Matter
solid
liquid
gas
Definite:
mass
volume
shape
Definite:
mass
volume
Definite:
mass
Fluid – state of matter in which
molecules and atoms are free to move
past each other.
liquid
will take the
shape of the
container
within its own
definite
volume
gas
will take the
shape and fill
the entire
volume of the
container (no
definite volume)
3
[m ]
volume –
-V
mass – [kg] - m
m
r=
V
Density – the concentration of
matter of an object.
3
density [kg/m ] – ρ
SG = Specific Gravity (no units!)
ro
SG =
rH O
2
What’s gold’s SG and what is its density in g/cm3 ?
19300 kg/m3 = density of gold is SI
Buoyant Force
The upward
force exerted
on a body
immersed or
floating in
the fluid
(liquid or
gas)
V f  Vo
The volume of the displaced fluid = the
volume of the object (part of the object
immersed in it)
Critical thinking
Two solid spheres of the same radius
are placed in two identical beakers
with water. One sphere is made of
gold (ρ = 19600 kg/m3) and the other
of iron (7860 kg/m3). Which sphere
will displace more water? Explain and
illustrate your answer.
Critical thinking
 Two solid spheres of the same mass are
placed in two identical beakers with
water. One sphere is made of gold (ρ =
19600 kg/m3) and the other of iron
(7860 kg/m3). Which sphere will
displace more water? Explain and
illustrate your answer.
Archimedes’s 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.
Fb = m f g = r f V f g
Sink or Swim?
If Fb  mo g
the object will stay
afloat or totally immersed in the
fluid without sinking all the way.
If Fb  mo g
the object will sink
Sink of Swim?
SINK
Fb = r f V f g
V f = Vo
mo g > m f g
roVo > r f V f
ro > r f
SWIM
Fb = r f V f g
mo g = m f g
roVo = r f V f
Vo ³ V f
ro £ r f
How do submarines ascend and
descend?
 Draw free body diagrams for each case and compare
density of the sub to that of a surrounding water.
Critical thinking
 Why do objects (people) appear to weigh less in water?
 If a 5.0 kg object appears to weigh 30.0 N in water,





what is:
A) buoyant force exerted on it?
B) volume of the displaced fluid?
C) volume of the object?
D) Density of the object?
If the same object is submerged into another fluid and
weighs only 15 N in it, what is the density of the fluid?
Lab – determine the density of
washers and density of a fluid
 You have: a cup of water, a set of nuts and washers, a
spring scale, a cup of unknown fluid.
 Design an experiment and determine the required
densities.
A floating raft
 The dimensions of the raft
are: 4.0m x 4.0m. It is
made of wood with SG =
0.55. How much height of
the raft is below the water?
How many people can get
on it without getting their
feet wet if the average
mass of a person is 55 kg?