Angular Momentum: Is L = Iw True for Extended Bodies?

Click For Summary
SUMMARY

The discussion centers on the validity of the angular momentum equation L = Iω for extended bodies, particularly in scenarios involving a massless stick with two attached masses. It is established that angular momentum does not always align with the angular velocity vector, especially when the system experiences torque. The inertia tensor, represented as I, plays a crucial role in accurately describing angular momentum in complex systems, as indicated by the equation ℓ = I · ω and its tensor form L_k = Σ(w_l I_{kl}).

PREREQUISITES
  • Understanding of angular momentum and its definitions
  • Familiarity with inertia tensor concepts
  • Basic knowledge of rotational dynamics
  • Proficiency in vector mathematics
NEXT STEPS
  • Study the properties and applications of the inertia tensor in rotational motion
  • Explore the implications of torque on angular momentum in non-uniform systems
  • Learn about the differences between point mass and extended body dynamics
  • Investigate classical mechanics textbooks for detailed explanations of angular momentum
USEFUL FOR

This discussion is beneficial for physics students, mechanical engineers, and anyone studying rotational dynamics and angular momentum in complex systems.

asdf60
Messages
81
Reaction score
0
Is angular momentum defined as L = Iw for extended bodies?

Take for example two masses attached to two ends of a massless stick. The stick is rotated at a constant frequency around the cm, except, at an angle with the stick, so the basically the stick revolves a cone. It's not hard to see that L does not point in the same direction as w, and torque must be applied to make it continue to rotate in the same fashion.

Why is this the case? Is it the (ficticious) centripedal force that puts a torque on the system?

Now, if we replaced the stick-mass object with, say a pencil, then it seems to me the analysis would be about the same, with the angular momentum NOT pointing in the same direction as w, so L = Iw is NOT true.

So then I'm confused as to what L = Iw even means and when it makes sense.
 
Physics news on Phys.org
In general, I is a tensor, known as the inertia tensor and the equation becomes:
\vec L = \mathbf{I} \cdot \vec \omega
or
L_k=\sum_{i=1}^3 w_l I_{kl}
as is (probably) explained in any classical mechanics textbook.
 
Last edited:

Similar threads

  • · Replies 30 ·
2
Replies
30
Views
3K
  • · Replies 6 ·
Replies
6
Views
769
  • · Replies 10 ·
Replies
10
Views
752
  • · Replies 2 ·
Replies
2
Views
3K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 1 ·
Replies
1
Views
1K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 6 ·
Replies
6
Views
2K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 3 ·
Replies
3
Views
3K