Study guide · 9 min read
How to Learn Physics
Physics is widely believed to be hard because of the mathematics. For most people, at most levels, that is not where the difficulty is. The arithmetic in school physics is easier than the arithmetic in school chemistry. What makes physics hard is that its correct answers frequently contradict what a lifetime of experience has taught you.
You have spent your whole life in a world with friction and air resistance, where things stop unless pushed. Physics starts by telling you that things only change motion when pushed, which is the opposite. Almost every classic misconception traces to that, and the subject does not get easier until the picture in your head is replaced rather than argued with.
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The equations are the last step, not the first
The reliable pattern among students who do badly is reaching for a formula immediately. They scan the question for numbers, find an equation containing those symbols, and substitute. This works on the easiest third of questions and fails completely on the rest, because physics questions are usually testing whether you have the right picture.
The order that works is: describe what is physically happening, draw it, identify what is constant and what is changing, and only then choose an equation. That feels slower and is dramatically faster on anything non-trivial, because most of the time the equation becomes obvious once the picture is right.
Draw the free-body diagram. Every time.
This is the single highest-return habit in the subject and the one most often skipped because it feels like a formality.
A free-body diagram forces you to state what is acting on the object, which is exactly where the errors live. Students routinely include forces that do not exist - a mysterious forward force on a coasting ball - or omit ones that do. Drawing it makes both visible in seconds.
The related idea is that forces come in pairs acting on different objects, which is why third-law pairs never cancel. The book pushes the table and the table pushes the book; those act on different things. Confusing that with the balanced forces on the book alone is one of the most common errors in the subject and a diagram prevents it.
Energy is an accounting system, and it always balances
When a mechanics problem looks impossible with forces, it is often trivial with energy, and vice versa. Knowing which tool to pick is most of the skill in the middle part of the subject.
Forces and acceleration are the right approach when you care about the details of the motion at each moment. Energy is right when you only care about the start and the end - how fast is it going at the bottom, how high does it get - because the messy middle cancels out.
The habit worth building is asking, before starting, whether the question cares about the journey or only the endpoints. That one question saves more time than any formula sheet.
Learn which graph you are reading
Motion graphs cause an outsized share of lost marks and the reason is simple: the same shape means different things depending on the axes.
On a distance-time graph, the gradient is speed and the area under it means nothing. On a velocity-time graph, the gradient is acceleration and the area is distance travelled. A horizontal line means stationary on one and constant velocity on the other - opposite situations, identical picture.
Before reading anything off a graph, say out loud what is on each axis. It sounds patronising and it eliminates a whole category of error.
Units are a free error check
Physics gives you a correctness check most subjects do not have. If the units of your answer are not the units of the thing you were asked for, the answer is wrong, and you know that before seeing the mark scheme.
Carrying units through every line of working, rather than adding them at the end, catches mistakes as they happen. It also frequently tells you which equation you need: if you want a value in metres per second squared and you have metres per second and seconds, there is only one sensible thing to do with them.
The related discipline is temperature. Gas law and thermodynamics calculations need absolute temperature, and using Celsius produces answers that are wrong in a way that looks completely reasonable.
Expect the modern material to feel wrong
Relativity and quantum mechanics contradict intuition even more thoroughly than mechanics does, and students often assume they have misunderstood when they have understood correctly and are simply uncomfortable.
The useful stance is that classical physics is the approximation. Relativistic and quantum effects are present at everyday scales and are far too small to notice, which is why your intuitions formed without them. Nothing is being broken; you are seeing the full version of rules you learned a simplified form of.
The test of whether you are learning physics rather than pattern-matching is whether you can handle an unfamiliar situation. If a question about a bicycle wheel is impossible because you only practised on discs, the understanding is not there yet, and more practice questions of the same type will not fix it.
Go back to the picture. Almost always, that is where it broke.
Practise Physics
18 units and 2,143 questions, every one with a written explanation.
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