Your Comments
Hooray for 8th grade science! this is fun!
I LOVED THIS TOPIC IN HIGH SCHOOL! Too bad we only spend one lecture on this :(
…on a bit of a tangent, pressure was defined as a F/A but stress is also a F/A so I'm
curious as to whether or not pressure is a special case of stress or stress a special case
of stress? Or do I just have them all messed up? I'm sure I'll be learning about this more
in further class but thought I'd ask.
Have you heard that entropy isn't what it used to be?
Since we are about 70% water, does 30 percent of our body float above water? Hello,
have a nice day! I'm the SQUISHER! WHOA!
Play some country music before lecture tomorrow.
Shout out to the Concrete Canoe Team! :D
Did Archimedes test buoyancy in one of those community bathing centers they had back
in the day?
What do professors do during the summer? Ive always assumed my teachers crawled in
a hole until mid-August
Please do a hard problem in class.
I wonder if I will see my comment on the board today. Probably not.
Do you want to hang out and get soft tacos sometime? I'll buy :D
Mechanics Lecture 25, Slide 1
Clicker Question
Combined exam: Tuesday May 7 at 7 pm.
Conflict exam: Thursday May 9 at 7 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.
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 a 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.
Mechanics Lecture 25, Slide 5
Pressure changes with depth
Think of it like this: The pressure at some depth is due to the
weight of everything above that depth.
Mechanics Lecture 25, Slide 6
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
A) It has more mass over it applying
pressure
B) The points are at the same depth so they
will have the same pressure.
Mechanics Lecture 25, Slide 7
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 8
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 9
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 10
Measuring Patm
Hwater ~ 10 m (33 ft) for normal Patm
Hmercury ~ 30” for normal Patm
Mechanics Lecture 25, Slide 11
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 13
Clicker Question
In general, how close together are
the atoms in solids, liquids & gases:
solid
liquid
gas
A)
B)
C)
Mechanics Lecture 25, Slide 14
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 15
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) Real life example, global warming will raise the see
level not reduce it.
B) Because ice is mostly air, the air will escape leaving
only a relatively small portion of water left.
C) The cube weighs the same whether it is melted or
frozen. If the weight of the cube is not causing the water
to over-flow now, then it shouldn't even when it melts.
Mechanics Lecture 25, Slide 16
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.
How is it that only about 1/3
or even 1/4 of the boat is
submerged if the boat is
made out of so much
metal? I'd imagine the boat
is extremely heavy, so is
that same volume of water
really equal to that boat's
weight?
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 17
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 18
PATM
PI
PB
Mechanics Lecture 25, Slide 19
PATM
PB
PB  PATM = w gh1
PB = PATM  w gh1
Mechanics Lecture 25, Slide 20
PI
PB
PB  PI = w gh2
PI = PB  w gh2
Mechanics Lecture 25, Slide 21
PATM
PI
PI  PATM = oil gh3
PI  PATM
oil =
gh3
Mechanics Lecture 25, Slide 22
PATM
PI
PI  PATM =  glycerin gh3
PI = PATM   glycerin gh3
Mechanics Lecture 25, Slide 23
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 24
PI
h2
PB = PI  w gh2
PB
We know both PI and h2 (prev. slides)
Mechanics Lecture 25, Slide 25