hello my name is Kaylee and this is a quick lesson on the units used in
physics before we start this lesson it will be helpful if you already
understand some of the equations for types of energy so make sure you've
already watched the equations for friction and thermal energy and the
video equations on kinetic and potential energy in physics and most Sciences
around the world we use the same units of measurement and we call this system
the metric system and sometimes it's called SI units which means system
international units and we use the metric system so that scientists all
around the globe can share their experiments with each other with ease so
when we need some way to quantitatively or numerically understand things that we
can measure we need units and units help us think about measurable like mass and
time and distance and height in a way that we can communicate universally so
for mass we use the unit kilogram when we're using the unit in an equation we
use the letters kg to represent kilogram and a kilogram is about two times
heavier than one pound for time we use the unit seconds and when we are using
the unit in an equation we use the letter s to represent a second and as
you probably already know one minute is made up of 60 seconds and for distance
and height we use the unit meters and when we're using the unit in an equation
we use the letter M to represent meter and one meter is a little bit more than
three feet so one meter is equal to three point two eight feet
we also have calculated units that are made up of our basic units and we use
calculated units for measurables like velocity and acceleration and forces
like friction and the normal force and all the types of energies we've already
talked about and these are called calculated units because they are the
mathematical combination a two or more of our basic units so for velocity we
use the unit's meters per second and we're using the unit in an equation we
use the letters m / s to represent meter per second and one meter per second is a
little over 2 miles per hour for acceleration we use the unit meters per
second squared we're using the unit and equation we use the letters M / S
squared to represent meters per second squared we'll go over acceleration in
more detail in a later video but it's helpful right now to know that
acceleration is how fast the velocity of an object is changing for force we use
the unit Newton when we're using the unit in the equation we use the letter
big n to represent Newton and a Newton is defined as mass times acceleration
so we ever we see kg times M / S squared or kilogram times meter per second
squared we can rewrite those units as a big n as a Newton for energy we use the
unit Joule and when we're using the unit in an equation we use the letter J to
represent Joule the definition of a Joule is Force Times distance so we can
shorten a Newton meter or kilogram times meters per second squared times meter -
a Big M a Joule whenever we see it in our math
all expressions so let's do a little experiment to prove why we use metric
units instead of standard units or the pound the foot the kind of units that
we're used to seeing in when we talk about measurables in the United States
and there's a good mathematical reason for choosing metric units over standard
units so let's use the Joule the metric unit for energy and we'll write it out
in all of its terms so it says a Joule is equal to a Newton meter or force
times distance and remember a Newton is equal to a kilogram times meters per
second squared and then if we times it by a meter we get a Joule again so when
we use the metric system it's really easy to see mathematically how they all
go together because for all of these measurables it's just a factor of 1 so
we could also say one Joule one Newton times one meter or one Joule one
kilogram times one meter per second squared times one meter and that's
really easy math but what happens if we convert those metric units to standard
units so remembering our conversions from earlier in the video 1 kilogram is
equal to two point two four pounds and one meter is equal to three point two
eight feet so let's plug those in so here we have one Joule is equal to two
point two four pounds times three point two eight feet per second squared times
three point two eight feet which is equal to 24 point oh nine pounds times
feet per second squared times feet and all of that equals one Joule so we see
how using our metric system is all ones and it makes that math are really easy
so instead of having to convert everything at the end back to see how
many factors of 24.09 pounds per feet per second squared times feet
we have we just have joules immediately so we don't have to convert it back to
our energy unit so while it may it might help make sense in the
beginning to use pounds and feet because you can think about measurables easier
in those terms in the long run it's going to cause you a lot more confusion
and a lot more mathematical work and not to mention you'll be the only scientist
with data and standard units so from this point point forward in our physics
lessons we will primarily be using metric units and if you ever come across
a question or an assignment where you do have one of those pesky standard units
make sure you convert it to a metric unit before plugging into your equation