so there are seven important base SI units the unit for length or distance is meters abbreviated lowercase m for Mass it's kilograms or lowercase kg for time it's seconds or lowercase s for temperature we use Kelvin abbreviated capital K for amount of substance we use moles or mol for short for electrical current we use amperes commonly referred to as amps also abbreviated as capital a and for light intensity we use Candelas abbreviated CD so these are the base units and just like we mentioned before you can combine these base SI units to form other SI units for example the SI unit for speed or velocity is meters per second combining the units for length and time the SI unit for volume is cubic meters for density it's kilograms per cubic meter and for Newtons which is the SI unit for force we use kilogram meters per second squared so we'll be using a lot of different units in physics but you'll find that pretty much all of them are made by combining the same three or four base SI units