 5\,\mathrm{\frac{rad}{s^2}} \)

Thus, the angular acceleration of the ceiling fan is \(5\,\mathrm{\frac{rad}{s^2}}\).

To find the fan's angular displacement, use the equation:

\( \Delta{\theta} = \omega_{o}t + \frac{1}{2}\alpha t^2 \)

Substitute the values:

\( \Delta{\theta} = 0 \times 4\,\mathrm{s} + \frac{1}{2} \times 5\,\mathrm{\frac{rad}{s^2}} \times (4\,\mathrm{s})^2 = 40\,\mathrm{rad} \)

Therefore, the angular displacement of the ceiling fan is \(40\,\mathrm{rad}\).

Example 3

For our last example, consider the torque required to accelerate a flywheel.

A flywheel with a moment of inertia of \( 50\,\mathrm{\frac{kg}{m^2}} \) experiences an angular acceleration of \( 3.2\,\mathrm{\frac{rad}{s^2}} \). Calculate the torque required to produce this acceleration.

Given the following information:

  • Angular acceleration
  • Moment of inertia

By applying the torque equation derived from Newton's second law, our calculations are as follows:\begin{align}\tau &= {I}\alpha \\\tau &= \left(50\,\mathrm{\frac{kg}{m^2}}\right)\left(3.2\,\mathrm{\frac{rad}{s^2}}\right) \\\tau &= 160\,\mathrm{N\,m}\end{align}

The torque required to rotate the flywheel about an axisis \( 160\,\mathrm{N\,m} \).

Rotational Motion Summary

In summary, rotational motion encompasses the movement of objects along a circular path and is characterized by several key concepts. These include the types of rotational motion—such as fixed-axis rotation and the combination of rotational and translational movements—and the core variables like angular velocity, angular displacement, and angular acceleration. The principles of rotational kinematics help in understanding the relationships between these variables, which can be analogously linked to linear motion equations.

Furthermore, rotational dynamics focuses on the forces and torques that govern rotational behavior, adhering to Newton's Second Law. When the net torque on a system is zero, it achieves rotational equilibrium, showcasing the balance of forces in rotational dynamics.

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