Download Chapter 13 Forces in Fluids

Chapter 13
Forces in Fluids
Fluid Pressure
Pressure
• Pressure- the result of a force distributed over
an area
Ex 1: A theater’s seat exerts less pressure
than a bicycle seat because the large padded
seat and back offer a larger area to support your
weight.
Ex 2: A sharp pencil point easily pokes a
sheet of paper, but an eraser does not. The
pencil point has a much smaller area than the
eraser, so it exerts a much greater pressure than
the eraser. The greater pressure exerted by the
pencil point allows it to pierce the paper easily.
Pressure Cont’d
• To calculate pressure, divide the force by the
area over which the force acts.
Pressure (N/m²) =Force (N) newtons
Area (m²) square meters
• Pascal (Pa)- the SI unit of pressure, equal to 1
newton per square meter (N/m²). The pascal is
named for French scientist Blaise Pascal.
Pressures are often stated in units of kilopascals
(kPa). 1 kPa=1000 Pa.
Pressure Cont’d
• Ex: A box with a weight of 2700 newtons is
resting on the ground. If the area of the
box touching the ground is 1.5 square
meters, what pressure does the box exert
on the ground?
Pressure=Force = 2700 N =1800 N/m²
Area
1.5 m²
1800 Pa= 1.8 kPa
1000
Pressure in Fluids
• Fluid- a substance or mixture that flows and has
no shape of its own. It assumes the shape of its
container. Liquids and gases: water, oil,
gasoline, air, and helium are fluids.
• A glass is filled with water. Because the water is
in contact with the walls and bottom of the glass,
it exerts pressure on these surfaces. The
amount of pressure exerted depends on several
factors.
Pressure in Fluids Cont’d
• Water pressure increases as depth
increases. The pressure in a fluid at any
given depth is constant, and it is exerted
equally in all directions.
• For a fluid that is not moving, depth and
the type of fluid are the two factors that
determine the pressure the fluid exerts.
Volume or weight does not affect
pressure.
Air Pressure and the Atmosphere
• Air pressure increases as the altitude
increases.
• When your ears pop, unbalanced
pressures (pressure between the inside of
your ears and the air outside) are
equalizing as air passes through a small
tube within your ear.
Forces and Pressure in Fluids
Pascal’s Principle
• According to Pascal’s principle, a change
in pressure at any point in a fluid is
transmitted equally and unchanged in all
directions throughout the fluid.
Hydraulic Systems
• Hydraulics is the science of applying Pascal’s
principles.
• Hydraulic system- a device that uses a
pressurized fluid acting on pistons of different
sizes to increase force
• In a hydraulic lift system, an increased output
force is produced because a constant fluid
pressure is exerted on the larger area of the
output piston.
• Because the pressure on each piston is the
same, the difference in forces is directly related
to the difference in areas.
Bernoulli’s Principle
• According to Bernoulli’s principle, as the speed
of fluid increases, the pressure within the fluid
decreases.
Ex: The air traveling over the top of an airplane
wing moves faster than the air passing
underneath. This creates a low-pressure area
above the wing and a lift is created.
Lift- an upward force due to a pressure difference
between the top and bottom of a wing
Buoyancy
Buoyant Force
• Buoyancy- the ability of a fluid to exert an
upward force on an object placed in it
• Buoyancy results in the apparent loss of
weight of an object in a fluid. Every object
in a fluid experiences buoyancy.
• Buoyant force- an upward force acting on
an object in a fluid
Archimedes’ Principle
• Archimedes’ Principle- the equivalence of the
buoyant force on an object and the weight of the
fluid displaced by the object
• Object is submerged: it pushes aside, or
displaces, a volume of fluid equal to its own
volume
• Object floats: it does not displace its entire
volume. It displaces a volume equal to the
volume of the part of the object that is
submerged
Density and Buoyancy
• If an object is less dense than the fluid it is in, it
will float. If the object is more dense than the
fluid it is in, it will sink.
• Different fluids can also float or sink in one
another. Oil floats on water because oil is less
dense than water.
• Two forces act on every object in a fluid- weight
and the buoyant force.
• When the buoyant force is equal to the weight,
an object floats or is suspended. When the
buoyant force is less than the weight, the object
sinks.