Your Comments
I like this stuff!!!!!! -said by me for the first time ever when talking about physics
Well this pre-lecture was alright. It was an ICE change of pace from the difficult wave lectures.
now I know why icebergs can be dangerous to ships because sailors can only see 10% of the
volume.
I'm an Atmospheric Science major and I just love how we are discussing atmospheric pressures.
It makes me feel all tingly inside! Ahhh I need this break to start soon.
This stuff semms kinda randomly tossed on the end.
I have never been on the board, so please answer this question! How do you
recommend studying for the final? Will doing practice midterm exams help? Assuming
the final is fairly equivalent to that of the midterms?
I thought this was the most straight forward lecture all semester.
I hope you have some great demos.
I already hate E&M before even registering for the class
Will you be teaching 212 as well? I heard that the concepts
are a lot harder than this, then I would need a good lecturer
for that as well...
"An ice-cube is floating in a class filled with water as shown above." I'll expect to see a
demo where we fill a classroom with water!
While watching this prelecture, I accidentally dropped my Oreo into my glass of milk. Oh
how I wish Oreos were less dense than milk.
Mechanics Lecture 25, Slide 1
Your Question
I looked at homework 26 and saw that there is only one
deadline, but one of the questions still has a clock next to it.
Wouldn't be more fair to remove the clock from this problem
because there is no 80% deadline?
80% deadlines for units 24, 25 & 26 extended to 8am Monday December 17
Mechanics Lecture 25, Slide 2
Your Question
Would you rather fight 100 duck-sized
horses, or one horse-sized duck?
A) 100 duck-sized horses
B) One horse-sized duck
Mechanics Lecture 25, Slide 3
Clicker Question
Combined exam: Friday Dec 14 at 1:30 pm.
Conflict exam: Wednesday Dec 19 at 1:30 pm.
A: I have checked my schedule already. I will take the combined final.
B: I have checked my schedule already. I will take the conflict final.
C: I have not checked my schedule but I will do so today.
Use your grade-book to check which one you
are signed up for and to switch if needed.
There is NO double conflict for final exams.
Mechanics Lecture 17, Slide 4
Physics 211
Lecture 25
Today’s Concepts:
A) Static Fluids
B) Archimedes Principle
Mechanics Lecture 25, Slide 5
Pressure due to a Liquid or Gas
Forces are caused
by the movement
and collisions of
molecules.
Mechanics Lecture 25, Slide 6
Pressure due to a Liquid or Gas
Pressure is the force
per unit area due to
these collisions.
“…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 7
Recall where this comes from
A
Doesn't depend on shape
Mechanics Lecture 25, Slide 8
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 don't understand how water has more weight as you go lower”
Mechanics Lecture 25, Slide 9
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
I think they're the same because they are
same position in height...however I wonder it
it's possible that the fact that the ship
displaces water has anything to do with it.
Mechanics Lecture 25, Slide 10
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
Mechanics Lecture 25, Slide 11
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 12
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 13
Measuring Patm
Hwater ~ 10 m (33 ft) for normal Patm
Hmercury ~ 30” for normal Patm
Mechanics Lecture 25, Slide 14
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 16
Clicker Question
In general, how close together are
the atoms in solids, liquids & gases:
solid
liquid
gas
A)
B)
C)
Mechanics Lecture 25, Slide 17
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.
Mechanics Lecture 25, Slide 18
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 ice cube contains water, it's particles are just
closer together. when the ice cube melts, the particles will
separate from each other and will cause the water level to go up…
B) ice is less dense than water so when it melts the
volume goes down.
C) The volume of water displaced by the ice cube is equal
in weight to the ice cube, meaning they have the same
mass and therefore the same volume when the ice melts.
“So if the water level doesn't change, why worry about global warming?”
Mechanics Lecture 25, Slide 19
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 20
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
Mechanics Lecture 25, Slide 21
PATM
PI
PB
Mechanics Lecture 25, Slide 22
PATM
PB
PB  PATM =  w gh1
PB = PATM   w gh1
Mechanics Lecture 25, Slide 23
PI
PB
PB  PI =  w gh2
PI = PB   w gh2
Mechanics Lecture 25, Slide 24
PATM
PI
PI  PATM =  oil gh3
 oil =
PI  PA T M
gh3
Mechanics Lecture 25, Slide 25
PATM
PI
PI  PA TM =  glycerin gh3
PI = PA TM   glycerin gh3
Mechanics Lecture 25, Slide 26
PATM
h1
PI
h2
PB = PATM   w gh1
PB = PI   w gh2
PB
h1  h2 =
h1  h2 = 0.5 (same as before)
So we know h1, h2, h3
PI  PA T M
wg
= 0 .3 9 1
Solve for h1, h2
d = h1  h2  h3
Mechanics Lecture 25, Slide 27
PI
h2
PB = PI   w gh2
PB
We know both PI and h2 (prev. slides)
Mechanics Lecture 25, Slide 28