In this video we will talk about fundamental and derived SI units.
We will see the reasons for using SI units, discuss the seven fundamental SI units, learn
about derived SI units, and finish with an example about finding the fundamental SI unit
combination of derived SI units.
Let's see what SI units are and why most of the world uses them.
SI is an abbreviation that comes from French and it stands for International System of
Units.
Before the French Revolution there were over 250,000 units used just in France.
After the Revolution the Academy of Sciences created the metric system in order to unify
and simplify the use of units.
The International System of Units grew out of this attempt.
So simply put, it allowed scientists to use a single system when communicating.
There are seven fundamental SI units.
These are the meter, the kilogram, the second, the ampere, the kelvin, the mole, and the
candela.
Only the first six are part of the IB physics syllabus.
Outside these seven units, all other SI units are called derived SI units.
This is simply because they are derived using one of the seven fundamental units.
An example of a derived quantity is speed.
Speed is equal to change in distance over change in time.
Note that V stands for speed, S for distance, and T for time.
The small triangle that you see in front of S and T is the Greek letter delta and means
change.
Distance is in meters, which is a fundamental SI unit.
Time is in seconds, which is also a fundamental SI unit, so speed will be meters per second,
which in physics we usually write as m, S to the power of negative one.
You probably remember from math that the power of negative one basically means division,
so S to the power of negative one means divided by S.
This is why m, S to the power of negative one, stands for meters per second.
Finally, let's see an example of finding the fundamental SI unit combination of derived
SI units.
We will find the unit of force expressed in fundamental SI units.
Force is equal to mass times acceleration.
Mass is a fundamental SI unit, however, acceleration is not.
Acceleration is equal to change in speed over change in time, so we can write m times delta
v over delta t.
Time t is a fundamental SI quantity, however, v speed is not.
Speed is equal to change in distance over change in time, so we can write m times and
as you see here I have replaced delta v by delta S over delta t, so we get delta S over
delta t divided by another delta t.
After combining the delta t's in the denominator, we get m times delta S over delta t squared.
S distance is a fundamental SI quantity, so now our equation only has fundamental SI quantities.
All we have left is to write out the units of these quantities.
M is mass and it is in kilograms, S is distance, it is in meters, and t is in seconds and since
delta t is in the denominator and is to the power of 2, we get S to the power of negative
2.
So the fundamental SI unit combination of force is kilograms times meters times seconds to
the power of negative 2, or in other words kilograms times meters per second squared.
Let's summarize what we've covered in this video.
We learned a bit about the history of the international system of units and that it
allowed scientists to use a single system when communicating.
We enlisted the seven fundamental SI units, meter, kilogram, second, ampere, kelvin, mole,
and candela.
A reminder that only the first six fundamental SI units are part of the IB physics syllabus.
You will not work with the candela in this course.
We discussed derived SI units, which are basically all other units outside the seven fundamental
SI units.
And finally, through our example, we have seen that derived SI units can be written
as a combination of fundamental SI units.
This wraps up our discussion of fundamental and derived SI units.
In the next video, we will learn about scientific notation and metric multipliers.