Angular Momentum and Torque

Oct 13, 2025 by wbrum.org 120 views

Introduction

Rotational dynamics is a fundamental concept in physics that deals with the motion of objects that rotate around a central axis. Angular momentum, torque, and rotation are key components of rotational dynamics, and understanding their relationships is crucial for analyzing and predicting the behavior of rotating systems. In this article, we will explore the possibility of a change in angular momentum without a net torque, a concept that may seem counterintuitive at first but has significant implications for our understanding of rotational dynamics.

Angular Momentum and Torque

Angular momentum is a measure of an object's tendency to continue rotating around a central axis. It is a vector quantity that depends on the object's mass, velocity, and distance from the axis of rotation. Torque, on the other hand, is a measure of the rotational force that causes an object to rotate. It is a vector quantity that depends on the force applied, the distance from the axis of rotation, and the angle between the force and the distance.

The relationship between angular momentum and torque is governed by the following equation:

L = r x F

where L is the angular momentum, r is the distance from the axis of rotation, and F is the force applied. The cross product (x) indicates that the direction of the angular momentum is perpendicular to both the distance and the force.

Net Torque and Angular Momentum

In classical mechanics, it is generally accepted that a net torque is required to change the angular momentum of an object. This is because the torque applied to an object causes a change in its angular momentum, and the net torque is the sum of all the torques applied to the object. However, there are certain situations where a change in angular momentum can occur without a net torque.

Example: Two Points on a Box

The picture below shows two points on a box. If torque is taken about point B, there is a net torque applied to the box. However, if we consider the motion of the two points separately, we can see that the angular momentum of each point changes even though there is no net torque applied to the box.

Image Description	Image
Two points on a box	[Insert Image]

Analysis

Let's analyze the motion of the two points on the box. Point A is moving in a circular path around point B, while point C is moving in a straight line. The angular momentum of point A is changing due to the circular motion, while the angular momentum of point C is changing due to the linear motion.

Although there is no net torque applied to the box, the angular momentum of each point is changing due to the motion of the points themselves. This is a classic example of a situation where a change in angular momentum can occur without a net torque.

Implications

The possibility of a change in angular momentum without a net torque has significant implications for our understanding of rotational dynamics. It suggests that the concept of net torque is not as absolute as previously thought and that there are certain situations where a change in angular momentum can occur without a net torque.

This concept also has implications for the design of rotating systems, such as gyroscopes and flywheels. In these systems, the angular momentum of the rotating components can change due to the motion of the components themselves, even if there is no net torque applied to the system.

Conclusion

In conclusion, the possibility of a change in angular momentum without a net torque is a fascinating concept that challenges our understanding of rotational dynamics. While it may seem counterintuitive at first, it is a well-established phenomenon that has significant implications for our understanding of rotating systems.

References

[1] Goldstein, H. (1980). Classical Mechanics. Addison-Wesley.
[2] Landau, L. D., & Lifshitz, E. M. (1976). Mechanics. Pergamon Press.
[3] Thornton, S. T., & Marion, J. B. (2003). Classical Dynamics of Particles and Systems. Brooks Cole.

Glossary

Angular Momentum: A measure of an object's tendency to continue rotating around a central axis.
Torque: A measure of the rotational force that causes an object to rotate.
Net Torque: The sum of all the torques applied to an object.
Gyroscope: A device that uses the principle of angular momentum to maintain its orientation in space.
Flywheel: A rotating wheel that stores energy in the form of angular momentum.
Q&A: Possible Change in Angular Momentum without Net Torque ===========================================================

Introduction

In our previous article, we explored the possibility of a change in angular momentum without a net torque. This concept may seem counterintuitive at first, but it has significant implications for our understanding of rotational dynamics. In this article, we will answer some of the most frequently asked questions about this topic.

Q: What is the relationship between angular momentum and torque?

A: The relationship between angular momentum and torque is governed by the following equation:

L = r x F

Q: Can a change in angular momentum occur without a net torque?

A: Yes, a change in angular momentum can occur without a net torque. This can happen when the motion of the object itself causes a change in its angular momentum, even if there is no external torque applied.

Q: What is an example of a situation where a change in angular momentum can occur without a net torque?

A: One example is the motion of two points on a box. If torque is taken about point B, there is a net torque applied to the box. However, if we consider the motion of the two points separately, we can see that the angular momentum of each point changes even though there is no net torque applied to the box.

Q: How does this concept affect the design of rotating systems, such as gyroscopes and flywheels?

A: This concept has significant implications for the design of rotating systems. In these systems, the angular momentum of the rotating components can change due to the motion of the components themselves, even if there is no net torque applied to the system. This means that designers must take into account the internal motion of the components when designing these systems.

Q: Can you provide more examples of situations where a change in angular momentum can occur without a net torque?

A: Yes, here are a few more examples:

A spinning top: As the top spins, its angular momentum changes due to the motion of its components, even if there is no external torque applied.
A rolling ball: As the ball rolls, its angular momentum changes due to the motion of its components, even if there is no external torque applied.
A rotating wheel: As the wheel rotates, its angular momentum changes due to the motion of its components, even if there is no external torque applied.

Q: What are the implications of this concept for our understanding of rotational dynamics?

A: This concept challenges our understanding of rotational dynamics by showing that a change in angular momentum can occur without a net torque. This means that we must re-examine our understanding of the relationship between angular momentum and torque.

Q: Are there any real-world applications of this concept?

A: Yes, this concept has significant implications for the design of rotating systems, such as gyroscopes and flywheels. It also has implications for the design of other systems, such as rotating machinery and vehicles.

Q: Can you provide more information on the mathematical treatment of this concept?

A: Yes, the mathematical treatment of this concept involves the use of vector calculus and differential equations. The equations of motion for rotating systems must take into account the internal motion of the components, as well as any external torques applied to the system.

Q: Are there any limitations to this concept?

A: Yes, this concept is limited to situations where the motion of the object itself causes a change in its angular momentum. It does not apply to situations where the change in angular momentum is due to an external torque.

Conclusion

In conclusion, the possibility of a change in angular momentum without a net torque is a fascinating concept that challenges our understanding of rotational dynamics. We hope that this Q&A article has provided a clear and concise explanation of this concept and its implications for our understanding of rotating systems.

References

[1] Goldstein, H. (1980). Classical Mechanics. Addison-Wesley.
[2] Landau, L. D., & Lifshitz, E. M. (1976). Mechanics. Pergamon Press.
[3] Thornton, S. T., & Marion, J. B. (2003). Classical Dynamics of Particles and Systems. Brooks Cole.

Glossary

Angular Momentum: A measure of an object's tendency to continue rotating around a central axis.
Torque: A measure of the rotational force that causes an object to rotate.
Net Torque: The sum of all the torques applied to an object.
Gyroscope: A device that uses the principle of angular momentum to maintain its orientation in space.
Flywheel: A rotating wheel that stores energy in the form of angular momentum.