Measurement
What is a unit?
A standard chunk you compare other things against — and why standards matter.
A unit is a standard amount we compare other quantities against. Without standard units, no number means anything.
Quick check
Why do we need standard units?
From king's feet to constants of nature
Early units were body-based — a 'foot', a 'cubit' — so they varied by person. Modern SI units are pinned to unchanging physical constants (the speed of light, the caesium atom's vibrations), so a metre is a metre forever.
Your turn
Your friend says 'it's 50 away'. Why is that useless?
Recap
- A unit is a standard amount other quantities are compared against.
- Modern units are defined from universal constants, not physical objects.
- Always write the unit next to the number.
The metre is now defined as the distance light travels in 1/299,792,458 of a second — derived from a universal constant.