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Physics · AFCAT

PA01 — Units, Dimensions & Basic Physics

✈ Physics – PA01  ·  AFCAT General Awareness AFCAT Level

Every Physics question in AFCAT traces back to units and measurement. Questions here are short, factual, and completely scoring — one right answer gets you a full mark in under 20 seconds. Know the 7 SI base units, 5 key derived units, and the scalar–vector distinction.

📌 AFCAT Focus: Direct questions — "SI unit of Force / Pressure / Power?", "Which is a vector quantity?", "Least count of Vernier caliper?" These three question types cover almost all AFCAT PA01 questions. Class 10 level only.
PART 1 — SI UNITS

1. The Seven Fundamental SI Units

Every physical quantity is measured using combinations of just 7 base quantities. All other (derived) units are built from these seven.

Fig. 1 — Seven Fundamental SI Base Units (memorise all 7)
SEVEN FUNDAMENTAL SI UNITS # Physical Quantity SI Unit Name Symbol 1 Length Metre m 2 Mass Kilogram kg 3 Time Second s 4 Electric Current Ampere A 5 Temperature Kelvin K 6 Amount of Substance Mole mol 7 Luminous Intensity Candela cd
💡 Memory aid — 7 base units: metre, kilogram, second, Ampere, Kelvin, mole, candela — think of them as the 7 pillars every measurement stands on.

2. Key Derived Units — Most Asked in AFCAT

Fig. 2 — Key Derived Units Built from Base Units
KEY DERIVED UNITS — AFCAT asks these directly Quantity SI Unit Name Symbol Made from Force Newton N kg · m · s⁻² Energy / Work Joule J kg · m² · s⁻² Power Watt W J/s = kg·m²·s⁻³ Pressure Pascal Pa N/m² = kg·m⁻¹·s⁻² Frequency Hertz Hz cycles / s = s⁻¹ Electric Charge Coulomb C A · s
PART 2 — SCALARS AND VECTORS

3. Scalars vs Vectors

AFCAT frequently asks: "Which of the following is a vector quantity?" — pure recall in under 5 seconds.

Fig. 3 — Scalar (magnitude only) vs Vector (magnitude + direction) with Examples
SCALAR Magnitude only — no direction ● Distance ● Speed ● Mass ● Time ● Temperature ● Energy / Work ● Power ● Volume, Density VECTOR Magnitude + Direction ● Displacement ● Velocity ● Acceleration ● Force ● Momentum ● Weight (= mg, downward) ● Torque / Thrust ● Electric Field
⚠ AFCAT Trap: Speed is scalar; Velocity is vector. Distance is scalar; Displacement is vector. Weight is a vector (it is a force — always acts downward); Mass is a scalar (no direction). This distinction is asked directly.
PART 3 — MEASURING INSTRUMENTS

4. Vernier Caliper & Screw Gauge

Fig. 4 — Vernier Caliper Reading: Main Scale + Vernier Scale = Total Measurement
VERNIER CALIPER — How to Take a Reading Main Scale (mm): 0 1 2 3 4 5 Vernier Scale (10 divisions) 0 1 2 3 4 ← coincides here MS = 2 mm Reading Calculation Least Count = 1 MSD − 1 VSD = 1 mm − 0.9 mm = 0.1 mm Main Scale Reading = 2.0 mm Vernier Reading = 4 × 0.1 = 0.4 mm Total Reading = 2.0 + 0.4 = 2.4 mm
Instrument Formulae:

● Vernier Caliper LC = 1 MSD − 1 VSD = 0.1 mm (standard)
● Screw Gauge LC = Pitch / Circular divisions = 0.5/50 = 0.01 mm
● Screw gauge is more precise than Vernier caliper (reads to 0.01 mm vs 0.1 mm)

📝 AFCAT PYQs — Units & Basic Physics

Q1. The SI unit of Force is: AFCAT PYQ
(a) Joule(b) Pascal(c) Newton(d) Watt
✔ Answer: (c) Newton
Force = mass × acceleration = [kg × m/s²] = Newton (N). Joule = energy; Pascal = pressure; Watt = power. This is the single most frequently asked unit question in AFCAT science. Named after Isaac Newton.
Q2. Which of the following is a vector quantity? AFCAT PYQ
(a) Speed(b) Distance(c) Mass(d) Displacement
✔ Answer: (d) Displacement
Displacement has both magnitude and direction (e.g., "5 km north"). Speed, distance, and mass have magnitude only — they are scalars. The key test: if the question "in which direction?" is meaningful for that quantity, it is a vector.
Q3. The SI unit of Pressure is: AFCAT PYQ
(a) Newton(b) Pascal(c) Joule(d) Bar
✔ Answer: (b) Pascal
Pressure = Force/Area = N/m² = Pascal (Pa). While bar and atm are used in daily life, the SI unit is Pascal. 1 atmosphere ≈ 101,325 Pa. Named after Blaise Pascal.
Q4. Which of the following is NOT a fundamental SI unit? ⚡ Tricky
(a) Kelvin(b) Mole(c) Newton(d) Candela
✔ Answer: (c) Newton
Newton is derived (= kg·m·s⁻²), not fundamental. The 7 base SI units are: metre, kilogram, second, Ampere, Kelvin, mole, candela. Students often include Newton — it is built from mass, length, and time.
Q5. The least count of a screw gauge with pitch 0.5 mm and 50 circular divisions is: AFCAT PYQ
(a) 0.5 mm(b) 0.1 mm(c) 0.01 mm(d) 0.001 mm
✔ Answer: (c) 0.01 mm
LC = Pitch / divisions = 0.5/50 = 0.01 mm. The screw gauge is more precise than the Vernier caliper (0.01 mm vs 0.1 mm). This formula is directly tested in AFCAT.

🧠 Quick Memory Chart — PA01

🏠 7 Base SI Units
  • Length → metre (m)
  • Mass → kilogram (kg)
  • Time → second (s)
  • Current → Ampere (A)
  • Temp → Kelvin (K)
  • Substance → mole (mol)
  • Luminosity → candela (cd)
⚡ Derived Units
  • Force → Newton (N)
  • Energy/Work → Joule (J)
  • Power → Watt (W)
  • Pressure → Pascal (Pa)
  • Frequency → Hertz (Hz)
  • Newton = DERIVED (not base)
📍 Scalar vs Vector
  • Scalars: speed, distance, mass, energy
  • Vectors: velocity, displacement, force
  • Weight = vector (force, downward)
  • Mass = scalar (no direction)
  • Screw gauge LC = 0.01 mm

📝 Practice Exercise

E1. The SI unit of Power is:
(a) Joule(b) Newton(c) Watt(d) Ampere
E2. Which one is a scalar quantity?
(a) Velocity(b) Acceleration(c) Force(d) Speed
E3. Screw gauge has pitch 0.5 mm and 50 circular divisions. Least count:
(a) 0.5 mm(b) 0.1 mm(c) 0.01 mm(d) 0.001 mm
E4. Which quantity has the same SI unit as energy?
(a) Power(b) Torque(c) Pressure(d) Force
Answers:
E1 → (c) Watt  |  E2 → (d) Speed [only magnitude, no direction]  |  E3 → (c) 0.01 mm [LC = 0.5/50]  |  E4 → (b) Torque [N·m = Joule; same dimension ML²T⁻², different physical meaning]
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