Section 9­1.notebook
January 24, 2012
3 States of Matter ­ solid, liquid, and gaseous
Fluid ­ a nonsolid state of matter in which the atoms or molecules are free to move past each other, as in a gas or a liquid.
Liquids have a definite volume, but not a definite shape.
Gases do not have a definite volume or a definite shape.
Mass Density ­ the mass per unit volume of a substance. (Greek letter ρ rho)
ρ = m
V
mass density = mass
volume
Table p 319
The SI unit is kg/m3
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Section 9­1.notebook
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Buoyant Force ­ a force that acts upward on an object submerged in a liquid or floating on the liquid's surface.
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 an object.
Buoyant Force FB = Fg(displaced fluid) = mfg
magnitude of buoyant force = weight of fluid displaced
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Buoyant Force FB = Fg(displaced fluid) = mfg
Buoyant Force FB = Fg(object) = mog
buoyant force = weight of floating object
The density of an object determines the depth of submersion. The net force acting on the object is zero. Recall m = ρV
Fnet = 0 = (ρfVf ­ ρoVo)g
rearranged formula
ρf = Vo
ρo Vf
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Objects can change its buoyancy by changing its average density. A submarine uses its ballest tank pump compressed air in and water out to make the submarine rise and vice versa to submerge. The weight depends on the density. As object's volume changes so does the density and its buoyant force. Fnet = 0 = (ρfVf ­ ρoVo)g
Fnet = 0 = (ρf ­ ρo)Vg
A relationship betweeen the weight and the density of the object can be rewritten as follows.
Fg(object) = ρoVg
ρfVg
FB
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Section 9­1.notebook
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A bargain hunter purchases a "gold" crown at a flea market. After she gets home, she hangs the crown from a scale and finds its weight to be 7.84 N. She then weighs the crown while it is immersed in water, and the scale reads 6.86 N. Is the crown made of pure gold?
Fg(object) = ρo
FB
ρf
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Section 9­1.notebook
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Density of some common substance
Substance ρ(kg/m3)
hydrogen
helium
steam (100℃)
air
oxygen
carbon dioxide
ethanol
ice
fresh water (4℃)
sea water (15℃)
iron
mercury
gold
0.0899
0.179
0.598
1.29
1.43
1.98
0.806 x 103
0.917 x 103
1.00 x 103
1.025 x 103
7.86 x 103
13.6 x 103
19.3 x 103
back
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