Physics Sensei questions & answers
How Do I Know Which Equation to Use in Physics?
You know the equations. They’re right there on your formula sheet. But when a problem shows up, you stare at the sheet and try them one by one, hoping something fits.
Sometimes you guess right. Often you don’t, and you can’t tell why.
Knowing which equation to use in physics isn’t about memorizing more. It’s about reading the problem in a way that points to the right equation. There’s a simple way to do that.
What’s really going on
An equation is a connection between a few quantities. It’s useful when it connects what you know to what you want, with nothing else unknown in it.
So the right equation isn’t hiding on the formula sheet. It’s hiding in the problem. Once you write down what you know and what you want, as symbols, the right equation is usually the one that contains exactly those.
Take motion with constant acceleration. There are five quantities: starting speed, final speed, acceleration, time, and distance. Each of the four main equations leaves out exactly one of them. If a problem never mentions the time, use the equation without time.
Why guessing makes it worse
Guessing feels faster. But every wrong guess burns time and confidence. And when a guess happens to work, you don’t learn why, so you can’t do it again on the test.
The other trap: hidden numbers. Problems often hide values in words. “Starts from rest” means the starting speed is zero. If you miss that, no equation seems to fit.
Why it’s hard to change
Writing out every known and unknown feels slow, especially when you think you can see the answer. Jumping straight to the formula sheet feels quicker.
But the list takes about a minute. Guessing can take ten.
The fix: list the symbols first
Before you look at a single equation, turn the problem into a short list of symbols.
Try this on your next five problems: the symbol list
- Ask: What are they asking? Write the unknown as a symbol with a question mark, like t = ?
- Ask: What do I know? Write every given as a symbol with its value and unit. Translate hidden words too: “from rest” means v₀ = 0.
- Ask: What’s the big idea? Motion, forces, energy, or momentum? That tells you which group of equations to look at.
- In that group, find the equation that contains your unknown and only quantities you know.
- If no equation fits, look for a hidden number you missed, or a second step you need first.
The list does the choosing for you. You’re not guessing anymore. You’re matching.
What changes
The first few times, the list feels like extra work. Soon it becomes automatic, and the right equation almost picks itself.
On the test, when your mind wants to panic and guess, the list gives you something to do in the first minute that always helps.
Don’t search the formula sheet. Read the problem.
Practice problems built for this
On Physics Sensei, every problem is solved step by step, and the second step is always “What do I know?”, the symbol list. Each one asks you to try it first, then gives you one more like it, with new numbers. You find out right away exactly where you slipped.
Your next step: on your next homework, write the symbol list before you look at any equation. Then check one problem on Physics Sensei.
More questions about studying and grades? See them all here.
Raul, Physics Sensei
