## Archive of posts with tag “classical mechanics”

### Defend our earth against aliens’ bullets!

The aliens intiated their attack to the earth! They shoot bullets with mass $m$ and speed $v$ from a far-awar planet. To defend, humans built a field $U=\alpha/r$ that can repel the bullets. What regions are safe? The answer turns out to be the interior of a circular paraboloid.

- Categories: physics
- Tags: calculus, classical mechanics, kepler problem, from zhihu

### How to construct mechanics in higher dimensions?

We can derive the equation of motion for mechanical systems in a Galileo universe with $\iota$ time dimensions and $\chi$ space dimensions by generalizing the principle of relativity and Hamilton’s principle.

- Categories: physics
- Tags: from zhihu, classical mechanics, imagination

### Kinetic energy, momentum, and angular momentum of rigid bodies

In this article, we will find that the inertia matrix naturally appears when we calculate the kinetic energy $T$ or the angular momentum $\mathbf M$ of a rigid body. Then, we introduce the concept of principal inertia $\mathbf J_{\mathrm{pri}}$. We also study how the inertia matrix changes under translations and rotations and how those transformations may lead to conclusions that can help us simplify the calculation of inertia matrices.

- Categories: physics
- Tags: rigid body, linear algebra, classical mechanics, from zhihu

### Mapping from Kepler problem to free particle on 3-sphere

There is a canonical transform of the Kepler problem which is the same as the problem of motion of a free particle on 3-sphere. The explicit formula of the transform as well as some links about this topic is written in the article. The explicit formula for $E<0$ is $\mathbf u:=\frac{p^2-p_0^2}{p^2+p_0^2}\hat{\mathbf n}+\frac{2p_0}{p^2+p_0^2}\mathbf p,$ where $\mathbf u$ is the position of the particle on 3-sphere (a 4-dimensional vector), $\mathbf p$ is the momentum of the original particle in Kepler problem, $\hat{\mathbf n}$ is a vector perpendicular to the 3-dimensional hyperplane where $\mathbf p$ lies, and $p_0:=\sqrt{-2mE}$.

- Categories: physics
- Tags: classical mechanics, canonical transformation, letter, kepler problem

### Use complex numbers as canonical variables

In this article, I try exploring an idea: using complex numbers to combine pairs of canonical variables into complex variables: $\mathbf c:=\alpha\mathbf q+\mathrm i\beta\mathbf p$. It turns out that we can write canonical equations $\frac{\mathrm d\mathbf c}{\mathrm dt}=-2\mathrm i\alpha\beta\frac{\partial\mathcal H}{\partial\mathbf c^*}$, Poisson brackets $\left\{f,g\right\}=-2\mathrm i\alpha\beta \left(\frac{\partial f}{\partial\mathbf c}\cdot \frac{\partial g}{\partial\mathbf c^*}- \frac{\partial f}{\partial\mathbf c^*}\cdot \frac{\partial g}{\partial\mathbf c}\right)$, and canonical transformations $\frac{\partial\mathbf c^*}{\partial\mathbf c’^*}= \frac{\partial\mathbf c’}{\partial\mathbf c}, \frac{\partial\mathbf c}{\partial\mathbf c’^*}= -\frac{\partial\mathbf c’}{\partial\mathbf c^*}$ in these complex numbers. Finally, I show two examples of using them in real problems: a free particle, and a harmonic oscillator.

- Categories: physics
- Tags: classical mechanics, canonical transformation, hamiltonian, complex, long paper