Density8th Grade Physical Science

Density• Density is the amount of matter (mass) compared to

the amount of space (volume) the object occupies.

• We will measure mass in grams and

volume in ml or cm3

Density Formula•Density is mass divided by volume.•Density = mass/volume•Remember, all fractions are division problems. •Since the unit for mass is grams, and the unit for volume is ml or cm3, then the unit for density is g/ml, or g/ cm3

Understanding Density• In the following illustrations, each will represent 1 cm3.• Each g will represent 1 gram.• Mass = 24g• Volume = 8 cm3

• Density = 3g/cm3 g g g

g g g

g g g

g g g

g g g

g g g

g g g

g g g

Density Problem 2g g

g g g g

g g

g g

g g

•Mass = 12 grams

•Volume = 6 cm3

•Density = 2 g/cm3

•In English we say the density of the object is 2 grams in

every cubic centimeter.

Density Problem 3g g g

gg g g

g g g

g g g g g g

•Our previous problems were materials of uniform density. They were the same stuff throughout. But many materials are not. Gravel is a great example.

•Mass = 16 grams

•Volume = 8 mL

•Density = 2 g/mL

Water and Density• Since 1-gram of water has a volume of 1-mL, then the

density of water will always be 1 gram/ml.

• 5o-mL of water will have a mass of 50 grams, so again the density of pure water will be 1 g/ml.

• A kg of water will have a volume of 1000-mL, so it’s density will be 1 gram/ml.

Comparing Densities

•Where is the most dense object?

•Where is the least dense object?

Floating and Sinking• Less dense materials will

float on top of more dense materials.

• Objects with a density of less than 1-g/mL will float on top of water.

• Objects with a density greater than 1 g/mL will sink in water.

Neutral Buoyancy• Objects with a density

equal to the density of water will float in mid water, at what ever level you place the object.

• Fish and submarines control their depth by changing their density.

Objects that Sink!•Objects with a density greater than 1 g/mL will sink in water.

Titanic Sails the Ocean Blue• The Titanic is sailing on

its maiden voyage.• What is the density of

this enormous, steel hulled ship, full of machinery, coal, people, and all sorts of heavy things?

• It’s floating, so it’s density must be less than 1 g/mL.

• How can this be?• The Titanic is a hollow

vessel full of air!

Titanic verses Iceberg• After HMS Titanic struck

the iceberg, she started to fill with water.

• What happened to her density?

• As she took on more and more water, her density got closer and closer to 1 g/mL.

• The denser the ship became, the lower she settled into the water.

Wreck of the Titanic• What is the density

of the Titanic resting on the ocean floor?

• Must be greater than 1 g/mL, as her steel hull is full of water instead of air.

Density Review• We don’t actually count g’s to find the mass of objects.

How would you find the mass of a rock?

• Use a balance scale.

• In real life, how would you find the volume of a rock?

• Use a graduated cylinder and see how much water the rock displaces.