Your Comments
MK
I thought that buoyancy and pressure in relation to depth were the most confusing. I just
don't understand where exactly all of the equations come from. Other that that, doesn't
seem to be too complicated.
I think this is Fairly easy compared to the previous material. Even though the homework was
LONG I still managed to finish it! I just don't understand why you would make our last
homework assignment that long. Oh yea will there be office hours after Monday?
Why can't the rest of physics be this easy?
AND THAT'S HOW TITANIC HAPPENED. THE END. (saw titanic right before i did the prelecture,
connects very well with the floating and sinking part)
I think one of my friends really wishes you were her grandpa. Do you get this kind of thing
often?
This stuff doesn't seem too bad, but I think I speak for most of us when I say that I'm feeling
pretty burnt out. Please have mercy in regards to homework and the final.
What do you call a pencil with a broken tip?? Nevermind.. It's pointless! haha.. Ha
Can we prove that Leonardo DiCaprio would have fit on the door also, because that is just
some foolishness.
I'm sure you've been receiving more and more "ZOMGGGG TWO LECTURES LEFT, PLEASE PUT
MY COMMENT UP B4 THE YEAR ENDS!!!!". Just wanted to let you know you can represent all
of us underdogs by putting this one up :D
Mechanics Lecture 25, Slide 1
Make sure you don’t have a double confict
Combined final: Wednesday May 9 at 1:30 pm.
Conflict final: Thursday May 10 at 8 am.
“If what really matters in college is what you learn
throughout the semester and not your grade, does that mean
an A on the final will get you an A in this course?”
If you want to know how to calculate your grade look at the first lecture
Mechanics Lecture 17, Slide 2
Physics 211
Lecture 25
Today’s Concepts:
A) Static Fluids
B) Archimedes Principle
Mechanics Lecture 25, Slide 3
Pressure due to an Liquid or Gas
Forces are caused
by the movement
and collisions of
molecules.
Mechanics Lecture 25, Slide 4
Pressure due to a Liquid or Gas
Pressure is the force
per unit area due to
these collisions.
“These concepts are understandable and not difficult. I was
wondering, however, if the shape of a glass is related to this
principle? I noticed that the shape of the glass tapers as it goes
down. Is this to reduce the force of the liquid on the glass?”
Mechanics Lecture 25, Slide 5
Recall where this comes from
A
Doesn't depend on shape
Mechanics Lecture 25, Slide 6
Pressure changes with depth
Think of it like this: The pressure at some depth is due to the
weight of everything above that depth.
“I dont understand how water has more weight as you go lower”
Mechanics Lecture 25, Slide 7
CheckPoint
A yacht floats in a large pool.
Compare the water pressure
directly below the hull of the
yacht (point A) to the
pressure at the same depth
beside the ship (point B).
The weight of the ship is
the same as the weight of
the displaced water
A
B
A) PA > PB
B) PA = PB
C) PA < PB
Points at the same depth have the
same pressure
Mechanics Lecture 25, Slide 8
Clicker Question
Two dams of equal height and width prevent water from
entering a central basin. Compare the net force due to
the water on the two dams.
A) FA > FB
B) FA= FB
C) FA< FB
“As a scuba diver, I find a lot of this rather interesting. As a
student I kind of just want to be studying for finals.”
Mechanics Lecture 25, Slide 9
Atmospheric Pressure
The weight of atmosphere.
Air molecules are colliding
with you right now!
Patm = 1x105 N/m2
= 14.7 lbs/in2
Example: Sphere with r = 0.1 m
A = 4pr2 = .125 m2
F = 12,000 Newtons (over 2,500 lbs)!
Hemisphere Demo
Crush can
Glass
Mechanics Lecture 25, Slide 10
Does Air Really Weigh That Much?
8 km
high
stack
weighs
105 N
Patm at 16 km its only
10% of Patm at sea level
1 mole N2 = 28 g
1 mole = 22.4 liters at STP
1m
1m
1 m3 N2 at STP
M = 1.25 kg
Weight = 12 N
Mechanics Lecture 25, Slide 11
Measuring Patm
Hwater ~ 10 m (33 ft) for normal Patm
Hmercury ~ 30” for normal Patm
Mechanics Lecture 25, Slide 12
Clicker Question
Can you drink water through a long straw from a
platform 40 feet above a glass of water.
A) Yes – you just have to suck hard enough.
B) No – it cant work no matter how hard you suck.
40 feet
Buoyancy
A floating object displaces its own weight
Mechanics Lecture 25, Slide 14
Excellent Question
Alright, call me naive, but why does the buoyant force always point up? I
understand it perfectly whenever the object is floating or suspended in the liquid,
but not when the object is at the bottom of the container. Since the buoyant force
is caused by the collisions of the molecules of liquid on the object, wouldn't the
buoyant force at the bottom not face upward since there's no liquid molecules to
collide in the upward direction? Would the buoyant force then just be the normal
force given off by the container?
CheckPoint
Suppose you float an ice-cube in a glass of water, and that
after you place the ice in the glass the level of the water is
at the very brim. When the ice melts, the level of the water
in the glass will:
A) Go up, causing the water to spill out of the glass.
B) Go down.
C) Stay right at the brim.
“Bring some icecubes and lets have some drinks :)”
Mechanics Lecture 25, Slide 16
CheckPoint
When the ice melts, the level of the water in the glass will:
A) Go up, causing the water to spill out of the glass.
B) Go down.
C) Stay right at the brim.
A) The water level will rise. I learned that from Al Gore
in "An Inconvenient Truth"
B) volume of ice is greater than volume of water
C) The melted water has exactly the same mass as
the ice cube, and the volume of water displaced is
equal to the mass of the ice cube.
“The ice melting in cup of water question confuses me. So if the
water level doesn't change, why worry about global warming?”
Mechanics Lecture 25, Slide 17
Clicker Question
Which weighs more:
A) A large tub filled to the brim with water.
B) A large tub filled to the brim with water
with a battle-ship floating in it.
C) They will weigh the same.
Tub of water + ship
Tub of water
Overflowed water weighs
the same as the ship
Weight of ship = Buoyant force = Weight of displaced water
Mechanics Lecture 25, Slide 18
CheckPoint
When you float an ice cube in water you notice that
90% of it is submerged beneath the surface.
Now suppose you put the same ice cube in a glass of
some liquid whose density is less than that of water.
How much of the ice cube will be submerged?
A) More than 90%
B) 90%
C) Less than 90%
A) It must displace more of the liquid to equal its weight
B) lower density meant that the ice cube will be heavier
C) If the density of the the liquid is less than the
density of water than there will be less buoyant force
on the ice cube and it will sink farther.
Mechanics Lecture 25, Slide 19
PATM
PI
PB
Mechanics Lecture 25, Slide 20
PATM
PB
PB  PATM = w gh1
PB = PATM  w gh1
Mechanics Lecture 25, Slide 21
PI
PB
PB  PI = w gh2
PI = PB   w gh2
Mechanics Lecture 25, Slide 22
PATM
PI
PI  PATM = oil gh3
oil
PI  PATM
=
gh3
Mechanics Lecture 25, Slide 23
PATM
PI
PI  PATM =  glycerin gh3
PI = PATM   glycerin gh3
Mechanics Lecture 25, Slide 24
PATM
h1
PI
h2
PB = PATM  w gh1
PB = PI  w gh2
PB
PI  PATM
h1  h2 =
w g
h1  h2 = 0.5 (same as before)
So we know h1, h2, h3
= 0.391
Solve for h1, h2
d = h1  h2  h3
Mechanics Lecture 25, Slide 25
PI
h2
PB = PI  w gh2
PB
We know both PI and h2 (prev. slides)
Mechanics Lecture 25, Slide 26