Density 8th 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
Density Problem 2 • 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 3 • 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 vesselfull 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.