- #1
Wavefunction
- 99
- 4
Homework Statement
A uniform right circular cone of height [itex]h[/itex], half angle [itex]α[/itex], and density [itex]ρ[/itex] rolls on its side without
slipping on a uniform horizontal plane in such a manner that it returns to its original position in
a time [itex]\tau[/itex]. Find expressions for the kinetic energy and the components of the angular momentum
of the cone. Hint: If [itex] \vec{v} = \vec{ω}\times\vec{r}[/itex] in the inertial frame, points on the cone instantaneously at rest
in this frame will lie in the direction of [itex]\vec{ω}[/itex].
Homework Equations
[itex]\mathcal{L}=T-U=T_{rot}[/itex]
[itex] T_{rot}=\frac{1}{2}I_{ij}\omega_{i}\omega_{j} = \frac{1}{2}L_{j}\omega_{j}[/itex]
The Attempt at a Solution
Setup: I'll start off by defining an inertial coordinate system [itex]\hat{x}'[/itex] Also successive rotations of this coordinate system will be given by [itex] \hat{x}'',\hat{x}''',...[/itex] etc. Eventually I want to build to a body frame [itex] \hat{x} [/itex].
Rotation 1: about the [itex] \hat{x_3}' [/itex] axis by an angle [itex] \theta [/itex] given by the rotation matrix [itex]\mathbf{A}[/itex]
[itex]\begin{pmatrix}x''_1\\x''_2\\x''_3\end{pmatrix} = \begin{pmatrix}\cos\theta&\sin\theta&0\\-\sin\theta&\cos\theta&0\\0&0&1\end{pmatrix}\begin{pmatrix}x'_1\\x'_2\\x'_3\end{pmatrix}[/itex]
Rotation 2: about the [itex] \hat{x_2}'' [/itex] axis by an angle [itex] \alpha [/itex] given by the rotation matrix [itex]\mathbf{B}[/itex]
[itex]\begin{pmatrix}x'''_1\\x'''_2\\x'''_3\end{pmatrix} = \begin{pmatrix}\cos\alpha&0&-\sin\alpha\\0&1&0\\\sin\alpha&0&\cos\alpha\end{pmatrix}\begin{pmatrix}x''_1\\x''_2\\x''_3\end{pmatrix}[/itex]
Rotation 3: about the [itex] \hat{x_1}'''[/itex] axis by an angle [itex] ψ[/itex] given by the rotation matrix [itex]\mathbf{C}[/itex]
[itex] \begin{pmatrix}x\\y\\z\end{pmatrix} = \begin{pmatrix}1&0&0\\0&\cos ψ&\sin ψ\\0&-\sin ψ&\cos ψ\end{pmatrix}\begin{pmatrix}x'''_1\\x'''_2\\x'''_3\end{pmatrix}[/itex]
So now I have [itex] \vec{x}=\mathbf{CBA}\vec{x}'[/itex] also [itex][\mathbf{CBA}]^{T}\vec{x}=\vec{x}' [/itex] which is a relationship between the inertial and body frames.
I can also get [itex] \vec{\omega} = \dot{\theta}\hat{x_3}'+\dot{ψ}\hat{x_1}''' [/itex]
Putting [itex] \vec{\omega} [/itex] into the body frame: [itex] \vec{\omega} = \begin{pmatrix}-\dot{\theta}\sin\alpha+\dot{ψ}\\\dot{\theta}\cos\alpha\sin ψ\\\dot{\theta}\cos\alpha\cos ψ \end{pmatrix}[/itex]
Okay now before I go any further I want to make sure what I have is correct. Also please if you find a mistake please explain in detail why it is wrong. I really want to get an understanding of rigid body rotations. I'm also attaching my drawing of the various transformations too, thanks in advance for your help.