In this guide
This sheet is for the last two or three weeks before the exam, after you have worked through the chapter guides once. It does not teach from scratch. It collects what NDA asks most often into a form you can read in forty minutes and test yourself on in twenty.
Physics carries the largest share of the GK section in the General Ability Test. The NDA & NA (II) 2026 notification gives it 25% of GK, ahead of history and geography at 20% each. Most questions are conceptual or one-step, so revision should be about speed and precision: the exact condition under which a law holds, the exact principle behind a device, the exact unit.
Use it like this: read one block, close the page, and write down everything you remember. Anything you missed goes into your error log. Revisit the log, not the whole sheet, in the final week.
Formulas with their conditions
| Topic | Formula | Condition or trap |
|---|---|---|
| Motion | v = u + at, s = ut + ½at², v² = u² + 2as | constant acceleration only; convert km/h to m/s (× 5/18) |
| Free fall | t = √(2h/g), v = √(2gh), max height u²/(2g) | ignores air resistance; independent of mass |
| Force, momentum | F = ma, p = mv, impulse = Ft = Δp | momentum conserved only with no external force |
| Circular motion | F = mv²/r | speed constant but velocity changing |
| Work, energy | W = Fs cos θ, KE = ½mv², PE = mgh, KE = p²/(2m) | zero work at 90° or with no displacement |
| Power | P = W/t = Fv; 1 hp ≈ 746 W | kWh is energy: 3.6 × 10⁶ J |
| Gravitation | F = Gm₁m₂/r², g = GM/R² | g/4 at height R; g/2 at depth R/2; zero at centre |
| Satellites | v₀ = √(gR) ≈ 7.9 km/s, vₑ = √(2gR) ≈ 11.2 km/s | vₑ = √2 × v₀; both independent of the body's mass |
| Pressure | P = F/A, liquid pressure = hρg | independent of the vessel's shape |
| Hydraulics | F₂ = F₁ × A₂/A₁ | Pascal's law |
| Heat | Q = mcΔT, Q = mL | temperature constant during a change of state |
| Scales | °F = (9/5)°C + 32, K = °C + 273 | equal at −40° |
| Waves | v = fλ, T = 1/f; echo distance = vt/2 | halve for there-and-back |
| Mirrors, lenses | f = R/2, P = 1/f, P = P₁ + P₂ | f in metres; convex +, concave − |
| Refraction | n = c/v, sin C = 1/n | TIR only denser to rarer |
| Electricity | V = IR, P = VI = I²R = V²/R, H = I²Rt | Ohm's law at constant temperature |
| Resistance | series R₁ + R₂; parallel R₁R₂/(R₁ + R₂); R = ρL/A | stretched to 2L gives 4R |
| Transformer | Vs/Vp = Ns/Np, VpIp = VsIs | AC only |
| Radioactivity | fraction left (½)ⁿ after n half-lives | α: Z − 2, A − 4; β: Z + 1 |
| Pendulum | T = 2π√(l/g) | independent of mass and (small) amplitude |
Units that trap people
- Light year is a distance (about 9.46 × 10¹⁵ m). Parsec is about 3.26 light years.
- kWh is energy; kW is power. Joule is energy; watt is power.
- Coulomb is charge; ampere is current. Volt is potential difference; ohm is resistance.
- Dioptre is the unit of lens power; decibel of sound level; hertz of frequency.
- Pascal is pressure; 1 atmosphere ≈ 1.013 × 10⁵ Pa ≈ 76 cm of mercury.
- Relative density, strain and refractive index have no unit.
- Nautical mile = 1,852 m; a knot is one nautical mile per hour.
How things work: devices and principles
The syllabus lists these by name. Each is one line to learn.
| Device | Principle |
|---|---|
| Simple pendulum, pendulum clock | T = 2π√(l/g); a clock loses time in summer (longer rod) and at altitude (smaller g) |
| Lever | moments; see-saw (class I), nutcracker (class II), tongs (class III) |
| Pulley | a fixed pulley changes the direction of effort (MA 1); a movable pulley halves it (MA 2) |
| Siphon | difference in liquid pressure; the outlet must be below the source surface |
| Balloon | Archimedes: it rises while the air displaced weighs more than the balloon |
| Lift pump | atmospheric pressure; cannot lift water more than about 10 m |
| Hydrometer, lactometer | law of flotation; sinks deeper in a less dense liquid |
| Pressure cooker | higher pressure raises the boiling point |
| Thermos flask | vacuum stops conduction and convection; silvering cuts radiation |
| Periscope | two plane mirrors or prisms at 45° |
| Microscope, telescope | convex lenses; telescope objective has a long focal length |
| Mariner's compass | a magnetised needle aligns with the Earth's magnetic field |
| Lightning conductor | pointed rod earthed by a thick strip gives charge a safe path |
| Safety fuse | heating effect; low-melting wire in series with the live wire |
| Electric bell, telegraph | electromagnet pulls an iron armature |
| Motor / dynamo | force on current in a field / electromagnetic induction |
| Transformer | mutual induction; AC only |
| Hydraulic brake, jack | Pascal's law |
| Quartz clock | regular vibrations of a quartz crystal driven by a circuit |
| Optical fibre | total internal reflection |
Scientists and discoveries
| Scientist | Remember for |
|---|---|
| Archimedes | buoyancy, levers |
| Galileo | falling bodies, inertia, the telescope in astronomy |
| Kepler | three laws of planetary motion |
| Newton | laws of motion, gravitation, dispersion of white light |
| Pascal | transmission of pressure in fluids |
| Torricelli | the mercury barometer |
| Oersted | magnetic effect of current |
| Faraday | electromagnetic induction |
| Ohm | V = IR |
| Röntgen | X-rays |
| Becquerel | radioactivity |
| J J Thomson | the electron |
| Rutherford | the nucleus |
| Chadwick | the neutron |
| Einstein | photoelectric effect, E = mc² |
| C V Raman | scattering of light (Raman effect), Nobel Prize 1930 |
Confusable pairs
- Mass vs weight: mass constant; weight = mg changes. Beam balance vs spring balance.
- Pitch vs loudness: frequency vs amplitude.
- Myopia vs hypermetropia: concave vs convex lens.
- Motor vs generator: Fleming's left vs right hand.
- Moderator vs control rods: slow neutrons vs absorb them.
- Fission vs fusion: reactors and atom bombs vs the Sun and the hydrogen bomb.
- Penetrating vs ionising: gamma most vs alpha most.
- Concave vs convex mirror: headlights and shaving vs rear view.
- Conduction vs convection: spoon in tea vs sea breeze.
- Evaporation vs boiling: surface, any temperature, cooling vs throughout the liquid, fixed temperature.
Worked mixed MCQs
Q1. The length of a simple pendulum is made four times. Its period becomes:
(a) four times (b) half (c) the same (d) double
T ∝ √l, and √4 = 2. Answer: (d).
Q2. A pendulum clock that keeps correct time at sea level is taken to a hill station. It will:
(a) gain time (b) lose time (c) keep correct time (d) stop
g is slightly smaller at altitude, so T = 2π√(l/g) increases. Each swing takes longer and the clock runs slow. Answer: (b).
Q3. A 2 kg body falls freely from 5 m (g = 10 m/s²). Its kinetic energy on reaching the ground is:
(a) 100 J (b) 50 J (c) 10 J (d) 20 J
KE = mgh = 2 × 10 × 5 = 100 J. Answer: (a).
Q4. Match the device with the principle: 1. hydraulic brake 2. optical fibre 3. hydrometer 4. transformer; A. total internal reflection B. electromagnetic induction C. Pascal's law D. law of flotation.
(a) 1-C, 2-D, 3-A, 4-B (b) 1-A, 2-C, 3-B, 4-D (c) 1-C, 2-A, 3-D, 4-B (d) 1-D, 2-A, 3-C, 4-B
Answer: (c).
Q5. Consider the statements: 1. Sound travels faster in steel than in air. 2. A convex mirror always forms a virtual image. 3. A transformer can step up DC voltage. Which are correct?
(a) 1 only (b) 1 and 2 (c) 2 and 3 (d) all three
Statement 3 is false: transformers need AC. Answer: (b).
Q6. Two 10 Ω resistors in parallel are connected across a 10 V battery. The total current is:
(a) 0.5 A (b) 1 A (c) 2 A (d) 4 A
Equivalent resistance = 5 Ω, so I = 10/5 = 2 A. Answer: (c).
Practice set
- A light year is a unit of: (a) time (b) speed (c) distance (d) energy
- Escape velocity from the Earth is about: (a) 11.2 km/s (b) 7.9 km/s (c) 3 × 10⁸ m/s (d) 340 m/s
- Water is densest at: (a) 0 °C (b) 100 °C (c) −4 °C (d) 4 °C
- A 250 W appliance runs for 8 hours. The energy used is: (a) 20 kWh (b) 0.2 kWh (c) 2 kWh (d) 2,000 kWh
- The sky appears blue because of: (a) reflection (b) refraction (c) dispersion (d) scattering
- The surest test of a magnet is: (a) repulsion (b) attraction (c) weight (d) colour
- A lens of focal length −50 cm has a power of: (a) +2 D (b) −2 D (c) −0.5 D (d) −50 D
- The seconds pendulum has a period of: (a) 1 s (b) 0.5 s (c) 60 s (d) 2 s
Answers:
- (c). About 9.46 × 10¹⁵ m.
- (a). 7.9 km/s is the orbital velocity near the surface.
- (d). The anomalous expansion of water.
- (c). 0.25 kW × 8 h.
- (d). Blue is scattered most by air molecules.
- (a). Only a magnet can be repelled by a magnet.
- (b). P = 1/f = 1/(−0.5 m).
- (d). One second for each swing from one side to the other; its length is about 1 m.
A ten-day revision plan
| Day | Revise | Test |
|---|---|---|
| 1 | Units, motion, laws of motion | 25 MCQs |
| 2 | Work, energy, power; gravitation | 25 MCQs |
| 3 | Properties of matter; heat | 25 MCQs |
| 4 | Sound; light | 25 MCQs |
| 5 | Electricity; magnetism | 25 MCQs |
| 6 | Modern physics; devices table | 25 MCQs |
| 7 | Error log, all chapters | one full GK mock |
| 8 | Formulas table, write from memory | 40 mixed physics MCQs |
| 9 | Devices, scientists, confusable pairs | 40 mixed physics MCQs |
| 10 | Error log only | light review, rest |
What to do next
- Photocopy or rewrite the formulas and devices tables and keep them in your pocket for the last ten days.
- For any row you get wrong twice, go back to the chapter guide: start from the NDA physics plan, and use electricity and light and optics for the most numerical-heavy chapters.
- Take at least two full GAT mocks in the last ten days and log every physics mistake by chapter.
A note on dates and numbers. Exam patterns, vacancies and schedules change from year to year. Always confirm the current details in the latest notification on the Union Public Service Commission website .
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