It's impossible to give you a single "formula to use in physics" because physics is a vast subject with many different areas of study, each with its own set of relevant formulas.

To help you, I need to know what specific area of physics you are interested in. For example:

* Mechanics:

* Newton's Laws of Motion:

* F = ma (Force = mass x acceleration)

* F_gravity = Gm1m2/r^2 (Force of gravity)

* Kinematics:

* v = u + at (Final velocity = initial velocity + acceleration x time)

* s = ut + 1/2 at^2 (Distance = initial velocity x time + 1/2 acceleration x time^2)

* Electricity and Magnetism:

* Ohm's Law: V = IR (Voltage = Current x Resistance)

* Faraday's Law: ε = -dΦ/dt (Induced electromotive force = negative rate of change of magnetic flux)

* Thermodynamics:

* First Law of Thermodynamics: ΔU = Q - W (Change in internal energy = heat added - work done)

* Ideal Gas Law: PV = nRT (Pressure x Volume = number of moles x ideal gas constant x temperature)

How to Find Formulas You Need:

1. Identify the topic: Determine the specific area of physics you're studying (mechanics, electricity, thermodynamics, etc.).

2. Consult textbooks and online resources: Physics textbooks, websites, and online resources like Khan Academy will provide formulas related to the topic.

3. Understand the concepts: Don't just memorize formulas. Understand the concepts behind them and how they are derived. This will help you apply the formulas correctly in different situations.

4. Practice problem solving: The best way to learn how to use formulas is by solving problems. Work through examples and practice exercises to solidify your understanding.

I hope this helps! Let me know if you have a specific area of physics in mind, and I can give you more specific formulas and guidance.