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Prentice Hall Pre-Algebra: Online Textbook Help13 chapters | 135 lessons

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Lesson Transcript

Instructor:
*David Wood*

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

Learn about how mass and volume are connected by a special formula, how to calculate volumes of common 3D shapes, and how to convert between different units of mass and volume. Complete some practice problems and perfect your skills.

Everything we've ever known or experienced is matter, and all matter has mass. The ground we walk on is matter; the air we breathe is matter; and there's even matter in space. **Mass** is the amount of 'stuff' inside an object; the more stuff there is, the more mass.

Matter is made up of atoms and molecules, and the more of them we have, the more the mass of an object is. We measure mass in units like kilograms, ounces, and pounds. In science, we prefer kilograms, which are the SI (Standard International) unit of mass. Your bathroom scale is calibrated to tell you your mass, but it only works properly on Earth. If you took it to the moon, the weight on the scale would change, but of course, your mass would not.

**Volume** is how much space something takes up. The standard unit of volume is meters cubed (or cubic meters). The classic question people ask to show the difference between mass and volume is, What has the greatest mass, a pound of feathers or a pound of nails? Well, it's a pound of each, so they both have the same mass. It's just that a pound of feathers will take up a huge amount of space; it has a greater volume.

Formulas are about relationships between numbers. Mass and volume relate to each other through a concept called density. **Density** is how tightly packed the atoms and molecules in a substance are, measured in kilograms per meter cubed. If you have a lot of mass in a small area, that's high density. If you have little mass spread over a large area, that's low density.

The formula that relates density, mass, and volume looks like this:

Here, *m* represents the mass of an object or material, *V* represents the volume, and the curly *p* (which is the Greek letter rho) represents the density.

There are also many formulas for volume, depending on the shape of an object. The way to calculate the volume of a sphere is different than how you calculate the volume of a cube or cuboid, for example. Here is a table of some common volume formulas that you can use to solve problems:

We've already talked about the standard units of density (kilograms per meter cubed), mass (kilograms), and volume (meters cubed). But sometimes people can use non-standard or non-scientific units. For example, if someone uses a mass in grams, and a volume in centimeters cubed, then the density will come out in grams per cubic centimeter. But, sometimes you'll want to be a good scientist and use standard units instead. In those situations, it can be helpful to convert between units. You might need to multiply or divide.

Here is some information that you might need:

- There are 1000 grams in a kilogram
- There are 100 centimeters in a meter
- There are roughly 2.2 pounds in a kilogram (or 0.45 kilograms in a pound)
- There are 16 ounces in a pound
- There are 0.0823 cubic meters in a cubic foot (or 5.31 cubic feet in a cubic meter)
- There are 1000 meters in a cubic liter
- There are a million cubic centimeters in a cubic meter

Let's take a look at some examples!

A box of apples is being imported, and the density needs to be declared in kilograms per meter cubed. The apples are put on a scale, and have a mass of 15 pounds. The box is a perfect cube, and each side has a length of 0.5 meters. What is the density of the box of apples in kilograms per meter cubed?

We'll have to use the density equation, and divide mass by volume. We're told the mass is 15 pounds, but we don't know the volume. The volume of a cube is length multiplied by width multiplied by height, which is 0.5 * 0.5 * 0.5. That gives us 0.125 meters cubed.

Now we could just divide mass by volume to get density. But there's a problem; if we do that, our answer will be in pounds per cubic meter. We're asked to give the answer in standard units: kilograms per cubic meter. So, we need to convert the mass in pounds into kilograms. There are 0.45 kilograms in a pound, so multiply 15 pounds by 0.45, which gives us 6.75 kilograms. Finally we can calculate the density, which is mass divided by volume. 6.75 divided by 0.125 gives us 54 kilograms per cubic meter. And that's it; that's our answer!

Lead has a density of 11,340 kilograms per meter cubed. How much will a cuboid of lead which measures 25 cm x 30 cm x 35 cm weigh on a scale?

If density equals mass divided by volume, then mass equals density multiplied by volume. We know the density already (11,340), but we don't know the volume. We can calculate the volume of the cuboid by multiplying the lengths of the sides together. That's 25 * 30 * 35. But since our density figure is in kilograms per meter cubed, we can make our job easier by converting into meters first. So the cuboid measures 0.25 meters by 0.3 meters, by 0.35 meters. Multiply those three sides of the cuboid together, and we get a volume of 0.0263 cubic meters.

Now, we're finally ready to calculate the mass of the cuboid. Multiply the density of 11,340 by the volume of 0.0263 cubic meters, and we find out that the cuboid will weigh roughly 298 kilograms. And, that's our answer.

**Mass** is the amount of 'stuff' inside an object. In science, we prefer to measure mass in kilograms, which is the SI (Standard International) unit of mass. **Volume** is how much space something takes up. The standard unit of volume is meters cubed (or cubic meters). **Density** is how tightly packed the atoms and molecules in a substance are, measured in kilograms per meter cubed.

The formula that relates density, mass, and volume looks like this:

Here, *m* represents the mass of an object or material, *V* represents the volume, and the curly *p* (which is the Greek letter rho) represents the density. Sometimes, people can use non-standard or non-scientific units. In those situations, it can be helpful to convert between units.

Mass | Volume | Density |
---|---|---|

*Amount of stuff inside an object | *Amount of space an object takes up | *How packed the atoms and molecules of a substance are (measured by dividing mass by volume) |

Complete the lesson on mass and volume so that you can confidently do the following:

- Differentiate between mass and volume
- Write the formula for mass and volume
- Remember how to convert between units
- Discuss the relationship between mass, volume and density

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Prentice Hall Pre-Algebra: Online Textbook Help13 chapters | 135 lessons

- Measuring the Area of a Parallelogram: Formula & Examples 4:02
- Area of Triangles and Rectangles 5:43
- Measuring the Area of a Trapezoid 4:38
- Circles: Area and Circumference 8:21
- Overview of Three-dimensional Shapes in Geometry 3:28
- Prisms: Definition, Area & Volume 6:12
- What Are Cylinders? - Definition, Area & Volume 5:09
- Pyramids: Definition, Area & Volume 7:43
- Cones: Definition, Area & Volume 8:59
- Spheres: Definition, Area & Volume 5:22
- How to Use Models to Solve Math Problems
- Mass and Volume: Formulas, Unit Conversion & Practice Problems 6:24
- Go to Prentice Hall Pre-Algebra Chapter 10: Area & Volume

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