- #1
Soren4
- 128
- 2
Consider the rotation of a rigid body about a fixed axis [itex]z[/itex], not passing through a principal axis of inertia of the body. The angular momentum [itex]\vec{L}[/itex] has a parallel component to the [itex]z[/itex] axis (called [itex]\vec{L_z}[/itex]) and a component perpendicular to it (called [itex]\vec{L_n}[/itex]). I have some doubts on [itex]\vec{L_n}[/itex].
From the picture we have that, taking a singular point of the rigid body [itex] P_i [/itex],
[itex]\mid \vec{L_{n,i}}\mid=m_i r_i R_i \omega cos\theta_i\implies \mid \vec{L_n}\mid=\omega \sum m_i r_i R_i cos\theta_i [/itex]
Is this correct? Now the quantity [itex]\sum m_i r_i R_i cos\theta_i [/itex] depends on the point [itex] O[/itex] used for the calculation of momenta, nevertheless it is a costant quantity, once calculated, right?
So is it possible to say that [itex]\mid \vec{L_n}\mid \propto \mid \vec{\omega} \mid[/itex] (1)?
If [itex]\vec{\omega}[/itex] is constant then there is an external torque [itex]\vec{\tau}=\frac{d\vec{L}}{dt}=\frac{d\vec{L_n}}{dt}=\vec{\omega}\times\vec{L_n}[/itex], perpendicular both to [itex]\vec{\omega}[/itex] and [itex]\vec{L_n}[/itex].
Now suppose that the direction of [itex]\vec{\omega}[/itex] is still the same (the axis is fixed) but [itex]\mid\vec{\omega}\mid[/itex] changes. Then the external torque has two components. For the component parallel to the axis we can write [itex]\vec{\tau_z}=\frac{d\vec{L_z}}{dt}=I_z \vec{\alpha}[/itex], once calculated the moment of inertia [itex]I_z[/itex] with respect to the axis of rotation, while for the orthogonal component we have [itex]\vec{\tau_n}=\frac{d\vec{L_n}}{dt}[/itex], but what are the characteristics of the component [itex]\vec{L_n}[/itex] in that case?
If (1) holds then [itex]\frac{d\mid \vec{L_n} \mid}{dt} \propto \mid \vec{\alpha} \mid[/itex]
But what about the direction of [itex]\vec{L_n}[/itex] and of its derivative?
From the picture we have that, taking a singular point of the rigid body [itex] P_i [/itex],
[itex]\mid \vec{L_{n,i}}\mid=m_i r_i R_i \omega cos\theta_i\implies \mid \vec{L_n}\mid=\omega \sum m_i r_i R_i cos\theta_i [/itex]
Is this correct? Now the quantity [itex]\sum m_i r_i R_i cos\theta_i [/itex] depends on the point [itex] O[/itex] used for the calculation of momenta, nevertheless it is a costant quantity, once calculated, right?
So is it possible to say that [itex]\mid \vec{L_n}\mid \propto \mid \vec{\omega} \mid[/itex] (1)?
If [itex]\vec{\omega}[/itex] is constant then there is an external torque [itex]\vec{\tau}=\frac{d\vec{L}}{dt}=\frac{d\vec{L_n}}{dt}=\vec{\omega}\times\vec{L_n}[/itex], perpendicular both to [itex]\vec{\omega}[/itex] and [itex]\vec{L_n}[/itex].
Now suppose that the direction of [itex]\vec{\omega}[/itex] is still the same (the axis is fixed) but [itex]\mid\vec{\omega}\mid[/itex] changes. Then the external torque has two components. For the component parallel to the axis we can write [itex]\vec{\tau_z}=\frac{d\vec{L_z}}{dt}=I_z \vec{\alpha}[/itex], once calculated the moment of inertia [itex]I_z[/itex] with respect to the axis of rotation, while for the orthogonal component we have [itex]\vec{\tau_n}=\frac{d\vec{L_n}}{dt}[/itex], but what are the characteristics of the component [itex]\vec{L_n}[/itex] in that case?
If (1) holds then [itex]\frac{d\mid \vec{L_n} \mid}{dt} \propto \mid \vec{\alpha} \mid[/itex]
But what about the direction of [itex]\vec{L_n}[/itex] and of its derivative?