A friend texts you directions: 'Walk 400, then wait 5.' 400 what? 5 what? A number by itself says nothing about the world.
Every measurement is a number plus a unit. And there are a lot of units to choose from.
What's the distance to the next town? A distance is a length, and you could measure a length in miles, kilometers, centimeters, or meters. Physicists pick one. For length, it is the meter, written m.
Time is the same story. A race could be timed in hours, minutes, or seconds. Physicists use the second, written s.
Mass too. A bag of flour could be measured in pounds, ounces, or kilograms. Physicists use the kilogram, written kg.
These agreed choices are called the SI base units. SI is short for the International System of units.
Which is the SI base unit for mass?
Motion problems in this course state lengths in meters, times in seconds, and masses in kilograms unless a question says otherwise.
A timekeeper records how long a crosswalk signal stays green. Which SI base unit does the record use?
Which pairing of quantity and SI base unit is correct?
A student's lab record for a cart run reads: distance 2.4 ___, time 3.0 ___, mass 1.5 ___. Which set of SI base units completes the record, in order?
A lab report states a backpack's mass as 3.5 s. What is wrong with the statement?
Three measurements are written on a whiteboard: 7.5 kg, 7.5 m, 7.5 s. Which one could be the mass of a backpack?