# Bobbie's Blog

## Fluid or Solid?

This is something I learned today from the Fluid Mechanics class in the Chemical Engineering department, class number CHE 527.

We have had the intuitive ideas about what a fluid is versus what a solid is. A solid is something that can resist shear stress, and will maintain its shape, while a fluid is something that will deform continuously when a shear is applied to it, without changing back to the original shape.

But there are many substances in the world that cannot be classified into solid or fluid with the above simple and intuitive definitions. Here are some examples:

## Human Brain Works Logarithmically

Do you know that our brain works logarithmically?

1. Do this exercise:
• Draw a number line
• Mark $0$ and $1,000,000,000$ on the line.
• Ask yourself a question: where is $1,000$?
2. Many people put $1,000$ at the $1/3$ position. Note that $1,000=10^3$, $1,000,000,000=10^9$, where $9$ is triple of $3$. This implies that we perceive number logarithmically!
3. Not only size of a number, but also weight of an object, loudness of a sound, brightness of a star, spiciness of a chili, all of these are perceived logarithmically by the human brain.
4. Such logarithmic law is known more than 100 years ago, under the name “Weber-Fechner law” in psychophysics.

## 管理引用文献

#### 市面上的文献管理软件

• EndNote：据说挺好用，但是（1）需要花稍多一点的时间学习使用，（2）收费，差不多 80 大洋。网络 basic 版免费，但是试用了一下并不方便，也有导入数量限制。
• Bookends：优势是可以找 Amazon 上的书，不用费神想到底要连哪个出版商的数据库。
• Mendeley：暂时觉得最好用的一个。免费。可以把 PDF 文献原文拖入软件窗口，就会帮你自动识别 citation 信息，虽然并不总是准确，但如果知道 DOI 的话可以自动更正。在 mendeley 中可以连接到本地 PDF，并且直接打开做笔记挺方便。
• BibDesk：神器，免费，原生支持 BibTeX (BibTeX 是基于 LaTeX 的文献管理软件)。适合习惯用 LaTeX 写论文的人 (理工科写论文基本标配用 LaTeX )。就用它了！

Remark: 如果不怎么用 LaTeX 写论文的话，Mendeley 就很好用。以前大部分人都用 Mendeley，现在很多科研人员不用的原因是因为“洁癖”： Mendeley 被 Elsevier (爱思唯尔出版社) 收购了，Elsevier 在学术界变得“臭名昭著”可以参考 Wikipedia: The Cost of Knowledge，中文参考 科学网：逾万科学家联名抵制爱思唯尔

## 不要计算

A computation is a temptation that should be resisted as long as possible.
- John P. Boyd (Michigan)

- John Wheeler

Six months in the lab can save you a day in the library.
- Albert Migliori

## Dimensional Analysis

Once one of my engineering friends asked me: “Why would you need all these dimensionless numbers? There are so many of them and they just complicate stuff!”

If something is not useful, it wouldn’t have been invented in science; this is particularly true for dimensional analysis which has made great success across all fields of science.

To put things simple, dimensional analysis has two major fundamental applications:

• Simplify equations and lead to new scientific branches/specialties.
• Guide scientific experiments with dynamic similitude.

## 写数学 Blog

• LaTeX 行间模式（displayed math）