Transcript Lesson 45 - Buoyancy and Displacement_2x

Buoyancy & Displacement
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What is Buoyancy?
How does Buoyancy affect us?
What are Bathyscaphes?
Bathyscaphes construction project
Did You Know?
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Did you know that we are floating right now?
The earths crust is floating on top of liquid molten rock
The different tectonic plates are shifting and moving right
now as we speak
The rock and crust
the we are on is
actually less dense
than the molten
rock beneath it… So
we float?
How Does it Work?
Buoyancy works by displacing a fluid that weighs
more than the object displacing it
EXAMPLE
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A chunk of wood and a chunk of copper have the same
volume and displace the same amount of water, but the
wood floats because the water it displaces is heavier than
itself. The opposite is true with the copper… same volume
but different weight
Archimedes Principle
Archimedes Principle states…
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The upward buoyant force acting on
an object equals the mass of the fluid
displaced by the object
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When Archimedes stepped into a tub
he would sink because the water he
displaced weighed less than he did,
when he stepped in a boat, the large
surface covered by the hull displaced
more water which far exceeded the
weight of him and the boat, so they
stayed a float
Displacement
The steel ball bearing does
NOT displace a mass of
water to equal its own
mass so buoyant force is
weaker than gravity and it
sinks
The steel bowl DOES
displace a mass of water
greater than itself so
buoyant force is greater
than gravity and it floats
Average Density
It is said:
When we step in the boat, our density is not less than the water,
the material that makes up the boat is not less dense than water…
The average density of us, the material of the boat, AND the air
within the hull of the boat do have an AVERAGE DENSITY less than
the water and therefore we float!
When we get in a hot air balloon, we do not become lighter than
cold air, the basket does not get lighter than cold air… but rather
the AVERAGE DENSITY of us, the material of the balloon, AND all
the hot air inside the balloon becomes less than the cold air so it
starts to float
Buoyancy & Mass
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If an object weighed 7 lb.
out of water, and only 4
pounds in water, then
the buoyant force
exerted 3 lb upward.
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That means the object
had to displace 3 lb
worth of water
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Lets prove Archimedes’
Principal
Buoyancy & Mass
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In our demo, when we dropped the 200 g weight in the water,
how much weight did the buoyant force of water take off of
the weight?
Buoyancy & Mass
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In our demo, when we dropped the 200 g weight in the water,
how much weight did the buoyant force of water take off of
the weight?
- It went from 200 g in air to 180 g in water
- 200 g – 180 g = 20 g
Buoyancy & Mass
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In our demo, when we dropped the 200 g weight in the water,
how much weight did the buoyant force of water take off of
the weight?
- It went from 200 g in air to 180 g in water
- 200 g – 180 g = 20 g
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Because it took 20 g off the weight of our object, how much
water would you expect it to displace if we measured it?
(NOTE – 1 ml of water equals 1 g)
Buoyancy & Mass
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In our demo, when we dropped the 200 g weight in the water,
how much weight did the buoyant force of water take off of
the weight?
- It went from 200 g in air to 180 g in water
- 200 g – 180 g = 20 g
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Because it took 20 g off the weight of our object, what mass of
water would you expect the object to displace if we measured
it?
- If it took 20 g off the weight, then that
would equal 20 g of water so it should
displace 20 g of water
Buoyancy & Mass
Copy this chart into you note book
Type of
Liquid
Water
Canola Oil
Motor Oil
Amount of weight
taken off of 200g mass
Mass of Liquid
Displaced
Do the two
weights match?
Wood vs. Metal
REMEMBER OUR EXAMPLE
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A chunk of wood and a chunk of copper have the same volume
and displace the same amount of water, but the wood floats
because the water it displaces is heavier than itself. The
opposite is true with the copper… same volume but different
weight
Your Assignment
The Worksheet
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Finish the short worksheet you are being given now. We will try
to go over this sheet before the end of the class so use your
time effectively.
Textbook Pages
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Some of you may already
have read Pages 65 – 67 in
the Science Focus 8
Textbook… those pages
cover Archimedes principle
which you need to know