Download Buoyancy - Mr-Durands

Good Morning! 7/8/2017
• Starter: why is it that a 1000 mega gram
cruise ship can float when a ½ kg rock
sinks? (there are two answers really)
• Today we will be…
– And discussing why and how things float
this is the beginning of our new unit on
Fluids and Gases.
Why is it that a huge heavy ship
can float but a small light rock
sinks?
• Answer this question to the best of your
ability in your group.
• The real answer is all about Buoyancy
Force.
What is the force?
• The force is called the
Buoyancy force, it is the force
pushing up on objects
keeping them afloat.
Where does it come
from?
• The force comes from the
liquid itself pushing up on
the floating object.
Buoyancy
• The buoyant force acts in the direction
opposite to the force of gravity, and that
is why you feel lighter in the water.
Buoyancy
• The pressure on the
bottom of a submerged
object is greater than
the pressure on the top
(pressure increases
the deeper you go).
• This is because there
is all that water above
pushing down to
increase the pressure.
• The result of all this is
a net force in the
upward direction. We
call this Buoyant
Force!
How much force does it push
up with?
• It pushes up with the same force
that it would push up to support
the water that was there before
the object moved it out of the
way. So the Buoyancy force is
equal to the weight of the water
that used to be there.
Buoyancy
• Achimedes’ principle states that the buoyant
force acting on a submerged object is equal to
the weight of the fluid the object displaces.
How can you increase it?
• The force is equal to the
weight of the water
displaced (moved out of
the way) so if you move
more water out of the way
then you will have more of
a Buoyancy force.
Buoyancy
• A solid block of steel sinks when placed in
water. A steel ship with the same weight
floats. The difference is the amount of water
that each
displaces that is
the important thing.
• A submarine changes the water it displaces by
bringing it in or pushing it out this allows it to
dive, rise, or float.
To make the submarine rise compressed
air is released to push water out of the
hull this displaces more water and
increase the buoyancy force.
Relating Cause and Effect
• As you read, identify the reasons why an
object sinks. Write them down in a graphic
organizer like the one below.
Causes
Weight is greater than
buoyant force.
Effect
Object is denser than
fluid.
Object takes on mass and
becomes denser than fluid.
Object is compressed and
becomes denser than fluid.
Object sinks
• Please write down the starter below, but
before we discuss it we will handout books
and a homework assignment on Pressure.
• Starter: Some guy on the street says he has
pure gold necklaces for $100. He says they
are worth $1000.00 and that this is a “steal”.
• Truth be told, if they are gold they would be
worth 1000 dollars.
• How do you KNOW if they are gold, how
can you tell?
You are not the first to have
face this dilemma, the first
recorded incident was posed to
Archimedes.
Archimedes
• Hiero, king of Syracuse in the third century
B.C., gave a jeweler a bar of gold and
ordered the jeweler to make it into a crown.
When the King got the crown he was
suspicious that the jeweler had cheated him
by substituted some less precious metal for
the gold.
• He measured the mass and found that the
crown had the same mass as the gold bar he
gave the jeweler, But this didn’t convince
him.
• So he asked Archimedes to figure it out how
He figured out that the jeweler had snuck in
some cheaper metals. How did he do it?
• Oh Yeah, the King
won’t let you cut
into it or destroy it
in any way?
• Want a hint?
• Well think about it
two metals can
have the same
mass but not the
same density.
Dude figured it out.
• Even though they had the same mass, they
couldn’t also have the same volume unless
they were both gold.
• So he put the crown and a bar of gold onto a
balance and as expected the equaled out.
• But then he put the whole thing under water
and the crown floated up.
• What did that tell him?
• Crazy story but true, he realized that the
crown went up because it was had a greater
volume and was displacing more water.
Buoyancy Force
• Before we can connect why having a larger
volume caused the crown to go up we need
to talk about why anything floats up in water.
• So what gives why do things float?
• Think about it like this, before you got into
the swimming pool what was in the space
that you now occupy?
Buoyancy Force
• Yea, Water, so what was supporting the
weight of that water?
• You’re right the other water, so if it could
support that weight of water how much of
your weight will it support?
• The same amount of weight ! Does that
make sense?
Almost there…
• So if the water could hold up the weight of the
water that you moved out of the way, It can
proved a force up on you that is equal to the
weight of the water you moved out of the way.
• So if you move 100 liters of water the water
will push up on you with a force equal to the
weight of 100 liters.
• It is this force that allows you and boats and
everything else to float.
• We call this force Buoyancy Force
The final answer!
• Now the question, did the crown gold and the
bar gold have the same Force pushing up on
them?
• NO!, so they must have moved different
amounts of water out of the way! This means
that the goldsmith used a metal that’s mass
takes up more space than gold’s mass.
The Final Answer Continued
• The fact that he figured it out is why we
remember his name.
• It doesn’t hurt that the story
goes…Archimedes made this discover
while in the bath and ran through the
streets naked screaming “Eureka” which
means “I’ve found it”