What is density?





Units -  kg/m3


Giancoli - p. 281) 2 - 4 densities found on p. 256


Specific Gravity - comparison of the density of a substance with the density of water

Specific Gravity =


Alcohol or lead, which has a specific gravity of 11?



Pressure = F/A


N/m2 or Pascal


Pressure under a Liquid 

P0 – pressure above liquid

(Density in liq. depends only on h)

(Density depends only on h)


P0 – pressure above liquid

(usually atmospheric pressure)


at sea level

1 atm = 1.013 x 105 N/m2

Pascal’s Principle

If a force is applied on one place in an enclosed liquid the pressure increases everywhere else in the liquid.


F2 = A2
F1 A1


Giancoli p. 281) 14, 16 - 17

density of water = 1000 kg/m3




"Archimedes Principle"

Why do some substances float in a liquid and some sink?


Objects that are less dense than the liquid sink.


The buoyancy force produced by a liquid is equal to the weight of the volume displaced


FB= Weight of fluid displaced= Weight of fluid displaced

FB = mg

Buoyancy force = submerged weight  

(rfluid)(Vsubmerged)g = mg



(rfluid)(Vsubmerged) = mobject


Displ. Massliq. = Massobject


Gauge Pressure - fluid pressure w/o atmospheric pressure

Absolute Pressure - fluid pressure with atmospheric pressure

Fb = Weightair - Weightliq


Apparent Mass or Weight

- mass or weight in fluid



mg - mg = rVg
in air

(apparent mass or weight)
  Mass of liquid displaced

 V of liquid displaced

= V of object submerged



Density / Part Submerged


Density Part Submerged
800 kg/m3

8/10 submerged
250 kg/m3

1/4 submerged
SG = .75

3/5 submerged

125 kg/m3

1/8 submerged


Partially Submerged /
Density and SG



Mass or Weight in Liquid
= SG
Mass or Weight in Air


©Tony Mangiacapre., St. Mary's H.S., Manhasset, NY
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