Measurement & Units

Science begins with measurement. Every experiment and conclusion in physics depends on accurate measurements in standard units. Let us learn the language of measurement that scientists around the world use.

SI Units (International System)

The SI system has seven base units:

| Quantity | SI Unit | Symbol | |----------|---------|--------| | Length | metre | m | | Mass | kilogram | kg | | Time | second | s | | Temperature | kelvin | K | | Electric current | ampere | A | | Luminous intensity | candela | cd | | Amount of substance | mole | mol |

Derived units combine base units: speed = m/s, force = Newton (kg m/s^2), energy = Joule (kg m^2/s^2).

Unit Conversions and Prefixes

Common Prefixes: kilo (k) = 1,000; centi (c) = 0.01; milli (m) = 0.001; micro = 0.000001; mega (M) = 1,000,000

Example: 5.2 km = 5.2 x 1,000 = 5,200 m

Example: 350 g = 350/1,000 = 0.35 kg

Significant Figures

Significant figures tell you how precise a measurement is.

Rules:

1. All non-zero digits are significant: 245 has 3
2. Zeros between non-zero digits are significant: 305 has 3
3. Leading zeros are NOT significant: 0.0042 has 2
4. Trailing zeros after a decimal ARE significant: 2.50 has 3

Errors in Measurement

• Absolute Error = |Measured value - True value|
• Percentage Error = (Absolute Error / True Value) x 100%

Example: A student measures a table as 1.52 m, but actual length is 1.50 m. Absolute error = 0.02 m. Percentage error = (0.02/1.50) x 100% = 1.33%

Did You Know? Nepal adopted the metric system officially, but traditional units like dharni (about 2.39 kg) and mana are still used in many bazaars. Converting between systems is a practical skill!

Key Takeaways

• The SI system provides standard units for all scientific measurements
• Learn the prefixes (kilo, centi, milli) for quick conversions
• Significant figures reflect measurement precision
• Always report percentage error when evaluating accuracy