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Demo. 1 cm aluminum cube (it ’ s weight is small) Measure its mass w balance. Kid ’ s baseball (hollow sphere of plastic) it ’ s weight is more than the cube. The heavy one floats!. Buoyancy. Why do boats float on water?. Demonstration.

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Demo

  • 1 cm aluminum cube (it’s weight is small) Measure its mass w balance.

  • Kid’s baseball (hollow sphere of plastic) it’s weight is more than the cube.

  • The heavy one floats!




Demonstration
Demonstration

  • Ping pong ball on the table. Why doesn’t the ball fall down?

  • Ping pong ball in the aquarium. Why doesn’t the ball fall down (“sink”) ?

  • In both cases, there is an upward force on the ball!

  • At its core, it is as simple as that, the water pushes up; we call it the “buoyant” force.


Hey are you saying that the water pushes up on the ball

Hey, are you saying that the water pushes up on the ball?”

How can that be?

Why would water do that to a ball?

Answer is coming up…


Have you ever jumped off of a diving board and gone down about 10 feet below the surface of the water?

What does that feel like?


The deeper you go, the greater the pressure. about 10 feet below the surface of the water?What is wrong about this drawing of the water pressure on a box that is underwater?


Is this better or worse
Is this better or worse? about 10 feet below the surface of the water?


Ok why is the pressure greater the lower you go

OK, about 10 feet below the surface of the water?why is the pressure greater the lower you go?

To see why, let’s build the marshmallow tower!


One marshmallow
One marshmallow about 10 feet below the surface of the water?


Add one on top
Add one on top. about 10 feet below the surface of the water?


Keep stacking
Keep Stacking! about 10 feet below the surface of the water?


Something is happening
Something is happening … about 10 feet below the surface of the water?


What s happening to the one on the bottom
What about 10 feet below the surface of the water?’s happening to the one on the bottom?


Did you ever build a human pyramid
Did you ever build a human pyramid? about 10 feet below the surface of the water?


What is the worst job in the human pyramid and why is that job so bad
What is the about 10 feet below the surface of the water?worst job in the human pyramid (and why is that job so bad)?


Why is the pressure greater at greater depth
Why about 10 feet below the surface of the water? is the pressure greater at greater depth?

  • What keeps the people at the top of the pyramid from falling?

  • What keeps the water at the top of the ocean from falling?

  • What keeps the air, high in the atmosphere, from falling to earth?


Why is pressure in a fluid greater at greater depths
Why about 10 feet below the surface of the water? is pressure in a fluid greater,at greater depths?

  • Deep in the ocean, the water molecules are pressed together by the weight of the molecules above. At the bottom the molecules are densely packed. It takes a large force to squeeze water, and in return, the molecules are pushing back with a large force. This is why the pressure down there is so crushingly huge.

  • The same thing is happening low in the atmosphere: The air molecules down here are being pressed together by the molecules up there. The density of oxygen at the earth’s surface is much greater than the density where airplanes fly (that’s why planes carry oxygen masks). The air pressure up there is much less than the air pressure down here.


So, since the pressure increases the lower you go, about 10 feet below the surface of the water?every object is “buoyed” up.There is a _ _ _ _ _ _ _ force on every object in a fluid.


So, since the pressure increases the lower you go, about 10 feet below the surface of the water?every object is “buoyed” up.There is a buoyant force onevery object in a fluid.


Every about 10 feet below the surface of the water? object in air or in water has an upward force on it.


Review fill in your blank spaces
Review … about 10 feet below the surface of the water?{Fill in your blank spaces}


Why about 10 feet below the surface of the water?

do objects

float?


Why about 10 feet below the surface of the water?

do objects

float?

The fluid

pushes up.


Why about 10 feet below the surface of the water?

do objects

float?

Why

does the fluid

push up?

The fluid

pushes up.


Why about 10 feet below the surface of the water?

do objects

float?

Why

does the fluid

push up?

The fluid

pushes up.

The pressure on the bottom

of the object is greater

than the pressure on the top.


Why about 10 feet below the surface of the water?

do objects

float?

Why

does the fluid

push up?

The fluid

pushes up.

Why

is the

pressure greater

on the

bottom?

The pressure on the bottom

of the object is greater

than the presure on the top.


Why about 10 feet below the surface of the water?

do objects

float?

Why

does the fluid

push up?

The fluid

pushes up.

Why

is the

pressure greater

on the

bottom?

The pressure on the bottom

of the object is greater

than the presure on the top.

The molecules that are lower

are squished by the molecules

that are higher.


Why about 10 feet below the surface of the water?

do objects

float?

Why

does the fluid

push up?

The fluid

pushes up.

Why

is the

pressure greater

on the

bottom?

The pressure on the bottom

of the object is greater

than the presure on the top.

The molecules that are lower

are squished by the molecules

that are higher.

This produces the

buoyant force that

holds up ships

and balloons.


What about a rock
What about a rock? about 10 feet below the surface of the water?

If there is always a buoyant force, then why do some things sink?


Why doesn about 10 feet below the surface of the water?’t every object float?


Why doesn about 10 feet below the surface of the water?’t every object float?(Remember that just because there is a force on an object, that does not mean the object will go in the direction of the force. For example, gravity is putting a downward for on you right now, but you are not moving down.)


Ok then why exactly do some objects sink and some float
OK then, why, exactly, do some objects sink, and some float? about 10 feet below the surface of the water?


Consider a brick that someone has just dropped into some water.Draw the forces thatact on the brick…



Ladies and gentlemen a sinker
Ladies and Gentlemen … water.A sinker!

Buoyant Force

Weight


Consider a basketball that someone has managed to get underwater. They release the ball.Please draw the forces…


Please label these forces1
Please label these forces: underwater. They release the ball.


A floater
A floater! underwater. They release the ball.

B

W = Mg

So, why do some objects

sink and some float?


Fill in the blanks with a or a
Fill in the blanks underwater. They release the ball.with a ‘<’ or a ‘>’.

If it floats, then B W.If it sinks, then B W.


Fill in the blanks with a or a1
Fill in the blanks underwater. They release the ball.with a ‘<’ or a ‘>’.

If it floats, then B>W.If it sinks, then B<W.


Checkpoint page
Checkpoint page: underwater. They release the ball.

  • The lower you are, the greater the P_ _ _ _ _ _ _.


Checkpoint page1
Checkpoint page: underwater. They release the ball.

  • The lower you are, the greater the Pressure.

  • B exists because the bottom and top of an object are at d _ _ _ _ _ _ _ _ depths.


Checkpoint page2
Checkpoint page: underwater. They release the ball.

  • The lower you are, the greater the Pressure.

  • B exists because the bottom and top of an object are at

    different depths.

  • The B on an object depends on its s _ _ _ (not its weight).


Checkpoint page3
Checkpoint page: underwater. They release the ball.

  • The lower you are, the greater the Pressure.

  • B exists because the bottom and top of an object are at

    different depths.

  • The B on an object depends on its size (not its weight).

  • An ice cube rises and a steel cube sinks, but the B is the s _ _ _.


Checkpoint page4
Checkpoint page: underwater. They release the ball.

  • The lower you are, the greater the Pressure.

  • B exists because the bottom and top of an object are at

    different depths.

  • The B on an object depends on its size (not its weight).

  • An ice cube rises and a steel cube sinks, but the B is the same.

  • What is different about an ice cube and a steel cube?


Checkpoint page5
Checkpoint page: underwater. They release the ball.

  • The lower you are, the greater the Pressure.

  • B exists because the bottom and top of an object are at

    different depths.

  • The B on an object depends on its size (not its weight).

  • An ice cube rises and a steel cube sinks, but the B is the same.

  • What is different about an ice cube and a steel cube?

    The weight of the cubes is different!

  • At the very bottom of the ocean is a rock. Is there a buoyant force on that rock?


Checkpoint page6
Checkpoint page: underwater. They release the ball.

  • The lower you are, the greater the Pressure.

  • B exists because the bottom and top of an object are at

    different depths.

  • The B on an object depends on its size (not its weight).

  • An ice cube rises and a steel cube sinks, but the B is the same.

  • What is different about an ice cube and a steel cube?

    The weight of the cubes is different!

  • At the very bottom of the ocean is a rock. Is there a buoyant force on that rock? YES, but its weight is greater than the buoyant force that acts on the rock.


Is there a buoyant force on you right now
Is there a Buoyant force on you right now? underwater. They release the ball.

  • Give a reason for or against.

  • Why do helium balloons float?

  • Why do air-filled balloons sink?

  • Why do you sink in air but float in water?


Beach ball why does the ball stop rising
Beach Ball. underwater. They release the ball.Why does the ball stop rising?

6

7

8

5

4

3

2

1


The ball stopped rising because
The ball stopped rising because: underwater. They release the ball.

  • The buoyant force decreased; it became equal to the weight.

  • The buoyant force decreased when less of the ball was underwater.

  • The buoyant force is proportional to how much volume is under water.

  • B~ V


An equation for buoyant force
An underwater. They release the ball.Equation for Buoyant Force

B = [Density]  [Volume]  [g]

  • D = density (mass/volume) of the fluid displaced

  • V = the submerged volume of the object

  • g = 9.8 m/s2

    B = DVg


Just before you jump in a pool, you take a deep breath of air. How does this change the buoyant force on you?B = DgV


Just before you jump in a pool, you take a deep breath of air. How does this change the buoyant force on you?B = DgV

  • You have changed the Density of the fluid.

  • You have changed g = 9.8 to a new value.

  • You have increased your Volume.

  • You have decreased your Volume.


Why is it impossible for an object to float on top of the ocean with none of the object under water
Why is it impossible for an object to float air. How does this change the buoyant force on you?on top of the ocean (with none of the object under water?


Calculate the buoyant force
Calculate the Buoyant Force air. How does this change the buoyant force on you?

  • A shoe box has a volume of about 0.005 m3.

  • The density of air is about 1.25 kg/m3.

  • The density of water is 1000 kg/m3.

  • What is the Buoyant force in air?

  • B = DVg = (1.25)(.005)(9.8) = 0.06 N

  • What is the Buoyant force in water?

  • B = DVg = (1000)(.005)(9.8) = 50 N

    If the box weighs 40 N, does it float?

    What would it need to weigh to float in air?


Float sink a different way

Float - Sink: A different way air. How does this change the buoyant force on you?

You don’t need to know the weight or the buoyant force!


Does ice ever sink in your glass of water
Does ice air. How does this change the buoyant force on you?ever sink in your glass of water?


Does ice ever sink in your glass of water1
Does ice air. How does this change the buoyant force on you?ever sink in your glass of water?

No. Even if the ice is very heavy (iceberg) it always floats in water.


Does a pebble ever float in a puddle
Does a pebble air. How does this change the buoyant force on you?ever float in a puddle?


Does a stone ever float in a puddle no no matter how light a stone always sinks in a puddle
Does a stone air. How does this change the buoyant force on you?ever float in a puddle?No. No matter how light, a stone always sinks in a puddle.


The pattern
The pattern: air. How does this change the buoyant force on you?

  • There is some quality of ice that always makes it float in water, and some quality of granite that always makes it sink:


The pattern1
The pattern: air. How does this change the buoyant force on you?

  • There is some quality of ice that always makes it float in water, and some quality of granite that always makes it sink:

Density


Density mass volume
Density = Mass / Volume air. How does this change the buoyant force on you?


Density mass volume1
Density = Mass / Volume air. How does this change the buoyant force on you?

  • Mass is the amount of _ _ _ _ _ _ an object has. [Think about protons and neutrons.]


Density mass volume2
Density = Mass / Volume air. How does this change the buoyant force on you?

  • Mass is the amount of matter an object has. [Think about protons and neutrons.]

  • Volume is the amount of _ _ _ _ _ that an object has (think gallons).


Density mass volume3
Density = Mass / Volume air. How does this change the buoyant force on you?

  • Mass is the amount of matter an object has. [Think about protons and neutrons.]

  • Volume is the amount of space that an object has (think gallons).

  • Low density materials (styrofoam) take up a lot of _ _ _ _ _ _ without much _ _ _ _.


Density mass volume4
Density = Mass / Volume air. How does this change the buoyant force on you?

  • Mass is the amount of matter an object has. [Think about protons and neutrons.]

  • Volume is the amount of space that an object has (think gallons).

  • Low density materials (styrofoam) take up a lot of volume without much mass.

  • High density materials (steel) have a lot of _ _ _ _ in a small _ _ _ _ _ _.


Density mass volume5
Density = Mass / Volume air. How does this change the buoyant force on you?

  • Mass is the amount of matter an object has. [Think about protons and neutrons.]

  • Volume is the amount of space that an object has (think gallons).

  • Low density objects (styrofoam) take up a lot of volume without much mass.

  • High density materials (steel) have a lot of mass in a small volume.


Sinking b w is the same as d f d o
Sinking (B < W) is air. How does this change the buoyant force on you?the same as DF DO.


Sinking b w is the same as d f d o1
Sinking (B < W) is air. How does this change the buoyant force on you?the same as DF < DO.


Sinking b w is the same as d f d o floating b w is the same as d f d o
Sinking (B < W) is air. How does this change the buoyant force on you?the same as DF < DO.Floating (B > W) isthe same as DF DO.


Sinking b w is the same as d f d o floating b w is the same as d f d o1
Sinking (B < W) is air. How does this change the buoyant force on you?the same as DF < DO.Floating (B > W) isthe same as DF > DO.


Sinking b w is the same as d f d o floating b w is the same as d f d o2
Sinking (B<W) is the same as D air. How does this change the buoyant force on you?F<DO.Floating (B>W) is the same as DF>DO.

B < W

DFVg < Mg

DFV < M

DF < M÷V

DF < DO


Steel ships float
Steel ships float air. How does this change the buoyant force on you?

  • A steel ship is mostly _ _ _.


Steel ships float1
Steel ships float air. How does this change the buoyant force on you?

  • A steel ship is mostly air .

  • The average Density of a ship is less than the density of water.


Why do hot air balloons rise in air
Why do Hot-Air Balloons rise in air? air. How does this change the buoyant force on you?


Why does smoke rise
Why does smoke rise? air. How does this change the buoyant force on you?


Still another view archimedes
Still air. How does this change the buoyant force on you?another view - Archimedes

  • B = gDV {V = volume submerged}

  • B = (DV)g

  • B = Mg !

  • The buoyant force is equal to the weight of the fluid that is moved aside by the object that is in the fluid.


Archimedes
Archimedes air. How does this change the buoyant force on you?

  • B = gDV

  • B = (DV)g

  • B = Mg !

  • The buoyant force is equal to the weight of the fluid that is moved aside by the object that is in the fluid.

  • “B = the weight of the water displaced.”

  • “Eureka!” Means “I found it !”


Archimedes1
Archimedes air. How does this change the buoyant force on you?

  • For a rock at the bottom of a river, how does the weight of the water displaced compare to the weight of the rock?

  • For a swimmer, how does the volume displaced compare to the volume of the swimmer?


Which ship has the greater buoyant force acting on it don t be fooled
Which ship has the greater buoyant force acting on it. air. How does this change the buoyant force on you?Don’t be fooled!


What is the volume of the displaced water equal to
What is the air. How does this change the buoyant force on you?volume of the displaced water equal to?


What is the weight of the displaced water equal to
What is the air. How does this change the buoyant force on you?weightof the displaced water equal to?


How does the weight of the displaced water compare to the weight of the stone
How does the air. How does this change the buoyant force on you?weight of the displaced water compare to the weight of the stone?


What s the message of these pictures
What air. How does this change the buoyant force on you?’s the message of these pictures?

First

Second


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