Chapter 19
Physical Quantities, Measurement & Fluid Mechanics
Unit VI · Physics. SI units, density, pressure, buoyancy, Bernoulli.
Full Chapter Notes
Source · FPSC Trap Decoder · CSS MPT Smart Notes (2026 Edition)
19.1 Context
| MPT Weightage | Difficulty Level | Confirmed Past Papers |
|---|---|---|
| 3–5 Marks | Medium | 2022 · 2023 · 2024 |
Trend Alert. Instruments appeared in every paper: Sphygmomanometer (blood pressure, 2022), Pulse Oximeter (oxygen level, 2022), Rain Gauge (rainfall, 2022). Light Year = distance appeared MPT 2023. Sky blue = scattering appeared MPT 2023. The kWh vs Watt distinction and 1 HP = 746 Watts are consistent FPSC targets. Isobars = equal pressure appeared MPT 2024.
19.2 High-Yield Fact Snapshot
| FPSC-Tested Fact | Correct Answer | Year Tested |
|---|---|---|
| Light Year is a unit of | Distance — NOT time | 2023 — Repeated |
| Sky appears blue due to | Scattering of light (Rayleigh) — not reflection | 2023 |
| Blood pressure instrument | Sphygmomanometer | 2022 — Repeated twice |
| Oxygen level in blood instrument | Pulse Oximeter (answer = None of these) | 2022 |
| Rainfall measurement instrument | Rain Gauge (answer = None of these) | 2022 |
| TV remote control waves | Infrared waves | 2022 |
| Isobars on weather maps | Lines connecting equal atmospheric pressure | 2024 |
| 1 Horsepower = | 746 Watts | Every paper trap |
| kWh is a unit of | Energy (NOT power) — Power × Time = Energy | Every paper trap |
| Terminal velocity — net force | Zero (weight = air resistance) | Every paper |
19.3 Instruments & Their Measurements
FPSC presents instruments in two formats: (1) 'What does X measure?' and (2) 'What measures Y?' Master the full two-way mapping.
| Instrument | Measures | FPSC Strategic Note |
|---|---|---|
| Sphygmomanometer | Blood Pressure | 2022 — Appears as option C when B is Hydrometer. If listed, always choose it. |
| Pulse Oximeter | Oxygen level in blood (O₂ saturation) | 2022 — NOT listed as option. Correct answer = 'None of these.' |
| Rain Gauge | Rainfall (precipitation) | 2022 — NOT listed as option. Correct answer = 'None of these.' |
| Barometer | Atmospheric Pressure | Sudden drop = storm approaching. Slow rise = fair weather. |
| Hygrometer | Humidity (moisture in air) | 'Hygro' = moisture. Trap: students pick Barometer. |
| Seismograph | Earthquake intensity (seismic waves) | Richter Scale is logarithmic — Magnitude 6 = 10x magnitude 5. |
| Altimeter | Altitude (height above sea level) | Used in aircraft. |
| Anemometer | Wind speed | 'Anemo' = wind. |
| Lactometer | Purity/density of milk | Measures specific gravity of milk. |
| Fathometer | Ocean depth | Uses sound waves to measure sea depth. |
Instrument trap (repeated 2022). FPSC presents options as 'Barometer / Hydrometer / Nanometer / None of these' for questions about sphygmomanometer, pulse oximeter, and rain gauge. The correct instruments are not in the options — so the answer is 'None of these.' This appeared THREE TIMES in the 2022 paper. Always check if the correct instrument name appears as an option before choosing.
19.4 Units — The Power vs Energy vs Force Trap
| Quantity | SI Unit | Common Trap Unit | FPSC Note |
|---|---|---|---|
| Force | Newton (N) | Joule (trap) | F = ma. Newton = kg·m/s². |
| Energy / Work | Joule (J) | Watt (trap) | Work = Force × Distance. |
| Power | Watt (W) | Joule (trap) | Power = Energy/Time. 1 W = 1 J/s. |
| Commercial Energy | kWh (Kilowatt-hour) | 'Must be power' (trap) | kWh = Power × Time = ENERGY. NOT a power unit. |
| Pressure | Pascal (Pa) | Newton (trap) | Pressure = Force/Area. |
| 1 Horsepower | 746 Watts | 500 or 1000 W (trap) | 1 HP = 746 W exactly. Every paper. |
| 1 kWh | 3.6 Million Joules | 'A power unit' (trap) | Energy = Power × Time = W × h = kWh. |
kWh is energy, NOT power. Students see 'Kilowatt' in kWh and answer 'power.' Adding 'hour' (time) to Watts converts it from power to energy. kWh = Kilowatt × Hour = Power × Time = Energy. This is what your electricity bill measures — total energy consumed, not power. 1 kWh = 3.6 million Joules.
19.5 Electromagnetic Spectrum — Wavelength Order
Confirmed MPT 2024 (repeated twice): 'Electromagnetic radiation with MAXIMUM wavelength = Radio waves.' Know the full spectrum order from longest to shortest wavelength.
| Wave Type | Wavelength | Frequency | FPSC Application |
|---|---|---|---|
| Radio Waves | Longest (meters to km) | Lowest | 2024: Max wavelength = Radio waves. TV, radio broadcasts. |
| Microwaves | cm range | — | Satellite communication, microwave ovens. |
| Infrared (IR) | ~700 nm to 1 mm | — | 2022: TV remote = Infrared. Night vision. Heat sensing. |
| Visible Light | 400–700 nm | — | VIBGYOR. Red = longest. Violet = shortest. |
| Ultraviolet (UV) | ~10–400 nm | — | Causes sunburn. Absorbed by ozone layer. |
| X-Rays | ~0.01–10 nm | — | Bone imaging. CT scans. Ionising radiation. |
| Gamma Rays | Shortest (<0.01 nm) | Highest | 2024: Most dangerous. Highest penetrating power. |
Solar energy is radiated in the form of ELECTROMAGNETIC WAVES — not 'heat waves' or 'light waves' alone. The Sun emits energy across the full electromagnetic spectrum including infrared, visible light, and ultraviolet. The correct answer is 'Electromagnetic waves.'
19.6 Mechanics — Key Concepts
| Concept | Key Fact | FPSC Strategic Note |
|---|---|---|
| Terminal Velocity | Net force = zero (weight = air resistance) | Speed constant. Parachutists and falling objects reach this. |
| Escape Velocity | 11.2 km/s (NOT km/h or m/s) | Unit trap: FPSC offers 11.2 km/h as distractor. Check the 's'. |
| Archimedes Principle | Upthrust = Weight of displaced fluid | 2022. Explains floating and iceberg physics. |
| Mass vs Weight | Mass = constant (kg). Weight = mg (varies with gravity) | Mass never changes. Weight = 0 in space. |
| Isobars | Lines on weather maps connecting equal pressure points | 2024. Sudden pressure drop = storm incoming. |
| Water Max Density | At 4°C — anomalous expansion | Below 4°C water expands. Ice floats because it is less dense. |
19.7 Battle Card — 5-Minute Revision
| Fact | Answer |
|---|---|
| Max wavelength radiation | Radio Waves (2024 — repeated twice) |
| TV remote waves | Infrared (IR) waves (2022) |
| Solar energy form | Electromagnetic waves (2024 — repeated twice) |
| Light Year | Unit of DISTANCE (not time) (2023) |
| Sky is blue due to | Scattering of light (Rayleigh Scattering) (2023) |
| Blood pressure instrument | Sphygmomanometer (2022) |
| Oxygen level in blood | Pulse Oximeter — answer = None of these (2022) |
| Rainfall instrument | Rain Gauge — answer = None of these (2022) |
| 1 Horsepower | 746 Watts (every paper trap) |
| kWh is a unit of | ENERGY (not power) — Power × Time |
| Isobars connect | Equal atmospheric pressure points (2024) |
| Terminal velocity net force | Zero — weight equals air resistance |
| Escape velocity of Earth | 11.2 km/s — unit trap: NOT km/h |
| Water max density | At 4°C — anomalous expansion |
19.8 Practice MCQs (FPSC Level)
Part A — Basic Recall
Light year, TV remote waves, EM spectrum, and the 1 HP = 746 W trap.
The term 'Light Year' refers to:
Show explanation
A light year is the distance that light travels in one year — approximately 9.46 × 10¹² km. It is a unit of DISTANCE, not time. The word 'year' refers to the travel duration used to calculate the distance.
Appeared MPT 2023
The waves used by TV remote controls are:
Show explanation
TV remote controls use Infrared (IR) waves — a specific region of the electromagnetic spectrum just below visible light. IR waves are invisible to humans but detected by the TV's receiver.
Appeared MPT 2022
Electromagnetic radiation with the maximum wavelength is:
Show explanation
Radio waves have the longest wavelengths in the electromagnetic spectrum — ranging from meters to kilometers. The order from longest to shortest: Radio → Microwave → Infrared → Visible → UV → X-Ray → Gamma.
Appeared MPT 2024 — Repeated twice
1 Horsepower (HP) is equal to:
Show explanation
1 Horsepower = 746 Watts exactly. FPSC offers 500 W, 764 W, and 1000 W as distractors. The exact value 746 is what is tested.
Appeared MPT 2024 — unit conversion trap
Part B — Trap-Based
Rayleigh scattering, instrument lookups, kWh vs power, and solar energy form.
The sky appears blue primarily because:
Show explanation
Rayleigh Scattering: Blue light has a shorter wavelength than red light and scatters more effectively off gas molecules in the atmosphere — spreading in all directions and making the sky appear blue. The answer per MPT 2023: 'scattering of light from atmosphere.'
Appeared MPT 2023
The instrument used to measure blood pressure is called a:
Show explanation
Blood pressure is measured by a Sphygmomanometer. Since this instrument does not appear among the listed options, the correct answer is D (None of these). Barometer measures atmospheric pressure; Hydrometer measures liquid density; Nanometer is a unit of length — none measure blood pressure.
Appeared MPT 2022 — if Sphygmomanometer listed, choose it; if not, choose D
kWh (Kilowatt-hour) is a unit of:
Show explanation
kWh = Kilowatt × Hour = Power × Time = Energy. Adding 'hour' (time) to the power unit Watt converts it into an energy unit. Your electricity bill measures energy consumed in kWh. 1 kWh = 3.6 million Joules.
Trap: Students see 'Watt' and choose Power — kWh = Energy.
The solar energy radiated by the Sun is in the form of:
Show explanation
The Sun emits energy across the full electromagnetic spectrum — including infrared, visible light, and ultraviolet radiation. All of these are electromagnetic waves. 'Heat waves' and 'light waves only' are both partially correct but incomplete. The correct answer is 'Electromagnetic waves.'
Appeared MPT 2024 — Repeated twice
Part C — Elite Simulation
Statement sets across the EM spectrum, terminal velocity, and weather-map line types.
Consider: (1) Radio waves have the longest wavelength in the EM spectrum. (2) Gamma rays have the highest penetrating power. (3) Infrared waves are used in TV remote controls. Which are correct?
Show explanation
All three statements are correct. Radio waves have the longest wavelength in the EM spectrum. Gamma rays have the highest penetrating power. Infrared waves are used in TV remote controls — they are invisible to humans but detected by the TV receiver.
Trap: All three correct — confirmed past paper facts.
When a body reaches terminal velocity:
Show explanation
Terminal velocity is reached when the downward weight force exactly equals the upward air resistance force. Net force = zero. From Newton's Second Law (F=ma), zero net force means zero acceleration — the object moves at constant speed.
Trap: Terminal velocity = weight equals drag = net force = zero.
Consider: (1) kWh is a unit of energy. (2) Watt is a unit of power. (3) 1 HP = 746 W. Which are correct?
Show explanation
All three are correct: kWh is energy (Power × Time). Watt is power (Energy/Time). 1 HP = 746 W (standard conversion). These are the three most-tested unit facts in FPSC physics.
Trap: All three correct.
Lines on a weather map joining points of equal atmospheric pressure are called:
Show explanation
Isobars are lines drawn on weather maps connecting all points of equal atmospheric pressure. They are used to identify high and low pressure systems. A sudden drop in isobar values = storm approaching.
Appeared MPT 2024
19.9 Answer Key & Trap Analysis
Physical Quantities & Measurement (Q1–Q12)
| Q | Correct | Type | Primary Trap | Why Others Fail |
|---|