Physics Section 8.1Apply the properties of fluids

General Phases of Matter1. Solid-constant shape and volume2. Liquid-constant volume, variable shape3. Gas/plasma-variable shape and volume

Fluids have the ability to flow. They change their shape to conform to a container. Both liquids and gases are fluids. Liquids have a definite volume. Gases will expand to fill a container.

Properties of fluids

mass density = _mass volume ρ = m m = mass (kg)

v v = volume (m3) ρ = density (kg/m3)

Gases can have a variable density, depending upon their pressure and temperature. It is very difficult to change the density of a solid or liquid.

The upward force exerted on an object in a fluid is called a buoyant force.

The buoyant force acts in a direction opposite gravity. The net force is lower when an object is in a fluid. The apparent weight of the object is the gravitational force minus the buoyant force.

Archimedes’ Principle: Any object completely or partially submerged in a fluid experiences an upward buoyant force equal in magnitude to the weight of the fluid displaced by the object.

buoyant force on floating object = weight of the fluid displaced FB = Fg = mf ·g mf = mass of fluid

g = acceleration due to gravity

http://www.youtube.com/watch?v=qILEEfXCanw

Buoyant force on a submerged object

Fnet = (ρf – ρo)V g ∙

If the buoyant force is equal to, or greater than the objects weight, the object will float.

If an objects mass density is greater than the mass density of the fluid, the object will sink.

Fg(object) = ρo

FB ρf

Fg = weight of object

FB = buoyant force

ρo = density of object

ρf = density of fluid

example

Will lead float in mercury? See table on page 869.

problem

A “gold” coin weighs .075 N in air. It weighs .045 N in Water. Is the coin pure gold?

problem

A rectangular piece of wood with dimensions of 1.0 m by 2.0 m by .25m has a mass of 70 kg. What is the maximum mass that can be supported by the wood when it is floating in water?

assignment

• Page 279• Problems 1-5