Physics Formula Sheet: What to Memorise vs What to Derive for Classes 11-12

Physics in Classes 11 and 12 often feels like an endless memory test. With hundreds of equations across mechanics, electromagnetism, and modern physics, trying to memorise every single expression is a guaranteed route to confusion in high-stakes exams like CBSE, ICSE, state boards, JEE Main, JEE Advanced, and NEET.

The secret of top scorers is not a photographic memory. It is a strict sorting system: memorising foundational core formulas while learning how to rapidly derive everything else. Here is your practical guide on how to divide your physics formula sheet for the 2026-27 academic session.

The Core Rule: Why Rote Memorisation Fails

If you rely purely on memorisation, the moment a question introduces a slightly modified boundary condition, your formula breaks down. Indian competitive exams and modern competency-based board questions test your ability to apply fundamental principles, not your capacity for recall.

Conversely, deriving every single relation from scratch during a three-hour exam wastes precious time. You need a balanced strategy.

Category 1: Formulas You Must Memorise Instantly

These are your bedrock equations. They represent fundamental laws of nature, definitions, or standard results that appear too frequently to waste time re-deriving. When you see a keyword in a problem, these formulas should trigger in your mind immediately.

Mechanics

• Kinematic Equations (Constant Acceleration): $v = u + at, \quad s = ut + \frac{1}{2}at^2, \quad v^2 = u^2 + 2as$
• Newton's Second Law: $F = \frac{dp}{dt} = ma$
• Centripetal Acceleration: $a_c = \frac{v^2}{r} = \omega^2 r$
• Work-Energy Theorem & Potential Energies: $W_{\text{net}} = \Delta K, \quad U_{\text{grav}} = mgh, \quad U_{\text{spring}} = \frac{1}{2}kx^2$

Electrodynamics & Magnetism

• Coulomb's Law: $F = \frac{1}{4\pi\varepsilon_0} \frac{q_1 q_2}{r^2}$
• Electric Field & Potential relation: $E = -\frac{dV}{dr}$
• Ohm's Law & Joule's Heating: $V = IR, \quad P = I^2R = \frac{V^2}{R}$
• Magnetic Force on a Moving Charge: $F = q(\vec{v} \times \vec{B})$
• Faraday's Law of Induction: $\varepsilon = -\frac{d\Phi_B}{dt}$

Optics & Modern Physics

• Snell's Law: $\mu_1 \sin i = \mu_2 \sin r$
• Lens Maker's Formula: $\frac{1}{f} = (\mu - 1)\left(\frac{1}{R_1} - \frac{1}{R_2}\right)$
• Einstein’s Photoelectric Equation: $K_{\max} = h\nu - \phi$

Category 2: Formulas You Must Know How to Derive

Never memorise specialized case results or lengthy expressions that stem directly from a standard calculus integration or a symmetrical setup. Instead, memorise the starting principle and practice the derivation step.

| Topic | Complex Result to Derive, Not Memorise | Starting Principle / Integration | | :--- | :--- | :--- | | Electrostatics | Electric field due to a uniform ring/disk at an axis point | Coulomb's Law + vector integration along the geometry | | Current Electricity | Drift velocity expression: $I = nAv_d$ | Counting free electrons in a cylindrical volume element | | Magnetism | Magnetic field at the center/axis of a circular current loop | Biot-Savart Law integration | | SHM | Time period of physical pendulums or liquid columns | Torque equation: $\tau = -C\theta$ or Newton's second law | | Wave Optics | Fringe width in Young's Double Slit Experiment ($\beta = \frac{\lambda D}{d}$) | Path difference approximation ($x \cdot d / D$) |

For instance, memorising the exact formula for the gravitational potential inside a solid sphere is risky. But if you remember Gauss's Law (or its gravitational equivalent) and how to integrate mass shells, you can derive it in 45 seconds on your rough sheet. This guarantees accuracy even if your memory blanks out.

Category 3: Shortcuts for JEE and NEET

Competitive exams reward time management. Here, you can memorise specific shortcut formulas, provided you know their exact constraints:

• Elastic Collision velocities: Memorise the direct velocity exchange formulas for bodies of equal mass, but only use them when explicitly stated.
• Time period of a spring-block system on an inclined plane: Independent of $\sin\theta$, remains $T = 2\pi\sqrt{m/k}$.
• Terminal velocity: $v_t = \frac{2r^2(\rho - \sigma)g}{9\eta}$.

Always check the validity conditions (e.g., small angle approximations, specific medium constraints) before applying shortcuts.

Quick Checklist: Building Your Formula Sheet

• [ ] One page per chapter: Keep your handwritten sheet concise. If it spills over two pages, you are writing too much background text.
• [ ] Define every variable: Next to every equation, note what symbols like $\mu$, $\rho$, or $\theta$ represent, including their SI units.
• [ ] List dimensional formulas: For Class 11 mechanics and units-and-dimensions chapters, tag tricky dimensional formulas right beside the main equation.
• [ ] Mark conditions of validity: Explicitly write when an equation fails (e.g., "valid only for small angles $\theta \ll 1$").
• [ ] Active revision weekly: Do not just read your formula sheet. Close it, pick a blank sheet of paper, and write down every core formula from memory every Sunday.