Angular acceleration and angular momentum are fundamental concepts in the field of physics, particularly in the study of rotational motion. These two quantities are closely related and play a crucial role in understanding the behavior of objects undergoing rotational motion. In this comprehensive guide, we will delve into the intricacies of angular acceleration and angular momentum, providing a detailed exploration of their definitions, formulas, applications, and realworld examples.
Understanding Angular Acceleration
Angular acceleration is the rate of change of angular velocity with respect to time. It is a vector quantity, meaning it has both magnitude and direction. The magnitude of the angular acceleration vector is directly proportional to the rate of change of angular velocity, and its direction is perpendicular to the plane in which the rotation takes place.
The formula for angular acceleration is:
α = dω/dt
where:
– α
is the angular acceleration (in radians per second squared, rad/s²)
– ω
is the angular velocity (in radians per second, rad/s)
– t
is the time (in seconds, s)
The direction of the angular acceleration vector is determined by the righthand rule. If the rotation is in the counterclockwise direction, the angular acceleration vector points in the direction of the thumb when the fingers of the right hand are curled in the direction of rotation.
Factors Affecting Angular Acceleration
The magnitude of the angular acceleration of an object depends on several factors:
 Net Torque: The net torque acting on the object is the primary factor that determines its angular acceleration. The greater the net torque, the greater the angular acceleration.
 Moment of Inertia: The moment of inertia of an object is a measure of its resistance to changes in its rotational motion. The greater the moment of inertia, the smaller the angular acceleration for a given net torque.
 Mass Distribution: The distribution of an object’s mass around its axis of rotation also affects its angular acceleration. Objects with a greater concentration of mass near the axis of rotation will have a smaller moment of inertia and, consequently, a larger angular acceleration for the same net torque.
Examples of Angular Acceleration

Spinning Top: When a spinning top is released, it experiences a constant downward force due to gravity, which creates a net torque about the top’s axis of rotation. This net torque results in an angular acceleration, causing the top to slow down and eventually topple over.

Bicycle Wheel: When a bicycle wheel is accelerated, the angular acceleration of the wheel is directed perpendicular to the plane of rotation, pointing in the direction of the wheel’s axis.

Rotating Disk Experiment: In the context of Experiment 1, the angular position and velocity of a rotating disk are measured using a rotary motion sensor. The sensor records the angular position and velocity as the disk undergoes a constant angular acceleration.
Understanding Angular Momentum
Angular momentum is a measure of the amount of rotation an object has, taking into account its mass, angular velocity, and the distribution of its mass around the axis of rotation. It is also a vector quantity, and its direction is the same as the direction of the angular velocity vector.
The formula for angular momentum is:
L = I * ω
where:
– L
is the angular momentum (in kilogrammeters squared per second, kg·m²/s)
– I
is the moment of inertia (in kilogrammeters squared, kg·m²)
– ω
is the angular velocity (in radians per second, rad/s)
The moment of inertia, I
, is a measure of an object’s resistance to changes in its rotational motion. It depends on the object’s mass and the distribution of its mass around the axis of rotation.
Conservation of Angular Momentum
The angular momentum of a system is conserved unless there is a net external torque acting on the system. This means that the total angular momentum of a closed system remains constant over time, unless an external force or torque is applied.
Examples of Angular Momentum

Spinning Figure Skater: When a figure skater pulls their arms in during a spin, their moment of inertia decreases, causing their angular velocity to increase in order to conserve angular momentum.

Dropping Ring on Rotating Disk: In Experiment 2, a nonrotating ring is dropped onto a rotating disk. The conservation of the angular momentum of the system is verified by measuring the linear speed before and after the torquefree collision.

Gyroscope: A gyroscope maintains its orientation due to the conservation of angular momentum. When a force is applied to the gyroscope, it resists the change in orientation, demonstrating the principle of angular momentum conservation.
Numerical Examples and Problems
 Calculating Angular Acceleration: A solid cylinder with a mass of 5 kg and a radius of 0.2 m is subjected to a constant net torque of 10 N·m. Calculate the angular acceleration of the cylinder.
Given:
– Mass (m) = 5 kg
– Radius (r) = 0.2 m
– Net Torque (τ) = 10 N·m
Moment of Inertia (I) for a solid cylinder:
I = (1/2) * m * r^2
I = (1/2) * 5 kg * (0.2 m)^2
I = 0.2 kg·m²
Angular Acceleration (α):
α = τ / I
α = 10 N·m / 0.2 kg·m²
α = 50 rad/s²
 Calculating Angular Momentum: A solid sphere with a mass of 3 kg and a radius of 0.1 m is rotating at an angular velocity of 20 rad/s. Calculate the angular momentum of the sphere.
Given:
– Mass (m) = 3 kg
– Radius (r) = 0.1 m
– Angular Velocity (ω) = 20 rad/s
Moment of Inertia (I) for a solid sphere:
I = (2/5) * m * r^2
I = (2/5) * 3 kg * (0.1 m)^2
I = 0.006 kg·m²
Angular Momentum (L):
L = I * ω
L = 0.006 kg·m² * 20 rad/s
L = 0.12 kg·m²/s
These examples demonstrate the application of the formulas and principles related to angular acceleration and angular momentum, providing a deeper understanding of these concepts.
Conclusion
Angular acceleration and angular momentum are fundamental concepts in the study of rotational motion. Understanding these concepts is crucial for analyzing the behavior of objects undergoing rotational motion, whether it’s a spinning top, a rotating bicycle wheel, or a figure skater in motion.
By exploring the definitions, formulas, and examples related to angular acceleration and angular momentum, we have gained a comprehensive understanding of these important physics principles. This knowledge can be applied to a wide range of realworld scenarios, from engineering applications to the analysis of sports and everyday activities.
As we continue to delve into the world of physics, the mastery of angular acceleration and angular momentum will undoubtedly prove invaluable in our pursuit of a deeper understanding of the physical universe.
References:
 Angular Acceleration  Physics – Lumen Learning
 What Is Angular Acceleration? – BYJU’S
 5.1 Angular Position and Displacement – Biomechanics of Human …
 What is angular acceleration (rotational acceleration)? – TechTarget
 Angular Momentum – WebAssign
Hi, I’m Akshita Mapari. I have done M.Sc. in Physics. I have worked on projects like Numerical modeling of winds and waves during cyclone, Physics of toys and mechanized thrill machines in amusement park based on Classical Mechanics. I have pursued a course on Arduino and have accomplished some mini projects on Arduino UNO. I always like to explore new zones in the field of science. I personally believe that learning is more enthusiastic when learnt with creativity. Apart from this, I like to read, travel, strumming on guitar, identifying rocks and strata, photography and playing chess.