Physics Basics Questions for RRB GROUP D

Units and Measurements

Every physical quantity is measured using a standard unit. The International System of Units (SI) is used worldwide. RRB Group D frequently asks which unit belongs to which quantity. The most common trap is mixing up derived units like Newton, Joule, and Watt.

• Length: metre (m), Mass: kilogram (kg), Time: second (s)
• Temperature: Kelvin (K), Electric current: Ampere (A)
• Force: Newton (N) = kg m/s squared
• Energy and Work: Joule (J) = N x m
• Power: Watt (W) = J/s
• Pressure: Pascal (Pa) = N/m squared

Newton's Laws of Motion

Newton gave 3 laws that explain how objects move. These are very commonly asked in RRB Group D as simple application questions. Think of them as: Law 1 = lazy object, Law 2 = force recipe, Law 3 = action and reaction.

• Law 1 (Inertia): An object stays at rest or in uniform motion unless an external force acts on it
• Law 2 (Force): Force = Mass x Acceleration (F = ma). Heavier object needs more force to accelerate
• Law 3 (Action-Reaction): Every action has an equal and opposite reaction
• Inertia is the tendency to resist change — heavier objects have more inertia
• Rocket propulsion and gun recoil are examples of Newton's Third Law
• SI unit of force is Newton (N)

Work, Energy and Power

Work is done when a force moves an object. Energy is the ability to do work. Power tells us how fast work is done. These three are linked and RRB Group D often asks simple numerical or unit-based questions on them.

• Work = Force x Distance x cos(angle). If force and displacement are in same direction, Work = F x d
• If object does not move, work done is zero (even if force is applied)
• Kinetic Energy = energy of motion; Potential Energy = stored energy
• Energy is conserved — it only changes form (Law of Conservation of Energy)
• 1 Horsepower = 746 Watts (common RRB fact)
• Power = Work / Time

Sound

Sound is a mechanical wave that needs a medium to travel. It cannot travel through vacuum. RRB Group D asks about speed of sound, properties of sound, and human hearing range.

• Speed of sound in air at 0 degree Celsius = 332 m/s; at room temperature approx 343 m/s
• Sound travels fastest in solids, then liquids, then gases
• Human hearing range: 20 Hz to 20,000 Hz (20 kHz)
• Ultrasound: frequency above 20,000 Hz — used in medical imaging (sonography)
• Infrasound: frequency below 20 Hz — elephants and whales use it
• Echo is reflection of sound; echo is heard when reflected sound comes back after 0.1 seconds or more

Light and Optics

Light is an electromagnetic wave and does not need a medium. It travels fastest in vacuum. RRB Group D tests reflection, refraction, and basic optical instruments.

• Speed of light in vacuum = 3 x 10 to the power 8 m/s (3 lakh km per second)
• Reflection: angle of incidence = angle of reflection
• Refraction: bending of light when it moves from one medium to another
• Convex lens converges light — used in magnifying glass, camera, human eye
• Concave lens diverges light — used in spectacles for short-sightedness (myopia)
• Total Internal Reflection is used in optical fibres

Basic Electricity

Electricity is the flow of electric charges (electrons). RRB Group D asks about Ohm's Law, units of electrical quantities, and safety devices like fuse.

• Current (I) is measured in Amperes (A); Voltage (V) in Volts (V); Resistance (R) in Ohms
• Ohm's Law: V = I x R — voltage equals current times resistance
• Power in a circuit: P = V x I (also P = I squared x R)
• Good conductors: copper, silver, aluminium. Good insulators: rubber, plastic, wood
• A fuse wire has high resistance and low melting point — it breaks the circuit if current is too high
• AC (Alternating Current) is used in homes in India at 220 V, 50 Hz

• Confusing Watt (power) with Joule (energy) — Watt is rate of doing work, Joule is the amount of work done
• Thinking work is done even when object does not move — if displacement is zero, work done is zero no matter how much force is applied
• Mixing up concave and convex lens uses — convex lens is thicker in middle and converges light; concave is thinner in middle and diverges
• Saying sound travels faster than light — light is always faster; sound cannot travel in vacuum at all
• Forgetting that g = 9.8 m/s squared on Earth but 0 in space and about 1.6 m/s squared on Moon — RRB sometimes asks Moon-based weight questions

• Newton's Laws shortcut: 1-Lazy (inertia), 2-Formula (F=ma), 3-Bounces back (action-reaction). Remember 'Lazy Formula Bounces'
• Solid Liquid Gas for sound speed: SLuG — Sound is Loudest in solids (S faster than L faster than G)
• For lens types remember: ConVEX = conVERGES light (both have V). ConcAVE = diVERGES (both deal with spreading/depression)
• Electrical units memory: Very Intelligent Robots — Voltage = V, Current = I (Intensity), Resistance = R. Ohm's law: VIR triangle — cover what you want to find

• RRB Group D asks 1-2 direct unit questions every paper — just memorise SI units of the 8 base quantities and common derived units like Newton, Joule, Watt, Pascal
• Newton's Third Law application questions are very popular — think rocket, gun recoil, swimming, walking on ground. All are action-reaction
• Speed of sound and speed of light are standard fact questions — no calculation, just recall 332 m/s and 3x10 to 8 m/s
• Electricity questions usually follow Ohm's Law with simple 2-step numbers — practice substituting into V=IR and P=VI quickly
• Human hearing range (20 Hz to 20 kHz) and ultrasound applications (sonar, medical imaging) are repeated in almost every RRB exam cycle — do not skip these