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?