Properties of Liquids: Density and Buoyancy

Definitions

Density:

The mass per unit volume of a material.

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.

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.

Calculating Densities

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?

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*g

Fb : Buoyant force

V  : Volume of fluid displaced

D  : Density of fluid

g  : Acceleration due to gravity

Why does it float?

For an object to float:

Wobject  =  Fbuoyant

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?

Activity

You will determine the density of a liquid and use that density to identify the liquid.

Good luck!

Questions?

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