U.S. customary units: fluid volume | 4th grade | Khan Academy
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Let’s do a survey about
how we would measure the volume of fluids
under US customary units. So the smallest volume of
fluid that you’ll hear people talk about– and this will
often be in cooking recipes or something like this–
you’ll talk about a teaspoon. And most of us have
teaspoons that are roughly the size of a teaspoon in
our cupboards someplace. So this recipe might call
for a teaspoon of sugar, or a teaspoon of salt,
or a teaspoon of oil. And you’ve seen
what it looks like. But those are the
smaller spoons that you might have in your cabinets
in your kitchen at home. So this might be a
teaspoon right over here. Now, if you were to take
3 teaspoons together, you have something else
that you would probably have in your cabinets. So if we multiply
this volume, so let’s say this right
over here is a teaspoon. This right over here is a
teaspoon of some substance. If you multiply that by 3,
then you get to the tablespoon. So 3 teaspoons
equal 1 tablespoon. So a tablespoon’s going
to be a little bit bigger. So a tablespoon
might look like this. These tend to be about the
size of the larger spoons that you have in your
cupboard, so a tablespoon, just like that. So if you have 3 times the
fluid, you get to a tablespoon. Now, if you take 2
tablespoons, put them together, then you get to the ounce. And I have to be careful here. You get to the fluid ounce. And the US customary units, they
aren’t designed to be super, super clear. Because you also have the
ounce as a measure of weight. You have the ounce that
is a measure of weight, which is equal to
1/16 of a pound. And now you have a fluid
ounce, which you could either view as 2 tablespoons or,
as we’ll see, 1/8 of a cup. Now, you might say, well, why
are they both called ounces? What’s the relationship
between the two? Well, there is somewhat of a
relationship between the two. If you took a little bit
over an ounce of water, so a weight of ounce
of water, slightly over an ounce of
water, that volume is going to be
about a fluid ounce. An ounce of water in
weight and a fluid ounce of water in volume are
very, very, very close, although they aren’t
exactly the same thing. Now, if you think about,
what would you measure here? We already talked about recipes,
and teaspoon, tablespoon, fluid ounce. You might be thinking
about how much medicine maybe someone might take. Maybe they need to take
2 tablespoons, which would be equivalent
to a fluid ounce. Now, if you take 8 fluid ounces
and put them together– so let me draw a fluid ounce here
just so we still have drawings. So you could imagine
some medicines have a little cap on the top that
you could put the medicine in. So if you do 2
tablespoons in it, maybe that will
be a fluid ounce. Now, if you take 8 fluid
ounces then you get to a cup. And many of us have
this in our kitchens. We have a measuring cup that
will measure exactly a cup. And you might have a
recipe for pancakes that say, hey, put a
cup of flour in there. And also a lot of the cups
that you have in your house might be around might be
around the size of a cup. If you look at,
say, a can of soda that you’re probably
familiar with, a can of soda, the typical can of soda,
is 12 ounces, not 8 ounces. So a typical can of soda
is a cup and a half. We see that a cup
is 8 fluid ounces. A typical can of soda
is 12 fluid ounces. So it is equivalent
to a cup and a half. Let me make sure this looks
like a can of some kind. So if this was 12 ounces,
this is 1 and 1/2 cups. But it gets you a sense of
how much fluid volume a cup actually is. Now, if we were to take 2 cups,
now you’re dealing with a pint. And so you might
have seen pints. Sometimes they’re in
these small cartons. So a pint might look something
like this in a carton. That looks more like a house. But I think, hopefully,
you get the picture that this is intended to
be a carton of some kind. So you have something like that. And so the pint, it’s 2 cups. And so let’s say this is
the fluid inside of it. Let’s say this was transparent,
and you might see it there. Or if you have a
very large mug, that might be the size
of a pint, so if you have a very large mug like this. So my best attempt at
drawing a large mug, this might be roughly
equal to a pint. So let me put some
fluid in here. So there you go. There’s my large mug. It’s got a pint. Now, if you take 2 pints, now
you are dealing with a quart. And you might have
found yourself going to your local
convenience store and buying a quart of milk. A quart of milk, those are kind
of those longer but still kind of skinny looking– so it
might look something like this. I’m trying to draw a
carton of a quart of milk. And then finally, if you were
to take 4 quarts together, you get to a gallon. So you take 4 quarts
together– so times 4– you get to a gallon. So we’re most familiar
with a gallon of milk. So let me see if I can
draw a gallon of milk here, my best attempt. Well, I’m sure someone could
draw a better gallon of milk here. But at least in the
US, it tends to be one of the most typical
ways that they sell milk. And it has, oftentimes, a
little red or orange thing right on top there,
so a gallon of milk. So just to review things,
right here, I just multiplied from the smaller
unit to the larger one. But if you want to think about
everything in terms of ounces, that’s one way to
think about it. A cup right over here
is equal to 8 ounces. 8 ounces is equal to a cup. If you multiply that by 2,
then you get to 16 ounces is equal to a pint. If you multiply that by
2, you get 32 ounces. 32 ounces is equal to a quart. And if you multiply
that times 4, you get 30 times
4 is 120, plus 8. You get 128 ounces per gallon. And so next time you go buy
something in the supermarket, I encourage you to look at
how it’s being measured. And you’ll see that oftentimes
you might buy a quart of milk, but they’ll also say that
this is a 32-ounce container. And in this context, they’re
talking about fluid ounces.

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