Memory Charts and Mnemonics

This high-yield revision module covers fundamental concepts of Physics. Designed for rapid retention, this guide utilizes memory charts, structural tables, and mnemonics to streamline complex physical laws, units, and phenomena.

Fundamental and Derived Quantities

Physical quantities are classified into fundamental (independent) and derived (dependent) quantities. The International System of Units (SI) standardizes seven base units.

SI Base Units and Dimensions

Mnemonic for 7 Base Quantities: My Large Tasteless Egg Tastes Almost Like-salt (Mass, Length, Time, Electric Current, Temperature, Amount of Substance, Luminous Intensity).

Derived Units with Special Names

• Force: Newton (N = kg·m/s²)
• Pressure/Stress: Pascal (Pa = N/m²)
• Energy/Work/Heat: Joule (J = N·m)
• Power: Watt (W = J/s)
• Frequency: Hertz (Hz = s^-1)

Mechanics and Newton’s Laws of Motion

Mechanics governs the motion of bodies under the action of forces. Newton’s three laws form the bedrock of classical mechanics.

Summary of Newton’s Laws of Motion

• First Law (Law of Inertia): A body remains at rest or in uniform motion unless acted upon by an external force.
  • Examples: Passengers falling forward when a bus brakes suddenly; dusting a carpet by beating it with a stick.
• Second Law (Law of Force and Momentum): The rate of change of momentum of a body is directly proportional to the applied force and takes place in the direction of the force (F = ma).
  • Examples: A cricket fielder pulling his hands back while catching a ball to reduce the impact force by increasing the time interval.
• Third Law (Action and Reaction): To every action, there is an equal and opposite reaction.
  • Examples: Recoil of a gun; propulsion of rockets; swimming in water.

Mnemonic for Newton’s Laws

Mnemonic: Indian Forces React

• I – Inertia (1st Law)
• F – Force/Momentum (2nd Law)
• R – Reaction (3rd Law)

Gravitation and Planetary Motion

Gravitation is a universal attractive force acting between any two masses. Planetary motion is governed by Kepler’s Laws.

Core Physics Facts on Gravity

• Acceleration due to gravity (g): Standard value is 9.8 m/s².
• Variation of g: * Maximum at the Poles.
  • Minimum at the Equator (due to centrifugal force and the Earth’s oblate spheroidal shape).
  • Decreases with altitude (going above Earth’s surface) and depth (going below Earth’s surface).
  • Zero at the center of the Earth.
• Escape Velocity (v_e): The minimum velocity required for a body to escape a planet’s gravitational field. For Earth, v_e ≈ 11.2 km/s.
• Orbital Velocity (v_o): The velocity required to keep a satellite in its orbit. For a low Earth orbit, v_o ≈ 7.9 km/s.
• Relationship: v_e = √(2) · v_o

Kepler’s Laws of Planetary Motion

• First Law (Law of Orbits): All planets move in elliptical orbits with the Sun at one of the foci.
• Second Law (Law of Areas): A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time (Linear speed varies; planet moves faster when closer to the Sun—Perihelion, and slower when farther—Aphelion).
• Third Law (Law of Periods): The square of the orbital period (T) of a planet is directly proportional to the cube of the semi-major axis (r) of its orbit (T² ∝ r³).

Properties of Matter: Surface Tension, Viscosity, and Capillarity

Surface Tension

Surface tension is the property of a liquid surface to shrink into the minimum surface area possible. It decreases with an increase in temperature and the addition of impurities (like soap).

• Examples: Spherical shape of rain droplets; insects walking on water; warm soup tasting better because lower surface tension allows it to spread over the tongue.

Capillarity

The rise or depression of a liquid in a narrow tube.

• Examples: Blotting paper absorbing ink; rise of sap in plants; oil rising up the wick of a lamp.

Viscosity

The internal friction of a fluid that opposes its relative motion.

• Fact: Viscosity of liquids decreases with an increase in temperature, whereas the viscosity of gases increases with an increase in temperature.

Optics: Light Phenomena and Applications

Optics frequently features in UPSC Prelims application-based questions. Understanding the distinction between refraction, scattering, and total internal reflection is critical.

Key Optical Phenomena Matrix

Human Eye Defects and Corrective Lenses

• Myopia (Near-sightedness): Nearby objects are clear, distant objects are blurry. Light focuses in front of the retina. Correction: Concave lens.
• Hypermetropia (Far-sightedness): Distant objects are clear, nearby objects are blurry. Light focuses behind the retina. Correction: Convex lens.
• Presbyopia: Aging-induced loss of elasticity of the crystalline lens. Correction: Bifocal lens.
• Astigmatism: Imperfect curvature of the cornea leading to distorted vision. Correction: Cylindrical lens.

Mnemonic for Vision Correction

Mnemonic: My Cave is Hyper Vexing

• Myopia → Concave lens
• Hypermetropia → Convex lens

Wave Mechanics and Sound

Waves transfer energy without transferring matter. They are categorized based on the direction of particle displacement relative to wave propagation.

Mechanical Waves Classification

• Transverse Waves: Particles oscillate perpendicular to the direction of wave propagation. They can travel only through solids and the surface of liquids (e.g., Light waves, electromagnetic waves, string waves).
• Longitudinal Waves: Particles oscillate parallel to the direction of wave propagation. They can travel through solids, liquids, and gases (e.g., Sound waves, seismic P-waves).

Speed of Sound: Key Variables

• Medium: Solids > Liquids > Gases. Sound cannot travel through a vacuum.
• Temperature: Speed of sound increases with an increase in temperature.
• Humidity: Speed of sound increases with an increase in humidity (humid air is less dense than dry air).
• Pressure: Speed of sound is independent of changes in atmospheric pressure at a constant temperature.

Ultrasonic and Infrasonic Waves

• Infrasonic: Frequency < 20 Hz (Produced by earthquakes, volcanic eruptions, whales, elephants).
• Audible Range: 20 Hz to 20,000 Hz (Human ear).
• Ultrasonic: Frequency > 20,000 Hz (Used in SONAR, echocardiography, bats navigation, cleaning internal parts of machinery).

Nuclear Physics: Fission vs. Fusion

Nuclear physics questions generally focus on the mechanisms driving nuclear reactors and stellar energy.

Comparative Analysis of Nuclear Reactions

Nuclear Reactor Components

• Moderator: Slows down fast-moving neutrons. Materials: Heavy water (D₂O), Graphite.
• Control Rods: Absorb excess neutrons to regulate the chain reaction. Materials: Cadmium, Boron.
• Coolant: Absorbs and transfers the heat generated in the reactor core. Materials: Water, Heavy water, Liquid Sodium.