Physics Sensei questions & answers
There Are So Many Physics Formulas. How Do I Know When to Use Each One?
Your formula sheet keeps growing. By the second exam, it’s a full page of tiny equations. Every one looks important, and they all start to blur together.
So you try to memorize them all. Then the test asks something slightly different, and you can’t tell which one applies.
When there are so many physics formulas, it feels like you need a perfect memory. You don’t. You need to see that most of them belong to just a few families, and that every formula has fine print.
What’s really going on
Almost every formula in intro physics comes from a handful of big ideas: motion, forces, energy, and momentum. The rest are special cases, or definitions that feed into those.
And every formula has fine print: when it’s true. v = v₀ + at only works when the acceleration is constant. Kinetic plus potential energy stays the same only when there’s no friction. Momentum stays the same only when nothing outside pushes during the collision.
Knowing a formula means knowing its family and its fine print, not just its letters.
Why memorizing the list makes it worse
A list of formulas has no structure. Each one is a separate thing to remember, with nothing tying it to the others. The longer the list, the harder it is to hold.
And a list without fine print is risky. You can use a formula perfectly in a situation where it simply isn’t true, and get the wrong answer with no idea why.
Why it’s hard to change
Your professor’s sheet, or the one in the back of the book, is usually just a list. Reorganizing it feels like extra work, and copying it feels like enough.
But building your own sheet is studying. Deciding where each formula goes is exactly the thinking the test asks for.
The fix: build a family sheet
Turn your formula list into a few families, with the fine print written right next to each formula.
Try this before your next exam: the family sheet
- Make four boxes on a page: Motion, Forces, Energy, Momentum.
- Put each formula from your list into the box where it belongs. If you’re not sure, that’s a good question for office hours.
- Next to each formula, write its fine print: when it works. For example: “only if acceleration is constant.”
- Under each box, write one clue that tells you this family is the big idea. For energy: “heights and speeds, no time.”
- Practice with mixed problems. For each one, ask: What are they asking? What do I know? What’s the big idea? Pick the family first, then the formula.
A page of four families with fine print is easier to remember than a page of thirty separate formulas. And it tells you when each one applies.
What changes
Building the sheet takes an evening. After that, new formulas have a place to go, and the page stops feeling endless.
On the test, you won’t scan thirty formulas. You’ll pick a family, check the fine print, and choose from just a few.
Fewer families. Read the fine print.
Practice problems built for this
On Physics Sensei, every problem is solved step by step, and the third step is always “What’s the big idea?”, the family. Each problem asks you to try it first, then gives you one more like it, with new numbers. You find out right away if you picked the right family.
Your next step: build your family sheet from your current formula list. Then try three mixed problems on Physics Sensei and pick the family before the formula.
More questions about studying and grades? See them all here.
Raul, Physics Sensei
