Transcript Pressure

Archimede’s Principle
• An object immersed in a fluid
has an upward (buoyant) force
equal to the weight of the fluid it
displaces. FB = gV
• FB = buoyant force
•  = mass density of the fluid
• g = acc. due to gravity
• V = volume of the fluid displaced
Floating
• Since apparent weight = 0, the weight
of the object = weight of the fluid
displaced (or buoyant force).
Wapp
= 0 so Wobj = FB = Wfd
• Volume of the fluid displaced =
volume of the submerged part of the
object. Vfd = Vsubmerged
• density of the object < density of the fluid
Sinking
• The object can’t displace enough fluid
to equal its own weight. Wobj > FB
• The volume of the fluid displaced =
the volume of the object. Vfd = Vobj
• Wapp = Wair - FB
Wapp = apparent weight
Wair
= weight of object in air (= mg) FB =
buoyant force (= Wfd = fVg)
Pressure
• Force per unit area
• Units: Pa (N/m2)
• P = pressure
• F=force
• A= area
F
P
A
Pascal’s Principle
• Pressure applied to a fluid is
transmitted throughout that fluid
unchanged in all directions.
• Pressure increases in a fluid with
depth
• P = g h, where  = density of
fluid, g= acc. due to gravity, and
h = height of the fluid column
• The shape of the container has no
effect on pressure. Only Depth!
The pressure
at the
bottom of
the each is
the same.
Hydraulics
• Machines using hydraulics use fluids to
multiply forces.
• These machines have a small piston and a
large piston connected by a chamber filled
with hydraulic fluid.
• P1=P2 so… F1/A1 = F2/A2
Pressure is constant
• MA = F2 / F1 = A2 / A1
Bernouilli’s Principle
• As the velocity of a
fluid increases, the
pressure exerted by
that fluid decreases.
• (not valid for
turbulent flow)
Applications of Bernouilli’s
Principle
• Lift on an airplane wing - air has to
travel farther over the top of the
wing, so velocity is greater and
pressure is less on top of the wing
than underneath.
• Spoilers on a race car - opposite of
the airplane wing.