Units

Units are the standard references we use to measure physical quantities.

When we say something is "5 meters long," the meter is our unit that gives meaning to the number 5. Without units, numbers alone cannot describe the physical world. There's a big difference between traveling at $v = 100$ miles per hour and $v = 100$ kilometers per hour!

The International System of Units (SI)

The SI system provides a globally standardized set of units that scientists and engineers use to ensure clear communication. It's built on seven fundamental base units from which all other units can be derived.

The three most common SI base units in physics are:

• Meter (m) for length
• Kilogram (kg) for mass
• Second (s) for time

The other four SI base units are:

• Ampere (A) for electric current
• Kelvin (K) for temperature
• Mole (mol) for amount of substance
• Candela (cd) for luminous intensity

Modern Definitions

Historically, units were arbitrary and varied between regions. Today, SI units are precisely defined using fundamental physical constants.

For example, the meter is now defined as the distance light travels in a vacuum in exactly $1/299{,}792{,}458$ of a second. This ensures that anyone, anywhere can reproduce these standards with sufficient precision. Even aliens!

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