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### Course: High school geometry > Unit 9

Lesson 3: Volume and surface area# Applying volume of solids

Calculate the volume of a grain hopper and then use the given rate to solve an applied problem. Created by Sal Khan.

## Want to join the conversation?

- Can't we also find the radius of the smaller cone by using the fact that the cones are similar? That is, if they are similar? Instead of taking a cross section and proving that the triangles in the cross section are similar... Is there any situation where this won't work?(10 votes)
- Sal does that at2:51when he sets up the equation r/10=6/8 to find r= 7.5. Since it is a single cone, each measure will be proportional(13 votes)

- when u have a can of beans and the height is 11 and the rades is 12 what will be an answer. my mom really needs to know because we r going to be using some beans tonight with dinner. yummmmmmmmmmmmmmy. also we r having hot dog wennies but I will save that problem for later.(5 votes)
- What can of beans has a diameter that is more than twice the height? V=πr^2 h(7 votes)

- At about3:50in the video, Sal multiplies 6 by 1/3. Wouldn't he have to do the same for the 7.5? Why doesn't he?

At5:58, he says that dividing by 1/10 is the same as multiplying by 10. How is this possible? Also,*why*does he multiply by 10?(1 vote)- Multiplication is commutative, so we can move things around to make it easy to multiply. So 1/3*(7.5)^2*6 can be commuted to (7.5)^2 * 1/3 * 6. You are trying to apply the distributive property which is not the same, you are trying to do 1/3((7.5)^2 + 6) where both terms would be multiplied by 1/3, but the problem has a single term.

As to the second issue, dividing by a tenth can be shown in two ways, 353.4/.1, multiplying both numerator and denominator by 10 (10/10=1) gives 3534/1, and dividing by 1 does not change a number, so it is not needed. The second way is 353.4/(1/10). To divide a whole number by a fraction, we reciprocate the fraction and multiply which gives 353.4*(10/1)=3534.(6 votes)

- At2:08, what triangles is he talking about

?(2 votes)- he's talking about the triangles on the cone, basically, he cut a image of a cone into two right similar triangles(3 votes)

- can someone tell me why he puts the r on top for the proportion @2:44?

why doesn't it go on the bottom?

I figure he's comparing the r to 10 so r:10 or r/10, but I need a deeper understanding as to why ?

Also, trying to conceptiualize if the ratio was 10/r in this problem @2:44how would the problem , the triangles, the question being asked, and answer differ?

not sure what im missing here, thanks(3 votes)- i think his decision to put the r on top was mostly arbitrary

if the ratio was 10/r, then it would just be set up as 10/r = 8/6 instead of r/10 = 6/8. it would still come out to 7.5 and the rest of the problem would be the same

hope this helps!(1 vote)

- from my proportion I think of it like 10/8=r/6 or 8/10=6/r am I correct in thinking of it this way?

is it always short side over long side when making a proportion for similar triangles ? so if 10 was the radius of the small triangle the 10 would be on top ??(2 votes)- yes, both ways you listed are correct

10/8=r/6, 8/10=6/r, and r/10=6/8 (what sal does in the video) are all valid ways of setting up this proportion. if you cross multiply & simplify, you should get the same answer (7.5) for any of these 3 ways.

there is no set rule for what to put on the top or bottom, as long as the whole thing is proportional. just pick whichever works best for you.

hope this helps :)(1 vote)

- how do you calculate the volume of a tough figure(2 votes)
- Hello, is it possible when calculating the volume of grain
*inside*the cone, to calculate the volume the cone holds in total and multiplying by 6/8 or 3/4 the portion of the cone filled up with grain? It's at2:14that he announces the similar triangle method, I just wanted to know if the method I explained above worked as well.(1 vote)- You are partially correct. Similar volumes can be determined through ratio of corresponding edges. However, multiplying by (6 / 8) isn't correct.

Note the following formula (It can be helpful sometimes)

For**2D**similar polygon area A_p and A_q with corresponding edges p and q.`A_p / A_q = (p / q)²`

For**3D**similar polyhedral volume V_p and V_q with corresponding edges p and q.`V_p / V_q = (p / q)³`

Now, lets try using this formula!

Volume of cone = (1 / 3) * 10² * 8 * π = 837.8 m³

Let the volume of grain be V.

V / 837.8 = (6 / 8)³

V = 353.4 m³(1 vote)

## Video transcript

- [Instructor] We're told that
a cone-shaped grain hopper, and they highlight hopper in
blue here, in case you want to know its definition on the exercise. It's something that would store grain, and then it can kind
of fall out the bottom, has a radius of 10 meters at
the top and is 8 meters tall. So let's draw that. So it's cone-shaped, and
it has a radius at the top, so the top must be where the base is. I guess one would think about it. It must be the wider part of the cone. So it looks like this,
something like that. That's what this first sentence tells us. It has a radius of 10
meters, so this distance right over here is 10 meters,
and the height is 8 meters. They say it's 8 meters tall, so this right over here is 8 meters. Then they tell us it is filled up to 2 meters from the top with grain. So one way to think about it,
it's filled about this high with grain, so it's filled
about that high with grain, so this distance is going
to be 8 minus these 2, so this is going to be 6 meters high. That's what that second sentence tells us. The hopper will pour the grain
into boxes with dimensions of 0.5 meters by 0.5 meters by 0.4 meters. The hopper pours grain at a rate of 8 cubic meters per minute. So, a lot of information there. The first question is what is the volume of grain in the hopper? Before we even get to
these other questions, let's see if we can answer that. So that's going to be this
volume right over here, of the red part, the cone
made up of the grain. Pause this video and try to figure it out. Well, from previous videos, we know that volume of
a cone is going to be 1/3 times the area of the
base, times the height. Now we know the height is
6 meters, but what we need to do is figure out the area of the base. Well, how do we do that? Well, we'd have to figure
out the radius of the base. Let's call that r, right over here. And how do we figure that out? Well, we can look at these two triangles that you can see on my screen, and realize that they
are similar triangles. This line is parallel to that line. This is a right angle. This is a right angle,
because both of these cuts of these surfaces are going
to be parallel to the ground, and then this angle's going
to be converted to this angle, 'cause you could view
this line as a transversal between parallel lines, and
these are corresponding angles. And then both triangles share this. So you have angle, angle, angle. These are similar triangles, and so we can set up a proportion here. We can say the ratio
between r and 10 meters, the ratio of r to 10, is
equal to the ratio of 6 to 8. Is equal to the ratio of 6 to 8, and then we could try to solve for r. R is going to be equal
to, r is equal to... Multiply both sides by 10. Multiply both sides by 10, and you're going to get 60 over 8. 60 over 8, 8 goes into 60
seven times with 4 left over. So it's 7 4/8, or it's also 7.5, and so if you want to know the area of the base right over here, if you wanted to know this b, it would be pi times the radius squared, so b in this case, is
going to be pi times 7.5, we're dealing with meters squared, and so the volume, to
answer the first question, the volume's going to be 1/3
times the area of the base, this area up here, which is
pi times 7.5 meters squared, times the height, so times 6 meters. And let's see, we could
simplify this a little bit. 6 divided by 3, or 6 times 1/3, is just going to be equal to 2, and so let me get my calculator. They say round to nearest
tenth of a cubic meter. So we have 7.5 squared times 2, times pi, is equal to, well if we
round to the nearest tenth, it's gonna be 353.4 cubic meters. So the volume is approximately
353.4 cubic meters. So that's the answer to the
first part right over there, and then they say how many complete boxes will the grain fill? Well, they talk about the
boxes right over here. The hopper will pour the grain into boxes with dimensions of 0.5 meters
by 0.5 meters by 0.4 meters, so we can imagine these boxes. They look like this, and they are 0.5 meters by
0.5 meters by 0.4 meters, so the volume of each
box is just going to be the product of these three numbers. So the volume of each box is going to be the width times the
depth times the height, so 0.5 meters times 0.5
meters times 0.4 meters, and we should be able
to do this in our head, because 5 times 5 is 25. 25 times 4 is equal to 100,
but then we have to think we have one, two, three
digits to the right of the decimal point, so one, two, three. So this is going to be a
tenth, 0.100 cubic meters, so a tenth of a cubic meter. So how many tenths of
cubic meters can I fill up with this much grain? Well, it's just going to be this number divided by a tenth. Well, if you divide by a tenth, that's the same thing
as multiplying by 10, and so if you multiply this by 10, you're going to get 3,534 boxes. Now once again, let's just appreciate. Every cubic meter, you can
fill 10 of these boxes, and this is how many cubic meters we have, so if you multiply this by 10, it tells you how many boxes you fill up, and one way to think about it, we've seen this in other videos, we're shifting the decimal
one place over to the right to get this many boxes, and,
it's important to realize, complete boxes, because
when we got to 353.4, we did round down, so
we do have that amount, but we're not going to fill up another box with whatever this rounding error, that we rounded down from. So the last question is,
to the nearest minute, how long does it take to fill the boxes? Well, this is the total volume, and we're going to fill 8
cubic meters per minute, so the answer over here is
going to be our total volume, it's going to be 353.4 cubic meters, and we're going to
divide that by our rate, 8 cubic meters per minute, and that is going to give
us 353.4 divided by 8, is equal to, and if we want to round to the nearest minute, 44 minutes. Is equal to approximately 44 minutes to fill all of the boxes, and we're done.