Angular Impulse

Angular impulse represents the effect of a moment of force, or torque ([math]\displaystyle{ \tau }[/math]), acting on a system over a certain period of time ([math]\displaystyle{ \Delta t }[/math]). Angular impulse helps indicate the direction that the system will rotate in (clockwise or counterclockwise) since it is associated with change in velocity and acceleration.

The Main Idea

Angular impulse is the torque acting over some time interval, or the change in angular momentum. Angular momentum can be changed by an angular impulse. There is no common symbol for angular momentum like how [math]\displaystyle{ \vec{F} }[/math] is for force and [math]\displaystyle{ \vec{p} }[/math] is for momentum, and as a result it is almost always referred to as [math]\displaystyle{ \Delta\vec{L} }[/math], since it is equal to the change in angular momentum [math]\displaystyle{ \vec{L} }[/math], just like how linear impulse ([math]\displaystyle{ J }[/math]) is equal to the change in linear momentum, [math]\displaystyle{ \Delta\vec{p} }[/math].

A Mathematical Model

The angular impulse is equal to the net cross product of a force vector, [math]\displaystyle{ \vec{F} }[/math], applied at a particular location a vector distance [math]\displaystyle{ \vec{d} }[/math] from a pivot point times a specified time interval [math]\displaystyle{ \Delta t }[/math]. This is also equal to the net torque [math]\displaystyle{ \sum{\vec{\tau}} }[/math] times a specified time interval [math]\displaystyle{ \Delta t }[/math].

[math]\displaystyle{ \Delta \vec{L} = \sum{(\vec{F}\times\vec{d})}⋅\Delta t = \sum{\vec{\tau}}⋅\Delta t }[/math]

The angular impulse is equal to the moment of inertia [math]\displaystyle{ I }[/math] times the change in angular velocity [math]\displaystyle{ \Delta\vec{\omega} }[/math].

[math]\displaystyle{ \Delta \vec{L} = I\Delta\vec{\omega} = I\vec{\omega_f} - I\vec{\omega_i} }[/math]

Angular Momentum Principle

The angular momentum principle directly involves angular impulse as shown in the image below:

Both sides are equal to the net angular impulse for a system.

Units

The units for angular impulse are the same as those for angular momentum: [math]\displaystyle{ kg⋅m^2/s }[/math] or [math]\displaystyle{ N⋅m⋅s }[/math].

A Computational Model

Angular impulse is often used to update angular momentum when there is a torque acting on an object. Much like how force times time (impulse) is used to update momentum by adding it to an initial momentum in order to obtain the final momentum when the force is constant, angular impulse can also be used to find final angular momentum or final angular velocity. This can be done by adding angular impulse to an initial angular momentum in a while loop and setting that equal to the final angular momentum. Below is part of a simple code example of a while loop that will update the final angular momentum by adding angular impulse. The final angular momentum is angm_f, the initial angular momentum is angm_i, torquenet is the net torque, deltat is some pre-defined time step, and t is time.

In order to find final angular velocity, one could simply divide the final angular momentum by the moment of inertia (a constant) within the while loop after updating the final angular momentum (angm_f in the example) and before updating the time (t in the example).

Examples

Be sure to show all steps in your solution and include diagrams whenever possible

Simple

The moment of inertia of an upright solid cylinder is 22.5 m ⋅ r^2. The cylinder is rotated from rest and has a final angular velocity of 5 rad/s. What is the angular impulse of the cylinder?

Solution

We are given the moment of inertia, final angular velocity, and deduce that the initial angular velocity is 0 rad/s since it began rotating from rest. As a result, we can find the angular velocity by multiplying the moment of inertia with the change in angular velocity, or 5 rad/s minus 0 rad/s times 22.5 m ⋅ r^2. This gives us the angular impulse, 112.5

Middling

A net force of 40 N is applied to the rim of a spinning wheel for 2 seconds. The radius of the wheel is 20 cm. Find the angular impulse that is applied to this wheel.

Solution

First, we need to find the torque acting on the wheel. Torque is equal to the applied force times the radius, or 40 N ⋅ .2 meters which equals 8 N ⋅ m. Currently, we know the torque applied as well as the duration of its application. Therefore, we can find the angular impulse, which is the applied torque times the duration, or (8 N ⋅ m) ⋅ (2 s) which equals 16 N ⋅ m ⋅ s, the angular impulse.

Connectedness

Angular impulse is present in so many things in daily life, from wheels turning on a bicycle to turning the steering wheel in a car and even a person just spinning around in place. Personally, I'm really interested in computers, desktop computers in particular. This topic relates to the turning of fans in my case, on my graphics card, and on my processor, so angular momentum is critical when it can mean a negative one would result in drastically lower fan speeds that would make a computer overheat or a postive one would result in an increase in fan speed which would likely result in the fans being really noisy and annoying.

As a Computer Science major, angular momentum relates to actual computers in the example I gave previously. Not only that, but in the branch of artificial intelligence, if robots are involved then angular impulse can be critical in their circular motion.

Angular impulse has numerous industrial applications, being critical in any rotating device, like cars (wheels, steering wheels), generators, and even in water/wind mills which can provide hydroelectric/wind power.

History

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See also

Are there related topics or categories in this wiki resource for the curious reader to explore? How does this topic fit into that context?

External links

Angular Momentum Impulse Video

Angular Impulse Practice

References

Chapter 11 of Matter & Interactions 4th Edition

List of Equations in Classical Mechanics

Lesson 13 Angular Impulse