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Properties of Liquids: Density and Buoyancy
Definitions
Density:
The mass per unit volume of a material.
Buoyancy:The ability of a fluid to exert an upward force on a floating object.
Density
The symbol for density is usually D and has units of g/cm3.
Density is a physical property of a material.
Often, a material’s density is used to identify it.
Water 1.00 g/cm3
Aluminum 2.7 g/cm3
Steel 7.9 g/cm3
Gasoline .70 g/cm3
Mercury 13.6 g/cm3
Air .0013 g/cm3
Densities of Common Materials
Density
Solids, liquids, gases, and other phases of matter all have densities.
Solids are most dense, liquids next dense, and gases least dense
MOST of the time, density is inversely related to temperature.
D = = mV
Which weighs more, A pound of feathers or a pound of lead?
Which is more dense?
Calculating Densities
Mass of the Sun:
2 x 1030 kg
Volume of the Sun: 1.4 x 1027 m3
Average Density of the Sun: 1.4 g/cm3
Calculating Densities
Mass of the Earth:
6 x 1024 kg
Does it make sense that the Earth is more dense than the Sun?
Volume of the Earth: 1.1 x 1021 m3
Average Density of Earth:5.5 g/cm3
A Density Problem:
You found this ring on the track at your school. You wondered what it was made of.
You calculated it’s density to be approximately 7.9 g/cm3.
What is it made of?
Does this help?
Metal Density g/cm3
Aluminum 2.70
Gold 19.30
Copper 8.63
Steel 7.87
Silver 10.40
Chromium 7.10
Lead 11.30
Forces
Newton’s Laws:
1.) The law of inertia: An object in motion stay in motion until it is acted upon. Or, an object at rest stays at rest until acted upon.
2.) F = m*a
3.) When two bodies interact by a force, the force on the first body is equal in magnitude and opposite in direction to the force on the second body.
Archimedes’ Principle
The buoyant force
exerted on an object
submersed in a fluid is
equal to the weight of
the fluid displaced.
But what does that mean?!?
Fb = D*V*gFb : Buoyant forceV : Volume of fluid displacedD : Density of fluidg : Acceleration due to
gravity
Fbuoyant
Wobject
Why does it float?
For an object to float:
Wobject = Fbuoyant
Fbuoyant
Wobject
Buoyancy Calculations
The polar bear on ice:A 500 kg polar bear sees a
slab of ice floating by and tries to hop on. The ice has a volume of 4 m3 and a density of .92 g/cm3. Seawater has a density of 1.035 g/cm3. Does the polar bear float away happily? Not this time!
Fb = 4140 N < Wbear = 4900N
Activity
You will determine the density of a liquid and use that density to identify the liquid.
Good luck!
Questions?