Latest post

follow me on Mastodon

Articles

• <<
• <
• 3
• 4
• 5
• 6
• 7
• >
• >>

• Nov 15, 2022

  Solving ODE by recursive integration

  By recursively integrating according to $x_{n+1}\!\left(t\right)\coloneqq\int_{t_0}^tf\!\left(x_n\!\left(s\right),s\right)\,\mathrm ds+C$ from $x_0\!\left(t_0\right)\coloneqq C$, we can get the solution of the ODE $x'\!\left(t\right)=f\!\left(x\!\left(t\right),t\right)$ with initial conditions $x\!\left(t_0\right)=C$ as the limit of the sequence of functions.

  • Categories: math
  • Tags: ode, calculus
• Nov 13, 2022

  An example of non-uniform elements: heavy elastic rope

  To illustrate the concept about non-uniform elements, we study a simple problem: suppose a uniform heavy elastic rope has mass $m$, original length $L_0$, and stiffness $k$, and find the mass distribution and length of it when hung vertically. We can use the element method to solve this problem, but the elements are non-uniform in terms of length. The elements add up to get the total length $L=\frac{mg}{2k}+L_0$.

  • Categories: physics
  • Tags: from zhihu
• Nov 12, 2022

  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
• Nov 11, 2022

  Hölder means inequality

  The Hölder mean of $\vec x$ with weights $\vec w$ and a parameter $p$ is defined as $M_{p,\vec w}\!\left(\vec x\right)\coloneqq\left(\vec w\cdot\vec x^p\right)^{\frac 1p}$, and the value at $p=-\infty,0,+\infty$ are defined by the limits. We can prove using Jensen’s inquality that the Hölder mean increases as $p$ increases. This property can be used to prove HM-GM-AM-QM inequalities.

  • Categories: math
  • Tags: calculus, from zhihu
• Nov 9, 2022

  A polynomial whose sum of coefficients is a factorial

  The function $\left(1-z\right)^{n+1}\sum_{k=1}^\infty k^nz^k$ is a polynomial of degree $n$ w.r.t. $z$, and the sum of its coefficients is $n!$. This turns out to be properties of Eulerian numbers.

  • Categories: math
  • Tags: combinatorics, number sequence, from zhihu
• Nov 9, 2022

  The image of a circular object through a thin lens

  The image of a circle with radius $r$ and centered at $C\left(-2f,0\right)$ through a thin lens at $x=0$ with focal length $f$ and centered at $O\left(0,0\right)$ is a conic section with the focus being $\left(2f,0\right)$, the directrix being line $x=f$, and the eccentricity being $\frac rf$.

  • Categories: physics
  • Tags: geometrical optics, from zhihu
• Nov 8, 2022

  The point on the circle farthest to two lines

  Suppose $P$ is a point on the circle $\odot C$. When is the sum of distances from $P$ to two edges of $\angle O$ extremal? It turns out to be related to angle bisectors (the intersections of $\odot C$ and the bisector of $\angle O$ or its adjacent supplementary angle are extremals), while the edge cases (at the intersections of $\odot C$ and edges of $\angle O$) are a little tricky: we need to use the bisectors to divide the plane into four quadrants, pick the two quadrants where the line intersecting $\odot C$ at $P$ lies, translate the region to make it center at $C$, and see whether $P$ is inside the translated region.

  • Categories: math
  • Tags: elementary geometry, trigonometry, from zhihu
• Nov 8, 2022

  Typesetting math in emails

  After some comparison among solutions, I use KaTeX to typeset math in my emails.

  • Categories: programming
  • Tags: jekyll, ruby, tex
• Nov 7, 2022

  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 $\mathbf u\coloneqq\frac{p^2-p_0^2}{p^2+p_0^2}\hat{\mathbf n}+\frac{2p_0}{p^2+p_0^2}\mathbf p,$ 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 $p_0\coloneqq\sqrt{-2mE}$.

  • Categories: physics
  • Tags: classical mechanics, canonical transformation, letter, kepler problem
• Nov 6, 2022

  Solving linear homogeneous ODE with constant coefficients

  By using power series, we can prove that the problem of solving linear homogeneous ODE with constant coefficients can be reduced to the problem of solving a polynomial with those coefficients. This article illustrates this point in detail, but it uses a very awful notation…

  • Categories: math
  • Tags: calculus, linear algebra, combinatorics, ode, long paper, from zhihu

• <<
• <
• 3
• 4
• 5
• 6
• 7
• >
• >>

subscribe via RSS