• Chapter 15, fluids
HW-8 due Tuesday, Nov 15
Wednesday: Read through Chapter 16
Physics 207: Lecture 21, Pg 1
Pressure (force per unit area)
SI unit for pressure is 1 Pascal = 1 N/m2
The atmospheric pressure at sea-level is
1 atm = 1.013 x105 Pa
= 1013 mbar
= 760 Torr
= 14.7 lb/ in2 (=PSI)
Physics 207: Lecture 21, Pg 2
Incompressible fluids (liquids)
What is the pressure at the bottom of the
Physics 207: Lecture 21, Pg 3
What if there is outside gas?
Physics 207: Lecture 21, Pg 4
Physics 207: Lecture 21, Pg 5
What is the pressure 10m down?
=P0+(1000 kg/m3)(10 m/s2) (10 m)
= approximately 2 atm
Home exercise: what is the pressure 6 miles
Physics 207: Lecture 21, Pg 6
Consider the open, connected container shown
below. How would the two heights compare?
Physics 207: Lecture 21, Pg 7
Pressure vs. Depth
In a connected liquid, the pressure is the same at
all points through a horizontal line.
Physics 207: Lecture 21, Pg 8
Pressure Measurements: Barometer
Invented by Torricelli
A long closed tube is filled with mercury
and inverted in a dish of mercury
The closed end is nearly a vacuum
Measures atmospheric pressure as
1 atm = 0.760 m (of Hg)
Physics 207: Lecture 21, Pg 9
Suppose we weigh an object in air (1) and
in water (2).
How do these weights compare?
a) W1 < W2
b) W1 = W2
c) W1 > W2
Physics 207: Lecture 21, Pg 10
=ρ g Vobject
=weight of the fluid
displaced by the
Physics 207: Lecture 21, Pg 11
Float or sink?
If we immerse the object completely in the liquid:
weight of the object < bouyant force
ρobject Vobject< ρfluid Vobject
ρobject < ρfluid
How does a steel ship float?
A)ρsteel < ρwater
B)overall density of the ship < ρwater
C) none of the above
Physics 207: Lecture 21, Pg 12
If the object floats, then we can find the portion of the
object that will be immersed in the fluid
Vimmersed ρfluid g =Vobject ρobject g
Vimmersed ρfluid =Vobject ρobject
Physics 207: Lecture 21, Pg 13
Any change in the pressure applied to an enclosed
fluid is transmitted to every portion of the fluid and to
the walls of the containing vessel.
Physics 207: Lecture 21, Pg 14
Pascal’s Principle in action:
Hydraulics, a force amplifier
Consider the system shown:
A downward force F1 is applied
to the piston of area A1.
This force is transmitted through
the liquid to create an upward
Pascal’s Principle says that
increased pressure from F1
(F1/A1) is transmitted
throughout the liquid.
P1 = P2
F1 / A1 = F2 / A2
A2 / A1 = F2 / F1
Physics 207: Lecture 21, Pg 15
To describe fluid motion, we need something that describes
Ideal fluid model:
No viscosity (no friction).
Physics 207: Lecture 21, Pg 16
Types of Fluid Flow
Physics 207: Lecture 21, Pg 17
Keep track of a small portion of the fluid:
Physics 207: Lecture 21, Pg 18
A1v1 : units of m2 m/s = volume/s
A2v2 : units of m2 m/s = volume/s
Physics 207: Lecture 21, Pg 19