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Overview
Measurement is one of the foundations of physics. A measurement tells how much of a physical quantity is present. Instead of saying an object is heavy, physics asks for its mass or weight using a defined unit. Instead of saying an event was fast, physics asks for speed, velocity, acceleration, time interval, or another measurable quantity.
Every useful measurement includes both a numerical value and a unit. The number alone is incomplete because it does not identify what kind of quantity was measured or how the value should be interpreted.
Physical Quantities
A physical quantity is a property of a system that can be measured or calculated. Common physical quantities include length, mass, time, temperature, electric current, force, energy, pressure, volume, speed, acceleration, and power.
Some quantities are measured directly with instruments. Others are calculated from measured values. For example, speed can be calculated from distance and time, while density can be calculated from mass and volume.
Measurement and Units
A unit provides the standard used to interpret a measurement. If the length of a table is recorded as 2, the measurement is incomplete. If the length is recorded as 2 meters, the measurement becomes meaningful.
Units also allow measurements from different people, places, instruments, and experiments to be compared. Without units, scientific communication becomes vague and unreliable.
Measurement Instruments
Measurement requires instruments suited to the quantity being measured. A ruler measures length, a stopwatch measures time, a balance measures mass, a thermometer measures temperature, a voltmeter measures voltage, and a pressure gauge measures pressure.
Instruments have limits. A measurement is only as useful as the instrument, procedure, calibration, and observer allow. Good measurement practice includes choosing the proper instrument, reading it carefully, recording the unit, and understanding its precision.
Precision and Accuracy
Accuracy describes how close a measurement is to the true or accepted value. Precision describes how closely repeated measurements agree with one another. A set of measurements can be precise without being accurate if the instrument is consistently wrong.
Physics depends on both concepts. Accurate measurements help describe reality correctly, while precise measurements help identify patterns and reduce random variation.
Measurement Uncertainty
No measurement is perfect. Every measurement has some uncertainty due to instrument resolution, calibration, environmental conditions, human reading limits, or natural variation in the system.
Uncertainty is not a failure of science. It is part of honest measurement. A useful measurement makes its limits visible instead of hiding them.