Measurement and Units in Physics

Why Measurement Matters

Physics is built on accurate measurement. Every observation in science becomes useful only when it is expressed as a number with a unit, so that other people can repeat the observation and compare results.

Fundamental Quantities and Their SI Units

• Length – metre (m)
• Mass – kilogram (kg)
• Time – second (s)
• Electric current – ampere (A)
• Temperature – kelvin (K)
• Amount of substance – mole (mol)
• Luminous intensity – candela (cd)

Derived Quantities

Derived quantities are obtained by combining fundamental quantities. Examples include area (m²), volume (m³), speed (m/s), density (kg/m³) and force (kg m/s², called the newton).

Instruments Used for Measurement

• Metre rule and tape – measuring length
• Vernier calipers – measuring small lengths and diameters accurately
• Micrometer screw gauge – measuring very small thicknesses
• Beam balance – measuring mass
• Stopwatch – measuring time
• Measuring cylinder – measuring volume of liquids

Errors in Measurement

No measurement is perfectly exact. Errors may be systematic (caused by a faulty instrument or wrong technique that affects every reading in the same way) or random (small, unpredictable variations from one reading to the next). Taking several readings and finding the average helps to reduce random error.

Summary

Measurement gives physics its precision. Knowing the correct SI unit and the right instrument for a quantity is the first step to solving any physics problem correctly.