DENSITY

What Is Density?

• Density is how heavy something is for its size.

• Density is mass divided by volume or

D=m/vD = density, M = mass

V = volume

Another Way of Looking at It.

• You will often hear density referred to as “mass per unit volume.”

• Let’s break this down:•Mass = how much matter the object has.•Per = means divided by•Unit volume = how much space it takes up

So…

• When you hear mass per unit volume, you should think…

Take the mass and divide by its volume!

What Are the Units of Density?

• Mass units would include – Grams for very small objects or– Kilograms for larger objects

• Volume units would include– Cubic centimeters for small objects or– Cubic meters for large objects– Milliliters for small amounts of liquids– Liters for larger amounts of liquid

• Lets stick to grams and cubic centimeters for now.

How to State Density Units

• You have grams for• You have cubic centimeters for

• These are not the same units, they don’t cancel so you have to include both units in your answer

How to State Density Units

• The units of density are:

• Grams/Cubic Centimeter or

g/cm3

Why Cubic Centimeters?

• Remember that volume is length multiplied by width multiplied by

height orl X w X h

• In each case, the distance is in cm and since you are multiplying it three times, you get cubic centimeters.

Now, How About an Example?

l=10 cm

h=5 cm

w=2 cm

Mass= 100 grams

Use the information provided to calculate the density of the object

What Did You Get?

• If you got 1g/cm3, you are correct!• Lets see how this works:

V=l x w x hV=10 cm X 5 cm X 2 cmV=100cm3

Mass = 100 g (given)D=m/vD=100 g/100cm3=1g/cm3

A Word About Irregularly Shaped Objects.

• If you are given an irregularly shaped object and told to find the density, you might have a problem.

• If the length, width, and height of the object are not uniform (the same), you cannot use l X w X h to determine volume.

A Word About Irregularly Shaped Objects.

• To determine the volume of an irregularly shaped object, start with a graduated cylinder of water filled about half full

• Record the volume of water in the cylinder• Drop in the irregularly shaped object• Record the volume of water in the cylinder with

the object in it• Subtract the original volume from the second

volume to determine the volume of the object

A Word About Irregularly Shaped Objects.

• You are measuring the amount of water displaced by the object which is equal to the volume of that object

• This is called:

Liquids

• So far we have been talking mostly about solids. Solids have densities that can be easily expressed in

• Liquids tend to be measured in

• This is not a problem because you can convert milliliters or liters into grams per cubic centimeter.

Liquids

• You can also weigh a liquid to determine its mass. Now we are going to use the word massing for weighing to avoid confusion

• To do this, measure your cylinder in grams and record its empty mass

• Add your liquid• Now mass the cylinder again• Subtract the mass of just the cylinder and you

have the mass of your liquid

Sink or Float?

• Solids can float or sink in a liquid.

Can you guess what kind of solid would sink in water?

Answer:

• Any solid will sink in water if it has a density greater than the density of water.

• Water has a density of

• Any solid with a density greater than 1g/cm3 will sink in water!