Physics

Physics for SSC is NOT about derivations — it's about concepts and real-life applications. Every exam asks you: why does a rainbow form? Why does sound echo? What instrument measures humidity? Which law explains rocket propulsion? This note covers the seven core physics topics tested in SSC CGL, CHSL, and MTS: Units & Measurement, Motion & Laws, Work-Energy-Power, Heat & Thermodynamics, Light & Optics, Sound, Electricity & Magnetism, and Modern Physics. Master the bold terms and the Quick-Fire table at the end — that's where 80% of marks come from.

UNITS & MEASUREMENT

The International System of Units (SI) is the global standard for measurement, adopted in 1960. It has 7 base units — everything else (speed, force, energy, pressure) is derived from these. SSC loves asking "what is the SI unit of X" — memorise all 7 base units cold.

MOTION & NEWTON'S LAWS

Newton's three laws of motion explain almost all movement you see in daily life. First Law (Inertia): a body keeps doing what it's doing — seatbelts exist because your body wants to keep moving when the car stops. Second Law (F = ma): the harder you kick a football (more force), the faster it accelerates — and a heavier ball needs more force to accelerate the same amount. Third Law (Action-Reaction): a rocket ejects gas downward, so the gas pushes the rocket upward with equal force. Gallileo originally described the first law, which is why it's also called Galileo's Law of Inertia.

Gravitation: Every object attracts every other object — this is Newton's Law of Universal Gravitation. The gravitational constant G = 6.67 × 10⁻¹¹ N m²/kg². Near Earth's surface, the acceleration due to gravity g = 9.8 m/s² (often rounded to 10). g is NOT constant — it is maximum at the poles and minimum at the equator (Earth is slightly flattened). g also decreases with altitude and depth. Escape velocity from Earth = 11.2 km/s — the minimum speed an object needs to escape Earth's gravity and never return. On the Moon, escape velocity is only 2.4 km/s, which is why the Moon has no atmosphere. Weight of a person on the Moon = 1/6 of weight on Earth (g on Moon = g/6).

WORK, ENERGY & POWER

Three concepts that SSC always tests together: Work is done only when a force actually causes displacement in the direction of the force — a coolie carrying luggage on his head while walking horizontally does ZERO work (force is vertical, displacement is horizontal → angle = 90° → W = FS cos90° = 0). Energy is the capacity to do work — it comes in kinetic (motion) and potential (position/height) forms, and the Law of Conservation of Energy says total energy of an isolated system never changes. Power is how fast work is done — a 100W bulb consumes 100 joules every second.

HEAT & THERMODYNAMICS

Heat is a form of energy (unit: Joule or calorie; 1 calorie = 4.18 J). Temperature is the measure of hotness or coldness — it tells you the direction heat will flow (always from hotter to cooler). The clinical thermometer reads 96°F to 110°F; normal human body temperature is 37°C = 98.4°F. Specific heat capacity is the heat needed to raise 1 kg of a substance by 1°C — water has the highest specific heat (4200 J/kg°C), which is why the sea stays cooler in summer. Latent heat is the heat absorbed or released during a change of state at constant temperature — why steam burns are more severe than boiling-water burns (steam releases extra 536 cal/g when condensing).

Temperature Scales: Three major scales are used globally. Celsius (°C) sets 0 at ice point and 100 at steam point. Fahrenheit (°F) used in the US and UK — 32°F = 0°C, 212°F = 100°C. Kelvin (K) is the SI unit and the absolute scale — 0 K (absolute zero) is the lowest possible temperature where all molecular motion stops. Formula: K = °C + 273, and °F = (9/5)°C + 32. The one magic temperature where Celsius and Fahrenheit read the same: –40°.

LIGHT & OPTICS

Light is an electromagnetic wave (transverse wave) that travels at 3 × 10⁸ m/s in vacuum — the fastest speed in the universe. It takes 8 minutes 19 seconds to reach Earth from the Sun, and 1.28 seconds from the Moon. Light shows three key behaviours: Reflection (bouncing off surfaces), Refraction (bending when entering a different medium), and Dispersion (splitting into colours). These three phenomena explain mirrors, lenses, rainbows, mirages, optical fibres, spectacles — and almost every optics question on SSC.

SOUND

Sound is a mechanical, longitudinal wave — it needs a material medium (solid, liquid, or gas) to travel, and cannot travel in vacuum. This is why space is silent (no medium). Sound travels fastest in solids and slowest in gases. Speed of sound in air at 20°C = 343 m/s (≈ 0.34 km/s). Speed of sound in water ≈ 1500 m/s. Speed of sound in steel ≈ 5000 m/s. When thunder and lightning occur simultaneously, you see lightning first because light (3 × 10⁸ m/s) is far faster than sound.

Doppler Effect: When the source of sound and observer are moving relative to each other, the apparent frequency changes. If the source moves toward you → pitch sounds higher (frequency increases). If source moves away → pitch sounds lower. Classic example: the pitch of an ambulance siren is higher as it approaches and drops as it passes. This effect is also used in radar guns (measuring car speed) and astronomy (red-shift tells us galaxies are moving away).

ELECTRICITY & MAGNETISM

Ohm's Law: Voltage (V) = Current (I) × Resistance (R). This single equation governs every circuit. The best conductor of electricity is Silver — then copper (used in wires), then aluminium. In series circuits, resistance adds up and the same current flows through all components — if one bulb blows, all go out (old fairy lights). In parallel circuits, each component gets the same voltage, and they work independently — which is why home wiring is parallel (so switching off one light doesn't affect others).

Magnetism: A solenoid (coil of wire) carrying current behaves like a bar magnet. Add a soft iron core → electromagnet (stronger, switchable). Electromagnets power electric bells, telephone receivers, MRI machines, electric cranes. An electric motor converts electrical energy to mechanical energy (fan, pump, mixer). A generator/dynamo does the reverse — converts mechanical energy to electrical energy (power plants, bicycle dynamos). Earth itself is a giant magnet — the magnetic North Pole is near (but not exactly at) the geographic North Pole.

MODERN PHYSICS

Radioactivity (discovered by Henri Becquerel in 1896, then studied by Marie Curie & Pierre Curie) is the spontaneous emission of radiation by unstable nuclei. Elements with atomic number ≥ 83 are naturally radioactive. Three types of radiation: Alpha (α) = helium nucleus (2 protons + 2 neutrons) — least penetrating, stopped by paper; Beta (β) = high-speed electron — stopped by aluminium; Gamma (γ) = electromagnetic radiation (like X-rays but shorter wavelength) — most penetrating, stopped only by thick lead or concrete. The end product of all natural radioactive decay series is Lead (Pb).

GRAVITATION & SATELLITES

Gravity is the invisible force that keeps us on Earth, makes things fall down when dropped, and keeps the Moon orbiting Earth instead of flying off into space. Every object in the universe pulls every other object — the bigger the mass, the stronger the pull. Earth's pull on objects near its surface gives them an acceleration of 9.8 m/s² downward — this is called the acceleration due to gravity (g). The key daily-life insight: regardless of whether you drop a feather (in vacuum) or a brick, both fall at the same rate of 9.8 m/s². Mass and shape do not matter for free fall.

Satellites are objects that orbit a planet. The natural satellite of Earth is the Moon. Artificial satellites are launched by rockets into specific orbits depending on their purpose. Two types are tested repeatedly in SSC: (1) Geostationary satellites — they orbit at exactly the right height and speed so they always appear stationary over the same spot on Earth; used for TV broadcasting and weather forecasting. (2) Polar satellites — they orbit at low altitude passing over the North and South poles; each pass covers a different strip of Earth as the planet rotates below, so they eventually map the whole surface.

PRESSURE, BUOYANCY & FLUID LAWS

Pressure is simply force spread over an area. The smaller the area, the higher the pressure — that's why knife blades are sharp (tiny area → huge pressure from same force) and why snowshoes are wide (large area → spread weight → less pressure on snow). This section covers three classic fluid laws that appear repeatedly in SSC: Pascal's Law, Archimedes' Principle, and Bernoulli's Theorem.

HUMAN EYE & DEFECTS OF VISION

The human eye works like a camera — the lens focuses light onto the retina at the back of the eye, creating an image that the brain interprets. The normal near point (least distance of distinct vision) is 25 cm. When the eye lens loses its ability to focus correctly, we get vision defects. SSC regularly asks: which defect is corrected by which lens? Learn the pattern: short sight → concave; long sight → convex; both → bifocal.

Instruments that use lenses: Simple microscope = single convex lens. Compound microscope = two convex lenses (objective + eyepiece). Astronomical telescope = two convex lenses (objective = large aperture + long focal length; eyepiece = small aperture + short focal length).

LATENT HEAT & SPECIFIC HEAT — KEY NUMBERS

You've felt this: stepping out of a swimming pool feels colder than standing in the water. Why? Evaporation takes heat from your body. Similarly, steam burns are far more painful than boiling water burns — because steam has stored a massive amount of heat that it releases when it condenses on your skin. This 'hidden heat' of phase change is called Latent Heat. It is 'hidden' because the temperature does not change during a phase change — all the energy goes into changing the state (solid to liquid, or liquid to gas).

DOPPLER EFFECT & LASER

You've experienced the Doppler Effect without knowing it: when an ambulance drives towards you, its siren sounds shriller (higher pitch); as it passes and drives away, the pitch drops. This change in perceived frequency when a source moves relative to the observer is the Doppler Effect. It is not just a sound phenomenon — it applies to light too, and is used in radar guns (traffic police), weather radar, and even to detect stars moving away from us (redshift).

QUICK-FIRE: Instruments, Units & Discoveries

SSC loves direct-recall questions: 'What instrument measures X?' or 'Who discovered Y?' or 'What is the unit of Z?' The table below covers the most-tested facts across all SSC exams.

COMMON EXAM TRAPS

1. "Sound travels faster than light" → FALSE. Light (3×10⁸ m/s) vs Sound (343 m/s in air) — light is ~900,000 times faster.
2. "Vacuum is needed for sound to travel" → FALSE. Sound CANNOT travel in vacuum — it needs a medium. Light and radio waves travel through vacuum.
3. "Convex mirror is used in microscopes" → FALSE. Convex LENS (converging) is used in microscopes and telescopes. Convex MIRROR diverges light and is used in rear-view mirrors.
4. "Alpha particles are the most penetrating" → FALSE. Gamma rays are most penetrating (need lead/concrete to stop). Alpha = least penetrating (stopped by paper/skin).
5. "H-bomb is based on fission" → FALSE. H-bomb (Hydrogen bomb) uses nuclear FUSION. Atom bomb uses fission.
6. "Mass and weight are the same" → FALSE. Mass is constant everywhere; weight = mass × g, so weight changes with location (less on Moon, zero in space).
7. "Electric current flows from negative to positive" → FALSE (for conventional current). Conventional current flows from positive to negative. Actual electron flow is negative to positive.
8. "Kinetic energy depends only on speed" → FALSE. KE = ½mv² — it depends on BOTH mass and velocity.
9. "Thermal expansion means only length increases" → FALSE. Solids expand in all three dimensions: linear, superficial (area), and cubical (volume).
10. "SONAR uses light waves" → FALSE. SONAR uses SOUND waves (ultrasound). RADAR uses radio waves (electromagnetic).