Physics Basics Questions for RRB NTPC

Units and Measurements

Every physical quantity is measured in a standard unit. The SI system (International System of Units) is the global standard used in all scientific work. RRB NTPC directly asks: what is the SI unit of pressure, force, energy, power, etc. Memorise the 7 base SI units and derived units cold.

• Length: metre (m), Mass: kilogram (kg), Time: second (s)
• Electric current: ampere (A), Temperature: kelvin (K)
• Force: newton (N) = kg m/s2
• Energy/Work: joule (J), Power: watt (W) = J/s
• Pressure: pascal (Pa) = N/m2
• Frequency: hertz (Hz) = cycles per second

Newton's Laws of Motion

Newton gave three laws that explain how objects move. These are directly tested in RRB NTPC as statement-based questions like 'Which law explains rocket propulsion?' or 'A body at rest stays at rest — which law?'. Understand the real-life example linked to each law.

• First Law (Inertia): A body stays at rest or in uniform motion unless acted on by external force. Example: Passenger jerks forward when bus brakes suddenly.
• Second Law (F=ma): Force equals mass times acceleration. More mass needs more force for same acceleration.
• Third Law (Action-Reaction): Every action has equal and opposite reaction. Example: Rocket moves forward by pushing gases backward.
• Inertia depends on mass — heavier object has more inertia.
• Impulse = Force x Time = Change in momentum (J = F x t)

Gravitation and Weight

Gravity is the force that pulls objects toward Earth. RRB NTPC asks about the value of g, difference between mass and weight, and how weight changes on moon. Gravity decreases as you go higher from Earth's surface or go deep inside the Earth.

• g on Earth surface = 9.8 m/s2 (approx 10 m/s2 in calculations)
• g on Moon = 1/6th of Earth's g, so weight on Moon = Weight on Earth / 6
• Mass is constant everywhere; Weight = m x g, so weight changes with location
• Weight is a force, measured in newtons (N), not kg
• G (Universal Gravitational Constant) = 6.67 x 10(-11) N m2/kg2

Work, Energy and Power

Work is done only when a force moves an object in the direction of the force. Energy is the capacity to do work. Power is how fast work is done. RRB NTPC frequently tests the units and simple calculations here.

• Work = Force x displacement x cos(angle); if force and motion are perpendicular, Work = 0
• Kinetic Energy (KE) = half m v squared — energy of a moving object
• Potential Energy (PE) = mgh — energy stored due to height
• Law of conservation: Total energy remains constant, KE + PE = constant
• 1 horsepower (hp) = 746 watts — common RRB conversion question
• 1 kilowatt-hour (kWh) = 3.6 x 10(6) J — unit on electricity bill

Light and Sound

Light travels in straight lines at 3 x 10(8) m/s in vacuum. Sound needs a medium and travels much slower. RRB NTPC tests laws of reflection, types of mirrors and lenses, speed comparisons, and echo conditions.

• Speed of light in vacuum = 3 x 10(8) m/s (3 lakh km/s)
• Speed of sound in air at 25 degrees C = approx 343 m/s; faster in solids than liquids than gases
• Echo: sound reflects back; minimum distance = 17.2 m (half of 34.4 m for round trip in 0.1 s)
• Concave mirror: converging, used in torches, headlights, shaving mirror
• Convex mirror: diverging, used in rear-view mirrors — gives wider field of view
• Convex lens: converging, used to correct hypermetropia (farsightedness); concave lens corrects myopia (nearsightedness)

Electricity Basics

Electricity questions in RRB NTPC focus on Ohm's law, series vs parallel circuits, and power/energy formulas. The concept of resistance and its relationship with voltage and current is heavily tested.

• Ohm's Law: V = I x R (Voltage = Current x Resistance)
• In series circuit: total resistance = sum of all resistances; same current flows
• In parallel circuit: 1/R_total = 1/R1 + 1/R2; voltage is same across each branch
• Electric Power P = V x I = I2 x R = V2 / R
• Electric Energy = Power x Time; unit is kWh (kilowatt-hour) for billing
• Good conductors have low resistance; insulators have very high resistance

• Confusing mass (in kg, constant) with weight (in N, varies with g). RRB often asks weight on Moon — always divide by 6.
• Forgetting that work done is zero when force is perpendicular to motion — like a coolie carrying load on head while walking horizontally.
• Mixing up concave and convex mirrors: Remember C for Concave = Converging = Cave-shaped (curves inward). Convex = bulges outward like a ball.
• In parallel circuits, total resistance is LESS than smallest individual resistance — many aspirants add them up like series.
• Confusing hertz (Hz) with watt (W): Hz is frequency (sound/light waves), W is power. Both are derived units but for different quantities.

• Newton's Laws mnemonic — IRF: Inertia, Rate of change (F=ma), Force-reaction. Think I Ride Fast.
• Mirror uses — TRSH: Torch/Reflector/Shaving = concave. Rear-view = convex (R for Road, R for Rear-view, both convex).
• Energy units ladder: 1 kWh = 3.6 MJ = 3.6 x 10(6) J. Think 3.6 as a constant friend for electricity billing.
• Speed comparison for sound: Solid faster than Liquid faster than Gas — SLG like Slowly Liquids Go (reversed to remind solids are fastest).

• RRB NTPC typically asks 3-5 Physics questions, mostly from units, Newton's laws, mirrors/lenses, and Ohm's law. Rarely goes into advanced topics.
• Statement-matching questions are common: Match the law to real-life example — rocket (3rd law), passenger jerks (1st law), F=ma (2nd law). Practise these associations.
• Numerical questions in Physics are straightforward — usually single-step substitution in F=ma, W=mg, P=W/t, or V=IR. Spend max 40 seconds on each.
• Mirror and lens questions often appear with ray diagrams described in words. Focus on uses: rear-view mirror is always convex, and concave is used where you want to focus or magnify.
• The 1 hp = 746 W conversion is a direct favourite in RRB papers. Similarly, know that 1 light year is a unit of distance, not time — a common trap question.