Chapter
9
Solids,
Liquids, & Gases
Hydrostatic
Pressure:
The
pressure P exerted by a fluid is the magnitude F of the force
acting perpendicular to a surface embedded in the fluid divided by
the area A over which the force acts:
(2)
Units: which
is a pascal (Pa).
Example
4
Find
the pressure exerted on the skin of a balloon if you press with
a force of 2.1 N using (a) your finger or (b) a needle. Assume
the area of your fingertip is , and the area of the needle tip is
. (c) Find the minimum force necessary to pop the balloon with
the needle, given that the balloon pops with a pressure of .
Solution:
(a)
(b)
(c)
Example
5
The
mattress of a water bed is 2.00 m long by 2.00 m wide and 30.0 cm
deep. (a) Find the weight of the water in the mattress. (b) Find
the pressure exerted by the water on the floor when the bed rests
in its normal position. Assume that the entire lower surface
of the bed makes contact with the floor.
Solution:
(a)
(b)
Gravity
is the principal cause of hydrostatic pressure.
Pressure
at some height in a liquid:
(3)
Total
pressure at any point within a liquid:
(4)
atm
=
Example 6
The
Titanic was found in 1985 lying on the bottom of the North Atlantic
at a depth of 2.5 miles. What is the pressure at this depth?
Solution:
Example
7
A
cubical box 20.00 cm on a side is completely immersed in a fluid.
At the top of the box the pressure is 105.0 kPa; at the bottom
the pressure is 106.8 kPa. What is the density of the fluid?
Solution:
Example
8
One
day, while swimming below the surface of the ocean, you let out
a small bubble of air from your mouth. As the bubble rises toward
the surface, does its diameter (a) increase, (b) decrease, or
(c) stay the same?
Solution:
(a)
The diameter of the bubble increases.
Pressure
in a Container
Points
A, B and C are at the same pressure.
The
pressure at a given depth is independent of the shape of the
vessal.
Points
A and C are at the same pressure and Points B and D are at the
same pressure.
Example
9
A
Ushaped tube is filled mostly with water, but a small amount
of vegetable oil has been added to one side, as shown in sketch.
The density of the water is ,
and the density of the vegetable oil is 920 .
If the depth of the oil is 5.00 cm, what is the difference in
level h between the top of the oil on one side of the U and the
top of the water on the other side?
Solution:
Find
the pressure at point A:
Find
the pressure at point B:
Set
the pressures equal and solve the :
Find
the difference between the heights:
On to Static Part 2
