General Science: Physics, Chemistry and Biology

1. Chapter at a Glance

General Science is the largest block of NDA GAT Part B: Physics (about 25 questions), Chemistry (about 15) and General Science/Biology (about 10), together worth nearly 200 of the 400 GK marks. CDS GK and AFCAT General Awareness ask 15–25 science questions at Class X level. Questions are conceptual and application-based ("why does ice float?", "which lens corrects myopia?") rather than numerical, though NDA includes a few simple calculations. This chapter covers the full syllabus in exam order with a formula table, facts tables and PYQ-pattern questions.

2. Units, Measurement and Motion

• The seven SI base units: metre, kilogram, second, ampere, kelvin, mole, candela.
• Scalars have magnitude only (speed, mass, energy); vectors have magnitude and direction (velocity, force, displacement, momentum).
• Equations of motion (uniform acceleration): v = u + at; s = ut + ½at²; v² = u² + 2as.
• Newton's laws: (1) inertia — a body stays at rest or in uniform motion unless acted on by a force; (2) F = ma = rate of change of momentum; (3) every action has an equal and opposite reaction (rocket propulsion, recoil of a gun).
• Momentum p = mv is conserved when no external force acts. Impulse = force × time = change in momentum (why a cricketer draws hands back while catching).
• Circular motion needs a centripetal force mv²/r towards the centre; banking of roads provides part of it.

3. Work, Energy, Gravitation and Pressure

• Work W = Fs cos θ; no work is done when force is perpendicular to displacement (a coolie carrying a load on a level road does no work against gravity).
• Kinetic energy = ½mv²; potential energy = mgh. If speed doubles, kinetic energy becomes four times.
• Power = work/time; 1 horsepower = 746 W; 1 kWh = 3.6 × 10⁶ J (the "unit" on electricity bills).
• Universal gravitation F = Gm₁m₂/r²; g on Earth ≈ 9.8 m/s²; on the Moon about 1/6 of that. Mass stays constant; weight changes.
• g is maximum at the poles and minimum at the equator; it decreases with height and with depth.
• Escape velocity from Earth ≈ 11.2 km/s; orbital velocity near the surface ≈ 7.9 km/s. A geostationary satellite has a period of 24 hours at about 36,000 km.
• Pressure = force/area (sharp knives cut better). Liquid pressure = hρg. Pascal's law is used in hydraulic brakes and lifts.
• Archimedes' principle: buoyant force = weight of liquid displaced. A body floats if its density is less than the liquid's; ice (density ≈ 0.92 g/cm³) floats on water. Ships float because their hollow shape displaces a large volume.
• Bernoulli's principle: faster-moving fluid exerts lower pressure — explains aeroplane lift and roofs blown off in storms.

4. Heat and Thermodynamics

• °F = (9/5)°C + 32; K = °C + 273. −40° is the same on both Celsius and Fahrenheit scales. Normal body temperature 37°C = 98.6°F.
• Heat transfer: conduction (solids), convection (fluids — sea breeze, land breeze), radiation (no medium needed — heat from the Sun).
• Water has a high specific heat, which is why coastal areas have a moderate climate.
• Latent heat is absorbed during melting and boiling without temperature change; steam at 100°C burns more than water at 100°C because of latent heat.
• Anomalous expansion of water: water is densest at 4°C, so lakes freeze from the top and aquatic life survives below.
• Boiling point falls with lower pressure (cooking is slow on mountains) and rises with higher pressure (pressure cooker).

5. Sound and Light

• Sound is a mechanical longitudinal wave; it cannot travel in vacuum. Speed: air ≈ 343 m/s, water ≈ 1500 m/s, steel ≈ 5000 m/s.
• Audible range 20 Hz–20 kHz; below 20 Hz is infrasound (earthquakes, elephants), above 20 kHz is ultrasound (bats, SONAR, medical scans).
• Pitch depends on frequency, loudness on amplitude, quality (timbre) on the waveform. Loudness is measured in decibels (dB).
• An echo needs a minimum distance of about 17 m (for 0.1 s persistence of hearing). The Doppler effect changes apparent frequency with relative motion.
• Light travels at 3 × 10⁸ m/s in vacuum. Refractive index n = c/v; light bends towards the normal when entering a denser medium.
• Total internal reflection: sparkling of diamond (critical angle about 24°), optical fibres, mirage.
• Scattering: the sky is blue because shorter (blue) wavelengths scatter most; the sun looks red at sunrise and sunset because blue light is scattered away over the long path. Danger signals are red because red scatters least.
• Dispersion of white light by a prism gives VIBGYOR; a rainbow forms by dispersion, refraction and internal reflection in raindrops.
• Mirror and lens formula: 1/f = 1/v + 1/u (mirror), 1/f = 1/v − 1/u (lens); power P = 1/f (in metres).
• Defects of vision: myopia (short-sightedness) — concave lens; hypermetropia (long-sightedness) — convex lens; presbyopia — bifocal lens; astigmatism — cylindrical lens.
• Rear-view mirrors are convex (wide field of view); shaving mirrors and headlight reflectors are concave.

6. Electricity and Magnetism

• Ohm's law V = IR. Resistance R = ρL/A — doubling length doubles resistance; doubling area halves it.
• Series: R = R₁ + R₂ + …; same current. Parallel: 1/R = 1/R₁ + 1/R₂ + …; same voltage. Household wiring is in parallel so each appliance gets full voltage and can be switched independently.
• Power P = VI = I²R = V²/R. Heating effect (Joule's law) H = I²Rt — used in heaters, irons and fuses.
• A fuse wire has low melting point and high resistance (tin–lead alloy); it is connected in series with the live wire. Earthing protects from shocks.
• Magnetic effect of current (Oersted); right-hand thumb rule gives field direction. An electromagnet uses a soft-iron core.
• Electromagnetic induction (Faraday): a changing magnetic field induces current — principle of the generator (dynamo). A motor converts electrical energy into mechanical energy.
• Transformers work only on AC and change voltage; power is transmitted at high voltage to reduce I²R losses. Indian domestic supply: 220–230 V, 50 Hz.
• Semiconductors: silicon and germanium; a diode allows current in one direction (rectifier); transistors amplify and switch.

7. Modern Physics in Brief

• Electromagnetic spectrum in increasing frequency: radio, microwave, infrared, visible, ultraviolet, X-rays, gamma rays.
• Radioactivity (Becquerel): alpha (helium nuclei, least penetrating), beta (electrons), gamma (EM waves, most penetrating).
• Nuclear fission (splitting heavy nuclei) powers reactors and atomic bombs; fusion (joining light nuclei) powers the Sun and hydrogen bombs.
• Heavy water (D₂O) is used as a moderator in Indian reactors; control rods are made of cadmium or boron.

8. Chemistry

• States of matter; sublimation (camphor, naphthalene, dry ice); diffusion is fastest in gases.
• Atom: protons and neutrons in the nucleus, electrons in shells (2, 8, 8…). Atomic number = protons; mass number = protons + neutrons. Isotopes: same atomic number, different mass number (protium, deuterium, tritium). Isobars: same mass number.
• Modern periodic table (Moseley): arranged by atomic number; 18 groups, 7 periods. Group 1 alkali metals, group 17 halogens, group 18 noble gases. Fluorine is the most electronegative element.
• Acids turn blue litmus red (pH < 7); bases turn red litmus blue (pH > 7). Neutralisation gives salt and water. Gastric juice contains HCl; bee sting is acidic (treated with baking soda); wasp sting is alkaline (treated with vinegar).
• Metals are lustrous, malleable, ductile and good conductors (silver is the best conductor, mercury is a liquid metal). Non-metals are generally poor conductors (graphite is an exception).
• Corrosion of iron (rusting) needs both oxygen and moisture; prevented by galvanisation (zinc coating), painting and alloying (stainless steel = iron + chromium + nickel).
• Alloys: brass (Cu + Zn), bronze (Cu + Sn), solder (Pb + Sn), duralumin (Al + Cu + Mg, used in aircraft).
• Carbon forms allotropes: diamond (hardest, non-conductor), graphite (soft, conductor, lubricant), fullerenes. Carbon compounds form chains because of catenation.
• Hard water contains calcium and magnesium salts; temporary hardness is removed by boiling, permanent hardness by washing soda or ion exchange.

9. Biology and Health

• The cell is the basic unit of life (Robert Hooke named it). Mitochondria — powerhouse; ribosomes — protein synthesis; chloroplasts — photosynthesis (plants only); lysosomes — "suicide bags".
• Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ in the presence of light and chlorophyll.
• Human blood: RBCs carry oxygen via haemoglobin (iron); WBCs fight infection; platelets clot blood. Universal donor O−, universal recipient AB+.
• The heart has four chambers; the largest artery is the aorta. Normal blood pressure about 120/80 mm Hg. The liver is the largest gland; the femur is the longest bone; the stapes (in the ear) is the smallest.
• Endocrine glands: pituitary (master gland), thyroid (thyroxine, needs iodine — deficiency causes goitre), pancreas (insulin — diabetes), adrenal (adrenaline — "fight or flight").
• Diseases by cause: bacteria — tuberculosis, cholera, typhoid, tetanus; viruses — polio, measles, AIDS, dengue, COVID-19; protozoa — malaria (Plasmodium, female Anopheles mosquito); fungi — ringworm.
• Vaccines: BCG (tuberculosis), OPV (polio); Edward Jenner developed the smallpox vaccine; Alexander Fleming discovered penicillin.
• Food chains: energy flows one way; only about 10% passes to the next trophic level. Biodiversity hotspots in India: Western Ghats, Himalaya, Indo-Burma, Sundaland (Nicobar).

10. Science Behind Defence Technology

NDA and AFCAT papers like to link everyday physics and chemistry to military equipment. If you understand the principle behind a device, you can answer a question on it even when the device is new to you. The table below links common defence systems to the science chapter they come from.

11. Master Formula Table

12. Master Summary Table

13. Mind Map

14. Tips and Tricks

15. Important One-Liners

• Inertia is the property by which a body resists a change in its state of rest or motion.
• Rocket propulsion is based on Newton's third law and conservation of momentum.
• Weight is maximum at the poles and minimum at the equator.
• A body weighs less in water because of buoyancy.
• Sound cannot travel through vacuum; light can.
• SONAR uses ultrasonic waves to locate submarines.
• The sky appears blue due to scattering of light; it would look black from the Moon.
• Diamond sparkles because of total internal reflection.
• A fuse works on the heating effect of current.
• Transformers do not work on DC.
• Gamma rays have the highest penetrating power among radioactive emissions.
• The Sun's energy comes from nuclear fusion.
• Pure water has pH 7; human blood is slightly alkaline (about 7.4).
• Stainless steel contains iron, chromium and nickel.
• Haemoglobin contains iron; chlorophyll contains magnesium.
• Malaria is caused by a protozoan, Plasmodium, spread by the female Anopheles mosquito.
• Insulin is secreted by the islets of Langerhans in the pancreas.

16. PYQ-Based Questions with Detailed Answers

17. Quick Revision Summary