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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!


Buoyancy


Why do boats float on water?


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?”

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.What is wrong about this drawing of the water pressure on a box that is underwater?


Is this better or worse?


OK, why is the pressure greater the lower you go?

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


One marshmallow


Add one on top.


Keep Stacking!


Something is happening …


What’s happening to the one on the bottom?


Did you ever build a human pyramid?


What is the worst job in the human pyramid (and why is that job so bad)?


Why 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?

  • 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, every object is “buoyed” up.There is a _ _ _ _ _ _ _ force on every object in a fluid.


So, since the pressure increases the lower you go, every object is “buoyed” up.There is a buoyant force onevery object in a fluid.


Every object in air or in water has an upward force on it.


Review …{Fill in your blank spaces}


Why

do objects

float?


Why

do objects

float?

The fluid

pushes up.


Why

do objects

float?

Why

does the fluid

push up?

The fluid

pushes up.


Why

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

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

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

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?

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


Why doesn’t every object float?


Why doesn’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?


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


Please label these forces:


Ladies and Gentlemen … 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 forces:


A floater!

B

W = Mg

So, why do some objects

sink and some float?


Fill in the blankswith a ‘<’ or a ‘>’.

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


Fill in the blankswith a ‘<’ or a ‘>’.

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


Checkpoint page:

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


Checkpoint page:

  • The lower you are, the greater the Pressure.

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


Checkpoint page:

  • 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 page:

  • 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 page:

  • 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 page:

  • 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 page:

  • 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?

  • 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?

6

7

8

5

4

3

2

1


The ball stopped rising because:

  • 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

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?


Calculate the Buoyant Force

  • 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

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


Does ice ever sink in your glass of water?


Does ice 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 stone ever float in a puddle?No. No matter how light, a stone always sinks in a puddle.


The pattern:

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


The pattern:

  • 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

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


Density = Mass / Volume

  • 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 / Volume

  • 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 / Volume

  • 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 / Volume

  • 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) isthe same as DF DO.


Sinking (B < W) isthe same as DF < DO.


Sinking (B < W) isthe same as DF < DO.Floating (B > W) isthe same as DF DO.


Sinking (B < W) isthe same as DF < DO.Floating (B > W) isthe same as DF > DO.


Sinking (B<W) is the same as DF<DO.Floating (B>W) is the same as DF>DO.

B < W

DFVg < Mg

DFV < M

DF < M÷V

DF < DO


Steel ships float

  • A steel ship is mostly _ _ _.


Steel ships float

  • 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 does smoke rise?


Still 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

  • 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 !”


Archimedes

  • 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!


What is the volume of the displaced water equal to?


What is the weightof the displaced water equal to?


How does the weight of the displaced water compare to the weight of the stone?


What’s the message of these pictures?

First

Second


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