Transcript Boy, Oh Buoyancy - Warren Township Schools

Boy, Oh Buoyancy
• Does it Float?
• Does it Sink?
What is density?
• A measure of how much material is
packed into a unit volume of the material
• The fewer particles packed into a given
volume, the less dense the material
• Density depends on
 mass of the atoms that make up the material
 distance between the particles
Things float if they are less dense
than their surrounding.
• Helium is less dense
than the surrounding
air.
The density of the ‘surroundings’
makes a difference!
• The Dead Sea in
Israel has very salty
water. People can
float very high with
very little effort.
What is a force?
• Any push or pull
• If a force in one direction is equal to a
force in the opposite direction, then there
is no movement
• If a force in one direction is stronger than
the force in the opposite direction, then
there will be movement
Buoyant force
• the upward force on an object in a fluid
exerted by a surrounding fluid
• Example: When you push a beach ball
under water, the water exerts an upward
force on the ball. When you push more of
the ball under water, more buoyant force is
exerted. When you let go, the ball pops
out of the water.
Archimedes said….
• In order to float, the
weight of the water
that an object
displaces must be
equal to – or greater
than – the weight of
the object.
Buoyant Force and Gravity
•A fluid exerts pressure on any object that is in
the fluid.
• Gravity is the downward force on the object due
to its weight.
•Archimedes’ principle: the buoyant
force on an object is an upward force
equal to the weight of the fluid that the
object takes the place of.
Weight Versus Buoyant Force
• Sinking An object in a fluid will sink if its weight is
greater than the buoyant force.
• Floating An object will float only when the buoyant
force on the object is equal to the object’s weight.
• Neutrally buoyant An object will be suspended in
water when its weight is exactly equal to the buoyant
force
Weight Versus Buoyant Force
• Will an object sink or float? That depends on the
whether the buoyant force is less than or equal to
the object’s weight.
Buoyant Force and Gravity
• There is more pressure at the bottom of an object
because pressure increases with depth. This results
in an upward buoyant force on the object.
But the ability to float is more than
density…
Water is “displaced” as
you enter a bathtub,
but you do not float!
So….
• In a pool, you spread
your arms, displacing
more water. The
water that is
displaced weighs as
much or more than
you do and you float!
Changing Overall Density
•The secret of how a ship floats is in the shape of
the ship.
•The overall density of the boat is made less
than the density of water by making the volume
of the boat larger.
CHANGING MASS
• Submarines have ballast tanks that can be opened
to allow sea water to flow in.
• As water is added, the submarine’s mass increases,
but its volume stays the same.
CHANGING VOLUME
•Like a submarine, some
fish adjust their overall
density to stay at a certain
depth in the water by
using an organ called a
swim bladder.
How does the surrounding fluid affect the
way a ship floats?
• Ships float because they
have air in their hulls.
• Due to density of the
surrounding fluid, they float
at different heights in
different types of water.
• Salt water is more dense
than fresh water so the oil
tanker floats high in the
water.)
In Fresh water the same oil tanker
floats lower in the water.
Temperature of the surrounding
water causes different densities
• In cold water, ships
float higher. In tropical
waters, ships float
lower in the water.
This Could be a Problem!!
• If a ship is loaded to the max
in a salt water port
• comes across the ocean to
unload in a fresh water port
• There could be a serious
problem if the ship was
loaded too heavily at the
beginning.
• Remember, salt water has a
higher density and a greater
buoyant force than fresh
water
Plimsoll Lines
Samuel Plimsoll
(1824-1898) a member of
the British Parliament
was concerned with the
loss of ships and crews
due to overloading.
He called them “coffin
ships”.
Plimsoll Lines
• To save sailors’ lives,
he persuaded
Parliament to provide
for the marking of a
line on ships’ sides
that would disappear
below the water line if
the ship was
overloaded.
Plimsoll Lines
•
•
•
•
•
•
T = Tropical
TF = Tropical Fresh
F = Fresh
S = Summer
W = Winter
WNA = Winter North
Atlantic
In summary,
Archimedes principle:
• If an object sinks, its volume = volume of
water displaced
• If an object floats, the weight of object =
the weight of the fluid it displaces
• the denser the fluid (example salt water
versus fresh water) the greater the
buoyant force exerted by the fluid