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Overview
Units are the measurement standards used in physics. A value such as 10, 25, or 100 has no physical meaning until a unit identifies what is being measured. Ten meters, ten seconds, and ten kilograms describe very different quantities.
Physics uses units to keep measurements clear and calculations valid. When units are handled correctly, they help reveal whether an equation makes sense and whether a result is physically reasonable.
The SI System
The International System of Units, often called SI, is the standard measurement system used in most scientific work. SI provides base units for fundamental quantities and derived units for quantities built from them.
Common SI base units include the meter for length, kilogram for mass, second for time, ampere for electric current, kelvin for temperature, mole for amount of substance, and candela for luminous intensity.
Derived Units
Many physics units are derived from combinations of base units. Speed uses meters per second. Acceleration uses meters per second squared. Force uses the newton, which is equivalent to a kilogram meter per second squared. Energy uses the joule, which is equivalent to a newton meter.
Derived units make physics more compact, but the underlying base units remain important because they reveal the structure of the quantity.
Unit Consistency
An equation must be unit-consistent. The units on one side of an equation must match the units on the other side. This does not prove that the equation is physically correct, but it can reveal many errors.
For example, a calculation intended to produce a distance should not produce a value with units of seconds or kilograms. Unit checking is one of the simplest ways to catch mistakes.
Metric Prefixes
Metric prefixes are used to express very large or very small quantities without writing many zeros. Common prefixes include kilo for one thousand, centi for one hundredth, milli for one thousandth, micro for one millionth, and mega for one million.
Prefixes are part of the unit. One kilometer is not the same as one meter. One millisecond is not the same as one second. Correct prefix handling is essential in calculations.
Unit Conversion
Unit conversion changes a measurement from one unit to another without changing the physical quantity. A length can be written in meters, centimeters, or kilometers as long as the conversion is correct.
Good unit conversion treats conversion factors as ratios equal to one. This allows unwanted units to cancel and desired units to remain.