Common Core: 8th Grade Math : Know and Use the Formulas for the Volumes of Cones, Cylinders, and Spheres: CCSS.Math.Content.8.G.C.9

Study concepts, example questions & explanations for Common Core: 8th Grade Math

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Example Questions

Example Question #1 : Volume Of A Three Dimensional Figure

Chestnut wood has a density of about \(\displaystyle 0.45\frac{g}{cm^3}\). A right circular cone made out of chestnut wood has a height of three meters, and a base with a radius of two meters. What is its mass in kilograms (nearest whole kilogram)?

Possible Answers:

\(\displaystyle 5,955 \; \textrm{kg}\)

\(\displaystyle 5,855 \; \textrm{kg}\)

\(\displaystyle 6,055 \; \textrm{kg}\)

\(\displaystyle 5,755 \; \textrm{kg}\)

\(\displaystyle 5,655 \; \textrm{kg}\)

Correct answer:

\(\displaystyle 5,655 \; \textrm{kg}\)

Explanation:

First, convert the dimensions to cubic centimeters by multiplying by \(\displaystyle 100\): the cone has height \(\displaystyle 300cm\), and its base has radius \(\displaystyle 200cm\).

\(\displaystyle 3m*\frac{100cm}{1m}=300cm\ \ \2m*\frac{100cm}{1m}=200cm\)

Its volume is found by using the formula and the converted height and radius.

\(\displaystyle V=\frac{1}{3} \pi r^{2}h\)

\(\displaystyle V=\frac{1}{3} \pi (200cm)^{2} ( 300cm )\approx 12,566,371 \; \textrm{cm}^{3}\)

Now multiply this by \(\displaystyle 0.45\frac{g}{cm^3}\) to get the mass.

\(\displaystyle m=V*\rho\)

\(\displaystyle 12,566,371cm^3*0.45\frac{g}{cm^3} \approx 5,654,867g\)

Finally, convert the answer to kilograms.

\(\displaystyle 5654867g *\frac{1kg}{1,000g} \approx 5,655kg\)

Example Question #3 : Volume Of A Three Dimensional Figure

A cone has the height of 4 meters and the circular base area of 4 square meters. If we want to fill out the cone with water (density = \(\displaystyle 1000\frac{Kg}{m^3}\)), what is the mass of required water (nearest whole kilogram)?

Possible Answers:

\(\displaystyle 4333\)

\(\displaystyle 5333\)

\(\displaystyle 2133\)

\(\displaystyle 10,000\)

6333

Correct answer:

\(\displaystyle 5333\)

Explanation:

The volume of a cone is:

\(\displaystyle Volume=\frac{1}{3}\pi r^2h\)

where \(\displaystyle r\) is the radius of the circular base, and \(\displaystyle h\) is the height (the perpendicular distance from the base to the vertex).

 

As the circular base area is \(\displaystyle \pi r^2\), so we can rewrite the volume formula as follows:

 

\(\displaystyle Volume =\frac{A\times h}{3}\)

 

where \(\displaystyle A\) is the circular base area and known in this problem. So we can write:

 

\(\displaystyle Volume =\frac{A\times h}{3}=\frac{4\times 4}{3}=\frac{16}{3}\approx 5.333 m^3\)

 

We know that density is defined as mass per unit volume or:

 

\(\displaystyle \rho =\frac{m}{v}\)

 

Where \(\displaystyle \rho\) is the density; \(\displaystyle m\) is the mass and \(\displaystyle v\) is the volume. So we get:

 

\(\displaystyle m=\rho v=1000\times 5.333=5333 Kg\)

Example Question #481 : Grade 8

The vertical height (or altitude) of a right cone is \(\displaystyle 2t\). The radius of the circular base of the cone is \(\displaystyle t\). Find the volume of the cone in terms of \(\displaystyle t\).

Possible Answers:

\(\displaystyle \frac{1}{3}\pi t^2\)

\(\displaystyle \frac{2}{3}\pi t^2\)

\(\displaystyle \frac{1}{3}\pi t^3\)

\(\displaystyle \pi t^3\)

\(\displaystyle \frac{2}{3}\pi t^3\)

Correct answer:

\(\displaystyle \frac{2}{3}\pi t^3\)

Explanation:

The volume of a cone is:

\(\displaystyle Volume=\frac{1}3{}\pi r^2h\)

 

where \(\displaystyle r\) is the radius of the circular base, and \(\displaystyle h\) is the height (the perpendicular distance from the base to the vertex).

 

\(\displaystyle Volume=\frac{1}3{}\pi r^2h =\frac{1}3{}\pi t^2\times 2t=\frac{2}{3}\pi t^3\)

Example Question #482 : Grade 8

A right cone has a volume of \(\displaystyle 8\pi\), a height of \(\displaystyle 3t\) and a radius of the circular base of \(\displaystyle 2t\). Find \(\displaystyle t\).

Possible Answers:

\(\displaystyle \sqrt[3]{3}\)

\(\displaystyle 2\)

\(\displaystyle \sqrt[3]{2}\)

\(\displaystyle \sqrt{2}\)

\(\displaystyle 2\sqrt[3]{2}\)

Correct answer:

\(\displaystyle \sqrt[3]{2}\)

Explanation:

The volume of a cone is given by:

\(\displaystyle Volume=\frac{1}{3}\pi r^2h\)

where \(\displaystyle r\)is the radius of the circular base, and \(\displaystyle h\) is the height; the perpendicular distance from the base to the vertex. Substitute the known values in the formula:

 

\(\displaystyle Volume=\frac{1}{3}\pi r^2h\Rightarrow 8\pi=\frac{1}{3}\pi\times (2t)^2\times 3t\)

\(\displaystyle \Rightarrow 8\pi=4\pi t^3\Rightarrow t^3=2\Rightarrow t=\sqrt[3]{2}\)

Example Question #2 : How To Find The Volume Of A Cone

A cone has a diameter of \(\displaystyle 12\:m\) and a height of \(\displaystyle 4\:m\). In cubic meters, what is the volume of this cone?

Possible Answers:

\(\displaystyle 144\pi\:m^3\)

\(\displaystyle 54\pi\:m^3\)

\(\displaystyle 36\pi\:m^3\)

\(\displaystyle 48\pi\:m^3\)

Correct answer:

\(\displaystyle 48\pi\:m^3\)

Explanation:

First, divide the diameter in half to find the radius.

\(\displaystyle \frac{d}{2}=r\)

\(\displaystyle \frac{12}{2}=6\:m\)

Now, use the formula to find the volume of the cone.

\(\displaystyle \text{Volume}=\frac{1}{3}\pi r^2 h\)

\(\displaystyle \text{Volume}=\frac{1}{3}\pi \times 6^2\times4=\frac{1}{3}\pi \times 36\times4=\frac{144}{3}\pi=48\pi\:m^3\)

Example Question #142 : Geometry

The height of a cylinder is 3 inches and the radius of the circular end of the cylinder is 3 inches. Give the volume and surface area of the cylinder.

Possible Answers:

\(\displaystyle V=84.78 in^3, A=103.04in^2\)

\(\displaystyle V=84.78 in^3, A=123.04in^2\)

\(\displaystyle V=84.82 in^3, A=113.10in^2\)

\(\displaystyle V=74.78 in^3, A=103.04in^2\)

\(\displaystyle V=74.78 in^3, A=113.10in^2\)

Correct answer:

\(\displaystyle V=84.82 in^3, A=113.10in^2\)

Explanation:

The volume of a cylinder is found by multiplying the area of one end of the cylinder (base) by its height or:

 \(\displaystyle Volume=\pi r^2h\)

where \(\displaystyle r\) is the radius of the circular end of the cylinder and \(\displaystyle h\) is the height of the cylinder. So we can write:

 

\(\displaystyle Volume=\pi r^2h=\pi \times 3^2\times 3=84.82in^3\)

 

The surface area of the cylinder is given by:

 

\(\displaystyle A=2\pi r^2+2\pi rh\)

 

where \(\displaystyle A\) is the surface area of the cylinder, \(\displaystyle r\) is the radius of the cylinder and \(\displaystyle h\) is the height of the cylinder. So we can write:

\(\displaystyle A=2\pi r^2+2\pi rh\)

\(\displaystyle A=2\pi (3)^2+2\pi \times 3\times 3\)

\(\displaystyle A=18\pi+18\pi\)

\(\displaystyle A=36\pi\)

\(\displaystyle A=113.10\)

Example Question #2 : Know And Use The Formulas For The Volumes Of Cones, Cylinders, And Spheres: Ccss.Math.Content.8.G.C.9

The height of a cylinder is two times the length of the radius of the circular end of a cylinder. If the volume of the cylinder is \(\displaystyle 16\pi\), what is the height of the cylinder?

Possible Answers:

\(\displaystyle 2\)

\(\displaystyle 2\sqrt{2}\)

\(\displaystyle 3\)

\(\displaystyle 4\)

\(\displaystyle 1\)

Correct answer:

\(\displaystyle 4\)

Explanation:

The volume of a cylinder is:

 \(\displaystyle Volume=\pi r^2h\)

where \(\displaystyle r\) is the radius of the circular end of the cylinder and \(\displaystyle h\) is the height of the cylinder.


Since \(\displaystyle h=2r\), we can substitute that into the volume formula. So we can write:

 

\(\displaystyle Volume=\pi r^2h\)

\(\displaystyle 16\pi=\pi r^2\cdot (2r)\)

\(\displaystyle 16\pi =2\pi r^{3}\)

\(\displaystyle 16=2r^{3}\)

\(\displaystyle 8=r^{3}\)

\(\displaystyle r=2\)

So we get:

 

\(\displaystyle h=2r=2\times 2=4\)

Example Question #483 : Grade 8

The end (base) of a cylinder has an area of \(\displaystyle 16\pi\) square inches. If the height of the cylinder is half of the radius of the base of the cylinder, give the volume of the cylinder.

Possible Answers:

\(\displaystyle 8\pi\)

\(\displaystyle 16\pi\)

\(\displaystyle 32\)

\(\displaystyle 32\pi\)

\(\displaystyle 16\)

Correct answer:

\(\displaystyle 32\pi\)

Explanation:

The area of the end (base) of a cylinder is \(\displaystyle \pi r^2\), so we can write:

\(\displaystyle \pi r^2=16\pi\Rightarrow r^2=16\Rightarrow r=4\ inches\)

 

The height of the cylinder is half of the radius of the base of the cylinder, that means:

 

\(\displaystyle h=\frac{r}{2}=\frac{4}{2}=2\ inches\)

 

The volume of a cylinder is found by multiplying the area of one end of the cylinder (base) by its height:

\(\displaystyle Volume=Area\times h=16\pi\times 2=32\pi\)

or

\(\displaystyle Volume=\pi r^2h=\pi\times 4^2\times 2=32\pi\)

Example Question #144 : Geometry

We have two right cylinders. The radius of the base Cylinder 1 is \(\displaystyle \sqrt{3}\) times more than that of Cylinder 2, and the height of Cylinder 2 is 4 times more than the height of Cylinder 1. The volume of Cylinder 1 is what fraction of the volume of Cylinder 2?

Possible Answers:

\(\displaystyle V_{1}=\frac{3}{4}V_{2}\)

\(\displaystyle V_{1}=\frac{4}{3}V_{2}\)

\(\displaystyle V_{1}=\frac{2}{3}V_{2}\)

\(\displaystyle V_{1}=\frac{3}{2}V_{2}\)

\(\displaystyle V_{1}=V_{2}\)

Correct answer:

\(\displaystyle V_{1}=\frac{3}{4}V_{2}\)

Explanation:

The volume of a cylinder is:

\(\displaystyle V=\pi r^2h\)

where \(\displaystyle V\) is the volume of the cylinder, \(\displaystyle r\)  is the radius of the circular end of the cylinder, and \(\displaystyle h\) is the height of the cylinder.

 

So we can write:

 

\(\displaystyle V_{1}=\pi (r_{1})^2h_{1}\)

and

\(\displaystyle V_{2}=\pi (r_{2})^2h_{2}\)

 

Now we can summarize the given information:

\(\displaystyle r_{1}=\sqrt{3}\cdot r_{2}\)

\(\displaystyle h_{2}=4\cdot h_{1}\Rightarrow h_{1}=\frac{h_{2}}{4}\)

 

Now substitute them in the \(\displaystyle V_{1}\) formula:

 

\(\displaystyle V_{1}=\pi (\sqrt{3}r_{2})^2\times \frac{h_{2}}{4}\Rightarrow V_{1}=\frac{3}{4}\pi (r_{2})^2\times h_{2}\Rightarrow V_{1}=\frac{3}{4}V_{2}\)

Example Question #145 : Geometry

Two right cylinders have the same height. The radius of the base of the first cylinder is two times more than that of the second cylinder. Compare the volume of the two cylinders.

Possible Answers:

\(\displaystyle V_{1}=2\cdot V_{2}\)

\(\displaystyle V_{1}=4\cdot V_{2}\)

\(\displaystyle V_{1}=\sqrt{2}\cdot V_{2}\)

\(\displaystyle V_{1}=3\cdot V_{2}\)

\(\displaystyle V_{1}=V_{2}\)

Correct answer:

\(\displaystyle V_{1}=4\cdot V_{2}\)

Explanation:

The volume of a cylinder is:

\(\displaystyle V=\pi r^2h\)

where \(\displaystyle r\) is the radius of the circular end of the cylinder and \(\displaystyle h\) is the height of the cylinder. So we can write:

\(\displaystyle V_{1}=\pi (r_{1})^2h_{1}\)

\(\displaystyle V_{2}=\pi (r_{2})^2h_{2}\)

 

We know that 

\(\displaystyle h_{1}=h_{2}\)

and 

\(\displaystyle r_{1}=2r_{2}\).

 

So we can write:

\(\displaystyle V_{1}=\pi (r_{1})^2h_{1}=\pi (2r_{2})^2\cdot h_{2}=4\pi (r_{2})^2h_{2}\Rightarrow V_{1}=4V_{2}\)

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