General Science Physics

🟢 Lite — Quick Review (1h–1d)

Rapid summary of Physics for NDA GAT paper.

Physics accounts for roughly 15–20 questions in the NDA GAT General Science section. High-yield topics for quick revision:

Must-Know Formulas:

• Equations of motion: v = u + at, s = ut + ½at², v² = u² + 2as
• Newton’s Second Law: F = ma
• Work: W = Fs cos θ; Kinetic energy: KE = ½mv²; Potential energy: PE = mgh
• Power: P = W/t
• Gravitational force: F = Gm₁m₂/r²; Escape velocity: v_esc = √(2gR)
• Ohm’s Law: V = IR; Power: P = VI = I²R
• Lens formula: 1/v – 1/u = 1/f
• Speed of sound in air: approximately 332 m/s at 0°C

Exam Tips:

• Dimension analysis questions are common — check which quantity has the right dimensions
• Numericals constitute about 40% of NDA physics questions
• Unit conversions (especially SI system) are frequently tested
• Diagrams in optics and mechanics questions help visualise problems

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🟡 Standard — Regular Study (2d–2mo)

For students who want genuine understanding of Physics for NDA.

Motion — Kinematics

Understanding motion requires three fundamental quantities:

• Speed is the rate of change of distance (scalar, no direction): Speed = distance/time
• Velocity is the rate of change of displacement (vector, has direction): Velocity = displacement/time
• Acceleration is the rate of change of velocity: a = (v – u)/t

Equations of Motion (for uniformly accelerated motion):

1. v = u + at — final velocity = initial velocity + acceleration × time
2. s = ut + ½at² — displacement = initial velocity × time + ½ × acceleration × time²
3. v² = u² + 2as — links velocity and displacement without time

Where: u = initial velocity, v = final velocity, a = acceleration, s = displacement, t = time.

Example: A car accelerates from rest (u = 0) at 2 m/s² for 5 seconds. Final velocity: v = 0 + (2)(5) = 10 m/s. Distance covered: s = 0 + ½(2)(25) = 25 m.

Free fall is motion under gravity alone. Here a = g = 9.8 m/s² (or 10 m/s² in simplified calculations).

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. Inertia is the resistance to change in state of motion. Mass is the measure of inertia — heavier objects have more inertia.

• Second Law: Force equals mass times acceleration: F = ma. One Newton is the force needed to accelerate a 1 kg mass by 1 m/s². This is the most frequently tested formula in NDA physics.

• Third Law: For every action, there is an equal and opposite reaction. A rocket expels gas downwards (action); the gas pushes the rocket upward (reaction). This is the principle behind jet propulsion.

Momentum (p = mv) is conserved in all collisions. Newton’s second law can also be written as F = Δp/Δt (rate of change of momentum).

Work, Energy, and Power

• Work is done when a force produces motion in the direction of the force: W = Fs cos θ where θ is the angle between force and displacement. Unit: Joule (J).

• Kinetic Energy (energy of motion): KE = ½mv². Doubling the speed quadruples kinetic energy.

• Potential Energy (energy of position): PE = mgh, where h is height above a reference point. For a body at height h above Earth’s surface.

• Conservation of Energy: Energy cannot be created or destroyed, only converted from one form to another. Total mechanical energy (KE + PE) remains constant in the absence of non-conservative forces.

• Power: Rate of doing work: P = W/t. Unit: Watt (W). 1 horsepower = 746 W (used in older texts). Also P = Fv for power as force × velocity.

Gravitation

Newton’s Law of Universal Gravitation: F = Gm₁m₂/r², where G = 6.674 × 10⁻¹¹ Nm²/kg² (gravitational constant).

Acceleration due to gravity on Earth: g = GM/R² ≈ 9.8 m/s². This decreases with altitude (g_h = g(R/(R+h))²) and latitude (higher at poles due to Earth’s oblate shape).

Escape velocity is the minimum velocity needed to escape Earth’s gravitational pull: v_esc = √(2gR) ≈ 11.2 km/s. This is derived by setting kinetic energy equal to gravitational potential energy.

Orbital velocity for a satellite orbiting close to Earth: v_orb = √(gR) ≈ 7.9 km/s.

Light — Reflection and Refraction

Laws of Reflection: (1) Angle of incidence = angle of reflection; (2) The incident ray, reflected ray, and normal all lie in the same plane.

Spherical Mirrors:

• Concave mirror: converging; used as shaving mirrors, torch reflectors, dentist’s mirrors
• Convex mirror: diverging; used as rear-view mirrors (wider field of view)
• Mirror formula: 1/v + 1/u = 1/f (sign convention must be applied carefully)
• Magnification: m = –v/u (negative m = inverted image)

Lenses:

• Convex (converging) lens: used in magnifying glasses, cameras, human eye
• Concave (diverging) lens: used to correct myopia
• Lens formula: 1/v – 1/u = 1/f
• Power of lens: P = 1/f (in metres), unit = Dioptre (D). Convex lens has positive power; concave has negative.

Electricity

• Ohm’s Law: V = IR — Voltage = Current × Resistance. Resistance depends on material, length, and cross-section: R = ρl/A.
• Series circuit: Same current flows through all components. Total R = R₁ + R₂ + R₃. Total V = V₁ + V₂ + V₃.
• Parallel circuit: Same voltage across all branches. Total current = I₁ + I₂ + I₃. 1/R_total = 1/R₁ + 1/R₂ + 1/R₃.
• Electric Power: P = VI = I²R = V²/R. Unit: Watt.
• Joule’s Law of Heating: H = I²Rt — heat produced in a resistor.
• Kirchhoff’s Laws (for complex circuits): Junction law (sum of currents entering = sum leaving) and Loop law (sum of EMFs = sum of potential drops).

Magnetism

• Magnetic field lines run from North to South outside a magnet.
• Earth’s magnetic field acts as if a bar magnet exists at Earth’s centre.
• Electromagnetism: Current-carrying wire produces a magnetic field (Oersted’s experiment). A coil of wire (solenoid) with a soft iron core forms an electromagnet.
• Force on a current-carrying conductor in a magnetic field: F = BIL sin θ.
• Electromagnetic induction (Faraday’s Law): Changing magnetic flux induces an EMF.

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🔴 Extended — Deep Study (3mo+)

Comprehensive theory for Physics mastery in NDA GAT.

Sound — Wave Phenomena

• Speed of sound in dry air at 0°C: 332 m/s. Speed increases by approximately 0.6 m/s per °C rise in temperature: v = 332 + 0.6T.
• Speed in water ≈ 1480 m/s; in steel ≈ 5000 m/s.
• Sound cannot travel through vacuum (unlike light).

Key sound phenomena for NDA:

• Echo: Audibly distinct reflected sound heard after the original sound. Minimum distance for echo at 20°C: 17.2 m (for reflected sound to return after 0.1 s).
• Reverberation: Multiple overlapping reflections creating a sustained sound after the source stops. Problematic in large empty halls; solved by sound-absorbing materials.
• Doppler Effect: Change in frequency when source or observer moves. Formula: f’ = f(v ± v_o)/(v ∓ v_s). Applications: radar speed guns, astronomical redshift.
• Pitch determined by frequency; Loudness by amplitude.
• Ultrasound (above 20 kHz): Used in sonar, medical imaging (SONAR = Sound Navigation and Ranging).

Heat and Temperature

Three temperature scales and their relationships:

• Celsius (°C) to Fahrenheit (°F): F = (9/5)C + 32
• Celsius (°C) to Kelvin (K): K = C + 273.15
• Absolute zero = 0 K = –273.15°C

Specific heat capacity (c): Heat required to raise 1 kg of a substance by 1°C. Water has the highest specific heat: c_water = 4186 J/kg·K. This is why coastal areas have milder climates than inland areas.

Latent heat: Heat absorbed or released during change of state without temperature change.

• Latent heat of fusion (ice → water): 336 J/g
• Latent heat of vaporisation (water → steam): 2260 J/g

Heat transfer: Conduction (through solids, e.g., metal spoon in hot tea), Convection (through fluids via circulation, e.g., sea breezes), Radiation (through electromagnetic waves, e.g., Sun’s heat reaching Earth — does not require a medium).

Magnetism — Detailed

• Magnetic poles: Unlike poles attract (N attracts S); like poles repel (N repels N).
• Magnetic field strength (B) measured in Tesla (T). Earth’s magnetic field ≈ 10⁻⁵ T.
• Magnetic flux (Φ = B × A) measured in Weber.
• Electromagnets are used in cranes for lifting scrap iron, in electric bells, and in MRI machines.
• electromagnetic spectrum: Radio waves → Microwaves → Infrared → Visible light → Ultraviolet → X-rays → Gamma rays (increasing frequency, decreasing wavelength).

Wave optics:

• Reflection: Angle i = Angle r
• Refraction: Light bends when passing between media of different optical densities. Snell’s Law: n₁ sin i = n₂ sin r, where n = refractive index.
• Total Internal Reflection: Occurs when angle of incidence exceeds the critical angle. Basis for fibre optic communication.
• Dispersion: White light splits into constituent colours through a prism (VIBGYOR — Violet bends most, Red bends least).

NDA Physics Question Patterns:

Based on analysis of NDA (I) 2023 and 2024 papers:

Topic	Approximate Questions	Difficulty
Mechanics (motion, laws, work-energy)	5–6	Moderate
Gravitation	2–3	Easy–Moderate
Light (reflection, refraction, wave optics)	3–4	Moderate
Electricity	3–4	Moderate
Magnetism	1–2	Easy
Sound	1–2	Easy
Heat	1–2	Easy–Moderate
General physics concepts	2–3	Easy

Key Exam Strategies for NDA Physics:

1. In numerical questions, identify given quantities first, write the formula, substitute values.
2. Check units before selecting answers — common trap is mixing units (e.g., km/h with m/s).
3. Dimension analysis is a quick elimination tool.
4. For lens/mirror questions, always draw a ray diagram first.
5. Remember: mass is constant (not changing with velocity — that’s a common confusion from relativity which is beyond NDA scope).