Idea 4 – Rereading lecture notes?

A physics guide for the semi-serious student

Answer: No! Do not reread lecture notes.
Or maybe do, but not right away.

In this post, we are talking about consolidating your knowledge of a module that you already know a bit because you studied it in the past. The idea presented here will be particularly useful before exams, but it can also be helpful whenever you finish a narrative arc (for lack of a better wording) within a module. Compared to Idea 0, where we tried to understand the main points of a course, this is a bit more advanced.

I suggest you choose a lecture you want to refresh your knowledge of, that you studied some time ago. This exercise should not take you more than 5 minutes per lecture, or about 20 minutes for a whole module. Without reading your notes beforehand (skim reading it would be ok in theory, but in practice, it is very likely that this will devolve into passively rereading the whole thing, which is exactly what we are trying to avoid), take out a piece of paper, and let’s get started:

  1. Write down the approximate title of your lecture/module.
  2. Answer, in your own words, the two simple questions: What (what is it about, in a nutshell)? and Why (what do you find interesting about this course)? [I am referring to Idea 0].
  3. Then, write down everything you remember about this lecture/module, as if you were trying to explain it in a few minutes to a panicking classmate before an exam. Try to spend at least a few minutes on this and do not give up, you remember more than you think. Do not forget to think, this is not just a memory game, there are certain things you can find by deriving them or by using your intuition. A word of caution, however: this is not about trying to rewrite the course exhaustively, with all the precise definitions, anecdotes, derivations, etc. While there is certainly a place for this – for the extremely motivated student – it is also very important to be able to extract the essentials for a module, the “tools” that it gives you. Again, think of a panicking classmate. What do they need to know?
  4. If everything comes out a bit mixed up on your piece of paper, this is normal, it happens when you brain dump. You can try and organize it somewhat, by writing a plan and drawing arrows, or just in your head.
  5. Now, and only now, you can open your notes and see what you missed.

What is the point of doing this rather than rereading your notes? Well, the problem with rereading notes is that you will recognize things, and your brain, lazy as it is (nothing personal here, we are all like that), will deduce that it knows them. The problem is… that is not true. There is a big difference between retrieval and recognition. There is a book whose title sums up the situation very well: “Everything Is Obvious: *Once You Know the Answer*”.

When trying to follow the above steps, you will inevitably make some mistakes, and even forget entire parts of your lecture/module. Great! When you open your notes, you will not waste time rereading what you already know and fall asleep before you get to what you do not. Instead, something much more useful will happen: what you had forgotten or remembered incorrectly will jump out at you and sink in much better. The frustration of having forgotten or misremembered entire swaths of your lectures will turn into an automatic desire to not forget about them again.
And all of this will probably take you much less time than if you reread passively.

I will leave you with an example:

Attempt without rereading the notes

Hydrostatics
What? It’s the study of fluids at rest.
Why? It’s funny because, in the end, just like water, air is a fluid, which obeys all these laws. This means that we humans are a bit like fish, but at the bottom of a vast ocean of air.

P(z) = P_0 + \rho g z
P(z) = P_0 + \rho(z) g z for a non-homogeneous fluid?

with P = F/S

Archimedes: \Pi = \rho g V with \rho the density of the fluid in which the object is placed.


After re-reading the notes

Hydrostatics

What? It’s the study of fluids at rest, basically the mass of the fluid above presses on the mass of the fluid below, and the lower you are, the harder it presses.
Why? It’s funny because, in the end, just like water, air is a fluid, which obeys all these laws. This means that we humans are a bit like fish, but at the bottom of a vast ocean of air.

Definition of pressure force: d \vec{F}(M) = - P(M) dS \vec{n}

P(z) = P_0 - \rho g z for an incompressible fluid
\cancel{P(z) = P_0 + \rho(z) g z}
\frac{d P}{dz} = - \rho(z) g for a non-homogeneous (or compressible) fluid.

There is continuity of pressure at the interface between two fluids: P_A(0^-) = P_B(0^+)

Archimedes’ Principle: \vec{\Pi} = - \rho \vec{g} V with \rho the density of the fluid in which the object is placed.



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