Notes

##### Cylinders and prisms

Calculating the volume of a cylinder or a prism is straightforward if we can easily calculate the area of the base.

- The
**volume of a cylinder or a prism**is given by*base area*x*height*(perpendicular to the base). - Example 1: Calculate the volume of a cylinder comprised of a circular base with a radius of 5 cm and a height of 6 cm. Volume = base area x height = (π x
*r*^{2}) x*h*= (π x 5^{2}) x 6 = 150π = 150(3.14) = 471 cm^{3}. - Example 2: Calculate the volume of a prism comprised of the following shape as the base and a height of 6 cm.

Half-circle area = (1/2) x π x*r*^{2}= (1/2) x π x (6/2)^{2}= 4.5π = 4.5(3.14) = 14.13 cm^{2}.

Triangle area = (1/2) x*b*x*h*= (1/2) x 3 x 4 = 6 cm^{2}.

Rectangle area =*l*x*w*= 6 x 4 = 24 cm^{2}.

Base area = 14.13 cm^{2}+ 6 cm^{2}+ 24 cm^{2}= 44.13 cm^{2}.

Volume = base area x height = 44.13 x 6 = 265 cm^{3}.

##### Cones and pyramids

Calculating the volume of a cone or a pyramid is also straightforward if we can easily calculate the area of the base.

- The
**volume of a cone or a pyramid**is given by (1/3) x*base area*x*height*(perpendicular to the base). - Example 3: Calculate the volume of a cone comprised of a circular base with a radius of 5 cm and a height of 6 cm. Volume = (1/3) x base area x height = (1/3) x (π x
*r*^{2}) x*h*= (1/3) x (π x 5^{2}) x 6 = 50π = 50 x 3.14 = 157 cm^{3}. - Example 4: Calculate the volume of a pyramid comprised of a square base with side length 5 cm and a height of 6 cm. Volume = (1/3) x base area x height = (1/3) x (l x
*w*) x*h*= (1/3) x (5 x 5) x 6 = 50 cm^{3}.

##### Spheres

The formula for the volume of a sphere isn’t quite as well-known as the formula for the area of a circle, but it’s somewhat similar.

- The
**volume of a sphere**is given by (4/3) x*π*x*r*.^{3} - Example 5: Calculate the volume of a sphere with a radius of 6 cm. Volume = (4/3) x
*π*x*r*= (4/3) x π x 6^{3}^{3}= 288π = 288 x 3.14 = 904 cm^{3}.

##### Scaling

Length, area, and volume problems sometimes involve one or more measurements changing or *scaling*.

- Example 6: In 9.3: Area and perimeter, we calculated the circumference and area of a circle with a radius of 5 cm to be 31.4 cm and 78.5 cm
^{2}, respectively. Use these values to calculate the circumference and area of a circle with a radius of 10 cm. The*scale factor*here is 10/5 = 2.- To calculate a new length (such as the circumference), we multiply by the scale factor. So, circumference of a 10 cm radius circle = 2 x circumference of a 5 cm radius circle = 2 x 31.4 cm = 62.8 cm.
- To calculate a new area, we multiply by the scale factor
*squared*. So, area of a 10 cm radius circle = 2^{2}x area of a 5 cm radius circle = 4 x 78.5 cm = 314 cm^{2}.

- Example 7: In 9.4: Volume and scaling, we calculated the volume of a cylinder comprised of a circular base with a radius of 5 cm and a height of 6 cm to be 471 cm
^{3}.- Use this value to calculate the volume of a cylinder comprised of a circular base with a radius of 10 cm and a height of 12 cm. The
*scale factor*here is 10/5 = 12/6 = 2. To calculate the new volume, we multiply by the scale factor*cubed*. So, volume of a 10 cm radius 12 cm tall cylinder = 2^{3}x volume of a 5 cm radius 6 cm tall cylinder = 8 x 471 = 3,768 cm^{3}. - What if just the radius doubles but not the height? Then the base area increases by a factor of 2
^{2}= 4 but the height stays the same. So, volume of a 10 cm radius 6 cm tall cylinder = 2^{2}x volume of a 5 cm radius 6 cm tall cylinder = 4 x 471 = 1,884 cm^{3}. - What if just the height doubles but not the radius? Then the height increases by a factor of 2 but the base area stays the same. So, volume of a 5 cm radius 12 cm tall cylinder = 2 x volume of a 5 cm radius 6 cm tall cylinder = 2 x 471 = 942 cm
^{3}.

- Use this value to calculate the volume of a cylinder comprised of a circular base with a radius of 10 cm and a height of 12 cm. The

The video below works through some volume and scaling calculations.

Video Tips

Practice Exercises

Do the following exercises to practice volume and scaling calculations.