Welcome to GattPrep!
Hello again, scientist! 👩🏽🔬👨🏽🔬
Imagine this: You go to buy fabric, and the seller says it’s “long.” You ask, “How long?” They say, “Just… long.” Would you be satisfied?
Of course not! You’d want exact figures—in metres, not guesses.
That’s why scientists use standard units for all measurements. No guessing, no confusion. In this lesson, we’ll explore the basic and derived quantities and their SI units—the universal language of science.
A quantity is anything you can measure: length, mass, time, temperature, etc. Every quantity has a unit, which tells us how much of it there is.
These are the seven quantities that form the foundation of all scientific measurements:
| Quantity | Symbol | SI Unit | Unit Symbol |
|---|---|---|---|
| Length | l | metre | m |
| Mass | m | kilogram | kg |
| Time | t | second | s |
| Temperature | T | kelvin | K |
| Electric current | I | ampere | A |
| Amount of substance | n | mole | mol |
| Luminous intensity | Iv | candela | cd |
These are the building blocks of all other measurements.
Derived quantities come from combining basic ones.
| Derived Quantity | Formula | SI Unit | Unit Symbol |
|---|---|---|---|
| Area | Length × Length | square metre | m² |
| Volume | Length × Width × Height | cubic metre | m³ |
| Speed/Velocity | Distance ÷ Time | metre/second | m/s |
| Acceleration | Velocity ÷ Time | m/s² | m/s² |
| Density | Mass ÷ Volume | kg/m³ | kg/m³ |
| Force | Mass × Acceleration | newton | N |
| Work/Energy | Force × Distance | joule | J |
| Power | Energy ÷ Time | watt | W |
| Pressure | Force ÷ Area | pascal | Pa |
| Electric charge | Current × Time | coulomb | C |
| Voltage (P.D.) | Energy ÷ Charge | volt | V |
| Resistance | Voltage ÷ Current | ohm | Ω |
Standardized worldwide — no confusion!
Helps communication in science and engineering.
Reduces errors in measurement and calculations.
Measuring speed: A car travels 60 km in 1 hour → Speed = 60 km/h = 16.7 m/s
Cooking: You boil water and need the temperature in kelvin, not just “hot”.
Electricity bill: Calculated using power in watts and time in hours.
A student walks 120 metres in 60 seconds. What is her average speed?
Step-by-step:
Speed = Distance ÷ Time
= 120 m ÷ 60 s
= 2 m/s
✔️ Answer: Her average speed is 2 m/s.
A box has a mass of 8 kg and occupies 0.02 m³. What is its density?
Step-by-step:
Density = Mass ÷ Volume
= 8 kg ÷ 0.02 m³
= 400 kg/m³
✔️ Answer: The density is 400 kg/m³
(a) The SI unit of mass is __________.
(b) Density is measured in __________.
(c) Speed is a __________ quantity.
Answers:
(a) kilogram
(b) kg/m³
(c) derived
| Quantity | SI Unit |
|---|---|
| (i) Force | A. J |
| (ii) Work | B. N |
| (iii) Power | C. W |
Answers:
(i) → B
(ii) → A
(iii) → C
Question: Why do scientists all over the world use the SI system instead of local units?
Sample Answer:
To avoid confusion, ensure accurate communication, and allow international cooperation in experiments and engineering.
Basic quantities like mass, length, time, and temperature have standard SI units.
Derived quantities are combinations of basic quantities (e.g., speed, density, power).
SI units help maintain accuracy, clarity, and uniformity in science worldwide.
Look around you! Choose any 3 items—your phone, water bottle, schoolbag—and list what quantities you’d need to measure them (e.g., mass, length, volume).
What are the correct SI units for those quantities?
Write your answers in your science journal or discuss them in a group.
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