Physics — NDA General Knowledge
GK Physics is not NDA Mathematics in disguise — there is no calculus, no vectors beyond a plain "push in this direction," no trigonometric ballistics. It is CBSE Class 9–10 general science: a closed set of laws, formulas and definitions that either you recall correctly under a four-mark, four-option, negative-marking clock, or you don't. Learn the definitions precisely, keep the half-dozen formulas automatic, and treat every "sounds right but isn't quite" option as the trap it's designed to be.
1. What NDA actually asks
Weightage: 25% of General Knowledge — the heaviest of GK's six sub-areas. GK itself is Part B of the GAT paper: 100 of the GAT's 150 questions, worth 400 of its 600 marks. A 25% share of GK Physics therefore works out to roughly 25 of the 100 GK questions — 100 of the 400 GK marks — ahead of Geography (20%), History & Freedom Movement (20%), Chemistry (15%), General Science (10%) and Current Events (10%). Every GAT question, English or GK, carries +4 for a correct answer and −1.3333 for a wrong one (1/3 of 4 marks); an unattempted question costs nothing.
Physics questions in NDA GK come in three recurring shapes:
- Factual recall — name the SI unit of a quantity, the instrument used to measure something, the correct statement of a law, the classification of a lever or a mirror.
- Correct-statement identification — four short statements about the same topic (heat transfer, eye defects, magnet behaviour), only one of which is scientifically accurate.
- Single-step numeric plug-ins — a value or two dropped into F = ma, W = F×d, P = W/t, V = IR, or a temperature-scale conversion; grade 9–10 arithmetic, no calculator needed, no multi-step derivation.
Nothing here goes beyond a CBSE Class 9–10 general science syllabus — this is the physics a generalist officer is expected to know, not the physics a science-stream Class 12 student uses in engineering entrance exams.
2. Measurement & units
SI base units — seven quantities, each with one fixed unit and symbol:
| Quantity | SI Unit | Symbol |
|---|---|---|
| Length | metre | m |
| Mass | kilogram | kg |
| Time | second | s |
| Electric current | ampere | A |
| Temperature | kelvin | K |
| Amount of substance | mole | mol |
| Luminous intensity | candela | cd |
Derived units matter just as much for NDA GK — force is measured in newton (N = kg·m/s²), work/energy in joule (J), power in watt (W = J/s), pressure in pascal (Pa = N/m²), and electric charge in coulomb (C). NDA's favourite unit trap is naming a quantity and offering the unit of a related but different quantity as a distractor — force's unit offered against energy's or power's, for instance.
Common measuring instruments — another closed, memorisable list:
| Instrument | Measures |
|---|---|
| Thermometer | Temperature |
| Barometer | Atmospheric pressure |
| Hygrometer | Humidity |
| Anemometer | Wind speed |
| Odometer | Distance travelled (vehicle) |
| Seismograph | Earthquake vibrations |
| Lactometer | Purity/density of milk |
| Hydrometer | Relative density of a liquid |
| Sphygmomanometer | Blood pressure |
| Ammeter / Voltmeter | Electric current / potential difference |
3. Mechanics — motion, force and machines
Motion basics. Distance is the total path length covered (scalar); displacement is the shortest straight-line change in position, with direction (vector) — displacement can never exceed distance. Speed = distance/time (scalar); velocity = displacement/time (vector). Acceleration is the rate of change of velocity.
Newton's three laws of motion:
- First law (law of inertia): a body remains at rest, or in uniform motion in a straight line, unless acted upon by an external force. Inertia is a body's resistance to a change in its state of motion, and it increases with mass.
- Second law: the rate of change of momentum of a body is proportional to the applied force and takes place in the direction of the force — giving the working formula F = ma (force = mass × acceleration).
- Third law: for every action there is an equal and opposite reaction — the two forces act on different bodies, so they never cancel each other out.
Work, energy and power:
- Work W = F × d (force applied in the direction of, and multiplied by, the displacement it produces), unit joule.
- Kinetic energy KE = ½mv² — energy of motion.
- Potential energy PE = mgh — energy of position, height h above a reference level, g ≈ 9.8 m/s².
- Power P = W/t — the rate of doing work, unit watt.
Simple machines trade force for distance (or vice versa) without changing the total work done (ignoring friction). Mechanical advantage (MA) = Load/Effort. Levers are classified by the position of the fulcrum (F), load (L) and effort (E):
| Class | Order | Example |
|---|---|---|
| Class I | Fulcrum between load and effort | See-saw, scissors, crowbar |
| Class II | Load between fulcrum and effort | Wheelbarrow, nutcracker |
| Class III | Effort between fulcrum and load | Fishing rod, human forearm, tweezers |
Pulleys (fixed — changes direction of force only, MA = 1; movable — MA = 2, halves the effort needed), the inclined plane, the wheel-and-axle, and the screw are the other simple machines NDA GK expects you to recognise by function.
4. Heat
Temperature scales. Three scales, two fixed reference points (freezing and boiling point of water):
| Scale | Freezing point | Boiling point | Conversion |
|---|---|---|---|
| Celsius (°C) | 0°C | 100°C | — |
| Fahrenheit (°F) | 32°F | 212°F | F = (C × 9/5) + 32 |
| Kelvin (K) | 273 K | 373 K | K = C + 273 |
Kelvin is the SI unit of temperature and is never negative in ordinary use (0 K is absolute zero); Celsius and Fahrenheit both go negative.
Modes of heat transfer:
- Conduction — heat passes through direct particle-to-particle contact; needs a medium, fastest in solids (especially metals).
- Convection — heat travels through the bulk movement of a fluid (liquid or gas) as warmer, less dense fluid rises and cooler fluid sinks; needs a fluid medium.
- Radiation — heat travels as electromagnetic waves; the only mode that needs no medium at all, which is how the Sun's heat crosses the vacuum of space to reach Earth.
5. Light
Laws of reflection: (1) the incident ray, reflected ray and normal all lie in the same plane; (2) the angle of incidence equals the angle of reflection, both measured from the normal (the perpendicular to the surface at the point of incidence) — not from the surface itself, a distinction NDA GK likes to test directly.
Refraction is the bending of light as it passes from one transparent medium into another of different optical density, caused by a change in the speed of light; a ray bends towards the normal when entering a denser medium, and away from the normal when entering a rarer one.
Mirrors and lenses:
- Concave mirror — converging, can form a real, inverted image (used in shaving/makeup mirrors, torches, headlights).
- Convex mirror — diverging, always forms a virtual, erect, diminished image with a wide field of view (used in vehicle rear-view/side mirrors).
- Convex (converging) lens — bulges outward, converges parallel rays to a real focus; corrects hypermetropia (long-sightedness/far-sightedness), where the image forms behind the retina.
- Concave (diverging) lens — curves inward, spreads parallel rays apart; corrects myopia (short-sightedness/near-sightedness), where the image forms in front of the retina.
The human eye and its defects:
| Defect | Problem | Corrected with |
|---|---|---|
| Myopia (near-sightedness) | Image forms in front of retina; distant objects blurred | Concave (diverging) lens |
| Hypermetropia (far-sightedness) | Image forms behind retina; near objects blurred | Convex (converging) lens |
| Presbyopia | Age-related loss of near focus (weakened ciliary muscles) | Bifocal lenses |
| Astigmatism | Irregular corneal curvature; blurred at all distances | Cylindrical lens |
6. Sound
Sound is a mechanical wave — it needs a material medium (solid, liquid or gas) and cannot travel through a vacuum, unlike light or radio waves.
Speed of sound is fastest in solids, slower in liquids, slowest in gases — the more tightly packed the particles, the faster vibrations transmit (roughly: steel ≈ 5,960 m/s, water ≈ 1,480 m/s, air ≈ 343 m/s at room temperature).
Frequency ranges:
- Audible range for the average human ear: 20 Hz to 20,000 Hz (20 kHz).
- Infrasonic: below 20 Hz (inaudible — elephants, earthquakes).
- Ultrasonic: above 20 kHz (inaudible to humans — bats, medical imaging, SONAR).
Echo is the reflection of sound off a hard, distant surface, heard as a distinct repetition; the human ear needs a minimum gap of about 0.1 seconds between the original and reflected sound to register them separately, which requires the reflecting surface to be at least roughly 17 metres away (at the speed of sound in air).
Ultrasound applications: medical imaging and diagnostics, industrial cleaning and flaw detection, and — most relevant to a defence GK paper — SONAR (Section 8).
7. Electricity & magnetism
Circuit basics. Ohm's Law: V = IR, where V is potential difference (volts), I is current (amperes), R is resistance (ohms) — valid at constant temperature.
- Series circuit: components share one single path; current is the same through each, and total resistance adds up: R = R₁ + R₂ + …
- Parallel circuit: components sit on separate branches; voltage is the same across each, and resistances combine as 1/R = 1/R₁ + 1/R₂ + … — total resistance is always less than the smallest individual resistor.
Magnets and electromagnets. Like poles repel, unlike poles attract. A current-carrying coil wound around a soft-iron core forms an electromagnet; its strength increases with the number of turns in the coil, the current flowing through it, and the use of a soft-iron (rather than air) core. Reversing the current's direction reverses the electromagnet's polarity but does not change its strength.
Household electrical safety: a fuse is a thin wire that melts and breaks the circuit if current exceeds a safe limit, protecting appliances and wiring from overheating; earthing (grounding) connects a device's metal body to the ground so a fault current is diverted safely away from a person touching it, rather than through them.
8. Physics in defence & everyday life
NDA's GK paper leans on physics that a defence officer actually encounters:
- RADAR (RAdio Detection And Ranging) uses radio waves to detect and track aircraft, ships and weather systems; radio waves travel well through air but are absorbed quickly in water.
- SONAR (SOund Navigation And Ranging) uses ultrasonic sound waves instead, because sound — not radio — travels efficiently underwater; submarines and naval vessels use it to detect other vessels and to measure ocean depth.
- Satellites (INSAT, IRNSS/NavIC, and international systems like GPS) apply orbital mechanics and radio-wave communication to weather forecasting, navigation and defence surveillance.
- Periscopes, used in submarines and trenches, rely on simple reflection — two mirrors (or prisms) angled at 45° let an observer see over an obstruction or above the waterline without exposing themselves.
Worked examples
Q1 (Unit matching). What is the SI unit of force?
Show explanation
Solution. Force = mass × acceleration → kg × m/s² = newton (N), not joule (unit of work/energy) or watt (unit of power) — each of those is the correct unit for a different quantity.
Q2 (Law identification). A block placed on a frictionless table stays at rest until pushed. Which law does this illustrate?
Show explanation
Solution. This is the law of inertia — Newton's First Law: a body at rest stays at rest unless acted on by an external force.
Q3 (Numeric — F = ma). A resultant force of 15 N acts on a 3 kg object initially at rest. Find the acceleration.
Show explanation
Solution. a = F/m = 15/3 = 5 m/s².
Q4 (Numeric — temperature conversion). Convert normal human body temperature, 37°C, to Fahrenheit.
Show explanation
Solution. F = (C × 9/5) + 32 = (37 × 1.8) + 32 = 66.6 + 32 = 98.6°F.
Q5 (Correct-statement identification — heat transfer). Which mode of heat transfer needs no material medium?
Show explanation
Solution. Radiation — it travels as electromagnetic waves and is how solar heat crosses the vacuum of space; conduction and convection both require particles of a medium to carry the heat.
Q6 (Numeric — Ohm's Law). A 5 Ω resistor carries a current of 3 A. Find the potential difference across it.
Show explanation
Solution. V = IR = 3 × 5 = 15 V.
10. Common traps
- Confusing the unit of one quantity with another's — force (newton) vs. energy (joule) vs. power (watt) vs. pressure (pascal) are four different units for four different quantities; NDA regularly swaps them as distractors.
- Naming the wrong Newton's law — "inertia" is always the First Law, "F = ma" the Second, "action-reaction" the Third; a description of one dressed up and attributed to another is a recurring trap.
- Measuring the angle of reflection from the mirror's surface instead of the normal — the law of reflection is always stated relative to the normal (the perpendicular), not the surface itself.
- Swapping myopia and hypermetropia's corrective lenses — myopia (near-sightedness, image falls short of the retina) needs a concave (diverging) lens; hypermetropia (far-sightedness, image falls behind the retina) needs a convex (converging) one. Mixing these up is the single most common light-section error.
- Assuming sound travels through a vacuum — it cannot; sound is a mechanical wave and needs a medium, unlike light or radio waves, which travel fastest through vacuum.
- Mixing up RADAR and SONAR's carrier wave — RADAR uses radio waves (works in air); SONAR uses ultrasonic sound waves (works underwater, where radio waves are absorbed almost immediately).
- Adding resistances in parallel the way you would in series — parallel resistance always comes out smaller than the smallest individual resistor (1/R = 1/R₁ + 1/R₂ + …), never a simple sum.
- Forgetting the "+32" step in Celsius-to-Fahrenheit conversion, or confusing it with the Celsius-to-Kelvin "+273" step — the two conversions use different formulas and are not interchangeable.
11. Revision protocol
GK Physics rewards a clean, memorised checklist far more than problem-solving — there are no multi-step derivations here, only a fixed set of laws, unit pairings and formulas repeated across differently-worded questions. Rebuild the seven SI base units, the ten common instruments, Newton's three laws, the myopia/hypermetropia lens pairing, and the RADAR/SONAR distinction from memory until each takes under five seconds to recall. Then run the six formulas — F = ma, W = Fd, P = W/t, V = IR, the Celsius-Fahrenheit-Kelvin conversions, and series/parallel resistance — as a five-minute daily drill using small, board-level numbers; at 25% of GK, this chapter alone is worth more raw marks than any other GK sub-area, and every one of its questions is a fast, four-mark gain for a well-drilled candidate.
