Unit Conversion the Easy Way (Dimensional Analysis)
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Learn Unit Conversion the Easy Way The method that we will be using to convert
between units is known as dimensional analysis or the factor-label method or even the unit-factor
method. But what we call it really doesn’t matter. What matters is the fact that this is a versatile
and powerful problem solving technique. So, let’s just do this. We’re going to start with a simple unit
conversion problem. A weightlifter can lift 495 lbs. How many kilograms is that? In order to solve a unit conversion problem
like this, we first need one more piece of information: the conversion factor. For pounds and kilograms, the conversion factor
is 1 kg equals 2.2 pounds. Now, we’re ready to solve this. The first thing you should always do is write
down the quantity that you want to convert. This is the number from the question, not
the conversion factor. Please also include the units. Next, we are going to multiply this number
by a fraction. Inside the fraction we are going to write
the two numbers from the conversion factor. But, how do we know which one goes on the
top, and which one goes on the bottom? To answer that question, all we need to do
is look at the units, which is why we always include the units in the calculation itself. The quantity we are starting out with has
the units of pounds, so we take 2.2 pounds from the conversion factor and write it on
the bottom. Next, because we want to end up with kilograms,
we take 1 kg from the conversion factor and write it on the top of the fraction. Notice that now, the pounds that we started
out with cancel out with the pounds on the bottom, and the units we have left on top
are kilograms, which is exactly what we want to convert to. The only thing left to do now is plug the
numbers in our calculator. You could, of course, put this in your calculator
exactly the way it appears here…but maybe you don’t have one of those fancy calculators
that can do fractions, or maybe you’re like me, and you just want to find a short cut. Because the number on the top of the fraction
is 1, this becomes a simple division problem. In your calculator, type 495 divided by 2.2,
and your calculator should tell you the answer is 225. Our final answer, therefore, is 225 kg. There is one more thing that we should notice
about this problem. The fraction, 1 kg over 2.2 lbs. actually
equals one because 1 kg equals 2.2 pounds. In fact, any time we do unit conversions,
we are simply multiplying our initial quantity by a conversion factor fraction that equals
one. Okay, now that we are experts at this technique,
let’s try a slightly harder problem. A certain car has a mass of 1920 kg. How many tons is that? Just like always, we need the conversion factor
before we can solve this, but this time we need two conversion factors: one to convert
from kg to lbs., and another to convert from lbs. to tons. So, this is going to be a two step problem. We start the problem by writing down the quantity
from the question, 1920 kilograms, and then we multiply this by a fraction. The two numbers that go in the fraction come
from one of the conversion factors, but what goes on the bottom? Because we are starting with kilograms, we
write 1 kg on the bottom of the fraction so that we can cancel out the kilograms. Next, the other half of that same conversion
factor, 2.2 lbs. has to go on the top. The kilograms cancel out leaving us with pounds
as the units of our answer. When you do the math in your calculator, simply
multiply 1920 by 2.2. This time we are multiplying the numbers because
the 1 of the conversion factor is on the bottom of the fraction. Our calculator tells us that the answer is
4224 pounds…but, we’re not done yet. We still need to convert the pounds to tons. The second step works exactly the same way. First, we write down the number that we want
to convert, that is 4224 pounds, and then we multiply this by a fraction. We want to have pounds in the denominator
of the fraction so that we can cancel out the pounds. But which pounds do we choose? 2.2 pounds or 2000 pounds? Remember that we want to convert to tons,
so choose the conversion factor between pounds and tons. We write 2000 lbs. on the bottom, and 1 ton
on the top. Our pounds cancel out, and we are left with
tons for the units of our answer. In our calculators, we type 4224 divided by
2000 because the one is in the numerator of the fraction. Our final answer works out to be 2.11 tons. If you are following in your calculator and
wondering why I rounded my final answer, the reason is that I should have only 3 significant
figures in my answer because the 1920 I started with has only 3 significant figures. Okay, we got the correct answer, but it turns
out that there is an even better way to solve problems that involve multiple conversion
factors. Rather than solving this in two separate steps,
we can combine those steps into one step with two conversion factors. Check this out. Once again, start the problem by writing down
the quantity that you want to convert. Multiply this by a conversion factor fraction,
putting what you want to cancel out on the bottom and what you want to convert it to
on the top. Notice that so far this is exactly the same
as the first step we just did. However, instead of solving this as it is,
we are going to multiply it by another conversion factor fraction. We now need to cancel out the lbs. that are
left on top, so we put 2000 lbs. on the bottom. We chose the 2000 lbs. rather than the 2.2
lbs. because we ultimately want to convert the quantity to tons. This gives us tons as our remaining units
on top while all the other units cancel out. We then proceed to calculate from left to
right. If the one is on the bottom, we multiply. If the one is on the top, we divide. So, we multiply 1920 by 2.2 and then divide
that answer by 2000. Our final answer is 2.11 tons, which is exactly
what we got the first time. But now we can see how powerful this method
is. No matter how many conversions you need to
do, putting the conversion factors in fraction form helps you to know when to multiply or
divide. Thank you for watching. Please comment, vote, subscribe, or check
me out at ketzbook.com.

100 thoughts on “Unit Conversion the Easy Way (Dimensional Analysis)

  1. How many kilograms of phosphorous are in a sample containing 4.00E+30 phosphorous atoms? Use “E” for “×10" and use significant figures. how would you solve this problem?

  2. 1 tonn is 1000 kilogramms if I am not mistaken…which means that 1920 kilogramms is 1.920 tonn and certainly not 2.11

  3. I just watched 6 different YouTube videos on help with conversion factors and now I can stop searching because you actually explained it thorough and simple way! Thank you!

  4. My chemistry teacher said when we walk into her class room there are no more pounds..deg. F……gallons and so on…..PROBLEM solved…….I hated conversions anyway….I found them useless and confusing

  5. THANK YOU I've been watching Khan Academy and this is much more my speed. Quick explanation and clean visuals! Looking forward to watching more of your vids.

  6. Thank you. You explained this well, another way to rember the unit conversion is
    What comes up must come down, 1920 kg must be below in the converison factor.

  7. doing 2 hour lectures on this topic in college.. But it only took me 5 minutes to actually understand how to do it and why it's important

  8. The only problem I have is I don’t know conversions like kilometers to miles what should the second step be? Miles to meters to kilometers

  9. THANK YOU!!! I am getting ready for nursing school and learning how to do conversions is a big deal. You made it sound so simple!!

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