Why Do Objects Float?
Chapter 12 – Section 2
Pages 348 - 354
The Buoyant Force
When you pick up an object underwater,
does it feel lighter or heavier than air?
Why?
Water exerts a force called the buoyant
force that acts on a submerged object.
The buoyant force acts in the upward
direction, against the force of gravity, so it
makes an object feel lighter.
What Causes the Buoyant
Force?
Remember – a fluid exerts pressure on all
surfaces of a submerged object.
Since the pressure in a fluid increases with
depth, the upward pressure on the bottom
of an object is greater than the downward
pressure on the top.
Floating & Sinking
Over 2000 years ago, a Greek
mathematician named Archimedes studied
buoyancy and the concept of displacement.
He found that a submerged object
displaces, or takes the place of, a volume
of fluid equal to its own volume.
Archimedes’ principle states that the
buoyant force on an object is equal
to the weight of the fluid displaced
by the object.
Floating & Sinking
If the weight of water displaced is MORE than
the weight of the object - it will FLOAT!
If the weight of the water displaced is LESS
than the weight of the object – it will SINK.
If the weight of the water
displaced is EQUAL to the
weight of the object – it
will be SUSPENDED
Density
By comparing the density of an object to the
density of a fluid, you can decide if it will float.
Density = Mass / Volume
Density of Lead = 180.8 g / 16 cm3
M
D
Density of Lead = 11.3 g/cm3
V
What is the density of water?
1 g/mL or 1 g/cm3
Buoyancy & Density
Which of these substances will float?
Density
Substance
Wood
Corn Oil
Plastic
Water
Tar Ball
Glycerin
Rubber Washer
Corn Syrup
Copper Wire
Lead
Mercury
3
(g/cm )
0.7
0.925
0.93
1
1.02
1.26
1.34
1.38
8.8
11.3
13.6
Changing the Buoyant Force
Changing the density of an object can make it
float or sink.
The density of a submarine is decreased
when water is pumped out of floatation tanks.
Since its volume remains the same, its
density decreases when its mass decreases.
Buoyancy & Density
Another way of changing density is to change
volume.
Why are large ships made of steel able to
float?
1) Boats have a large volume that is filled
with air that has little mass.
2) The shape of boats allows for a large
amount of water to be displaced creating
added buoyant force.
Practice Problem #1
An object has a mass of 500 grams and
occupies a volume of 40 cm3. What is the
density of the object? If it is submerged in
water, will the object float?
o
o
Density = Mass / Volume
o
Density = 500 grams / 40 cm3
o
Density = 12.5 g/cm3
o
No – the object will sink
Practice Problem #2
If an object has a density of 0.9 g/cm3 and
it occupies of volume of 3.5 cm3, what is the
mass of the object. Will the object float?
o
o
Mass = Density * Volume
o
Mass = 0.9 g/cm3 * 3.5 cm3
o
Mass = 3.15 grams
o
Yes – the object will float