Name: Unit Conversion & Significant Figures: Crash Course Chemistry #2
Uploaded: 2022-06-05T10:23:57.000Z
Duration: 11 min 24 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

2640 lumens. 1 foot. 2.3 kilograms. 9 volts. Aaah!

Adverbs of degree

I just closed the circuit with my tongue and I felt all 9 of the volts.

So what do all these things have in common?

They're units. Yes, but they're also absolutely, completely arbitrary.

You know who decides how much a kilogram weighs?

A hunk of platinum and iridium known as the International Prototype Kilogram or IPK.

The IPK isn't just how much a kilogram weighs. In a very real sense the IPK is the kilogram.

Every other kilogram is exactly the same as the IPK,

and the IPK is the lump of metal that decides what that mass is.

A kilogram is defined as being the same mass as the IPK.

We made kilograms up just like we made up seconds and weeks and volts and newtons.

There's nothing about these things that makes them them.

Someone just decided one day that that was a kilogram.

Now the fact that I find units fascinating probably says more about me then it does about units,

For example, did you know that the International System of Units only includes seven base units

and every other unit is derived from those units?

Acceleration is speed divided by time again, so meters per second per second.

Force is that acceleration multiplied by mass, cause F=ma remember?

Work done in joules is force multiplied by distance.

And power is work divided by time, so how much work can be done per unit of time. Makes sense.

It goes pretty deep, and it's absolutely correct to say that there are an infinite number of possible derived units,

just most of them aren't useful enough to name.

But here's a bit of trivia for you. When I say watts or hertz, those things are just regular words.

But Hertz and Watt, they were real people with like last names that were capitalized.

So what's up with that? Well, getting a unit named after you is kind of the holy grail of science.

"True greatness is when your name - like hertz and watt - is spelled with a lowercase letter."

Of course when these geniuses were first piecing together how the world works

they had no idea that there were fundamental basic units beneath it all.

They were basing all of their units on arbitrary values because, well,

how could there possibly be a fundamental amount of mass or distance.

Interestingly, one of the standard base units is derived from an actual value though not a universal one.

The second is 1/60th of 1/60th of 1/24th of the time it takes for the Earth to rotate a single time.

That's something, at least but it also illustrates an interesting point.

As fundamental as that seems, when you get down to the dirty details things start to get kind of cloudy.

The Earth's rotation for example is slowing down.

Does that mean that seconds should also slow down?

No. That would mess up every calculation ever.

So seconds are slowly becoming less and less based on reality.

Now don't worry. It's gonna take forever for the Earth to slow down noticeably.

And when it does we'll just keep adding leap seconds to keep things balanced.

But units are extremely important in chemistry and in sciences in general,

as we learned when the Mars Climate Orbiter crashed into Mars

because instructions were inputted in the wrong units.

Next time you get a B instead of an A because you didn't keep track of your units,

just remember at least you didn't destroy a 300 million dollar mission to Mars.

But what do I mean when I say keep track of your units?

Well. I mean watch them. Do not let them do anything you didn't tell them to do because they're sneaky.

And a lot of chemistry is just converting between units.

So say you are in a car, and the car is going 60 miles per hour.

Now right now everyone who doesn't live in America is like:

"Boo, miles are terrible. Convert to kilometers Hank!"

Well I'll do you one better. From a scientific perspective, kilometers are terrible too.

They're just as arbitrary. We should use something more universal.

Like lightyears. The amount of distance light can travel in a year. And hours, hours is no fun.

So let's convert to lightyears per second. 60 miles per hour.

When you say it it sounds like a whole number with a single unit.

But it's not. It's actually a fraction. 60 miles over 1 hour.

Let's start with the easy part. Getting to the seconds.

So first we've got to get to minutes. So there's 60 minutes per hour. And also 1 hour per 60 minutes.

That fraction once we have it can flip either way.

We want it with the hours on the top, on the numerator. Why?

Because we want the units to cancel. We want to destroy the hours.

We don't want them in our units when we're done.

And then the same thing happens again with 1 minute per 60 seconds. Now we go to lightyears.

I asked Google, and there's 1 light-year in every 5.9 * 10^12 miles.

Looking at this we see that the hours cancel and the minutes cancel and the miles cancel.

Leaving us with lightyears per second. That's really what matters.

The rest is just hammering at the calculator to discover that a car going 60 mph is also going

Now we perform an important test. The "does this make sense?" test.

And yes indeed it does because 9.3 * 10^-12 is a very, very, very, very small number.

Which makes sense because when you're traveling in a car you're going

a very, very, very, very, very, very, very tiny fraction of a light-year every second.

Now there are probably gonna be fifty to a hundred thousand people that watch this video.

And I'm gonna guess that maybe a solid seven of you did the math along with me with your calculator out.

Now I'm not giving you a hard time. That's just my guess.

If you want to follow along with your calculator in the future that might be helpful.

But if you have been following along with your calculator, you might maybe have noticed something interesting.

I said 9.3 * 10^-12. When your calculator...

Your calculator probably said something like 9.3487658140029 * 10^-12.

So why, when I had so many more numbers to give, did I only give two? Was I trying to save time?

Well obviously not, because now I appear to be wasting time talking about it.

Do you think that it would be too hard for me to remember all those numbers?

Well obviously not, because I just did it. So I will tell you why.

When you're doing experimental calculations, there's two kinds of numbers. There's exact and measured.

Exact numbers are like the number of seconds in a minute or the number of eggs in a dozen.

They're defined that way and thus we know them in effect all the way out to an infinite number of decimal places.

If I say that there are a dozen eggs you know that that's 12. It's not 12.0000000001

But that's not true for the number of miles per hour my car was going.

That car wasn't going 60.0000-out into infinity mph.

I only know the speed of my car to two decimal places because that's all I get from the speedometer.

So the car could have been going 59.87390039 mph or 60.49321289 mph; the speedometer would still say 60.

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Unit Conversion & Significant Figures: Crash Course Chemistry #2

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