SCIENCE NOTES
PRESSURE AND BUOYANCY
PRESSURE
Substances that don’t have a rigid shape are considered fluids. Examples of this
are liquids and gas.
Pressure, what is it? All matter is made up of tiny particles. The forces that exist
in fluids are caused by the mass and the motion of the particles making up the fluid. In a
solid, the particles are packed very tightly together. There is very little movement. In
liquids and gasses, the particles are not packed together so tightly. Therefore, they are
able to move about more freely. As each particle moves it pushes against other particles
and against the walls of its container with force. The “push”, or force, particles exert
over a certain area is called pressure. Fluid pressure is exerted equally in all directions.
Scientists define pressure as force per unit area. Pressure is calculated by--pressure =
force / area
Force is measured in newtons (N) and area is measured in square centimeters (cm2).
Pressure is measured in newtons per square centimeter (N/cm2).
Hydraulic Devices
The transmission of pressure equally in all directions in a liquid is the principal of
hydraulic devices. In a hydraulic device, the force experienced by the larger piston is
greater than the force experienced by the smaller piston. Hydraulic devices multiply
force. In a hydraulic device, the pressure exerted on every square centimeter of area of
the larger piston is equal to the pressure exerted on every square centimeter of area of the
smaller piston. In a hydraulic brake system, the larger piston is connected to the brake
pads. Many animals and things we use everyday are hydraulic. Animals like the
earthworm, some spiders and sea anemone use hydraulics. Barber chairs, rescue ladders,
automobile lifts are all devices that use hydraulics.
Pressure and Gravity
The pressure exerted by a fluid is the same throughout the fluid if there are no
forces acting on the fluid. There is one force that is always present, that force is gravity.
Gravity pulls downward on all the particles of a fluid.
Your ears ache as you swim to the bottom of the pool because the pressure of
water is increasing rapidly and your body wants to even out the pressure. Your body
exchanges air through your ears to keep you inside pressure equal to the outside pressure.
But when you are in water your ears do not let water in, so your body can not even out
the pressure.
Due to the force of gravity, the pressure of any fluid varies with its depth, the
greater the depth the greater the pressure. The increase in pressure that accompanies an
increase in depth has some important effects. A submarine can only dive so deep
because it goes beyond what it can stabilize the pressure it will be crushed. Divers
cannot go to deep without experiencing serious problems caused by the increased
pressure. Under high pressure conditions, more nitrogen gas than usual dissolves in a
diver’s blood. When the diver resurfaces, the pressure on the body decreases and the
nitrogen leaves the blood. If the nitrogen gas leaves the body too quickly, it forms tiny
bubbles that are painful and quite dangerous. This condition is sometimes referred to as
the bends. In order to prevent the bends a diver must rise slowly to the surface to allow
the dissolved nitrogen to be released gradually from the blood.
Water is not the only fluid in which pressure varies with depth. Our planet is
surrounded by a fluid atmosphere. The pressure of our atmosphere varies, the higher the
altitude, the lower the pressure. When you are at traveling at high elevations you might
have had your ears “pop”. This occurs because when you are at high elevations the
pressure inside your body becomes greater than the pressure outside your body. Your
eardrums feel the pressure and the release of pressure through your ears cause them to
“pop”.
BUOYANCY
Force increases with depth. Thus, the force at the bottom of an object in a fluid is
greater than the force at the top. This causes an upward push making the object feel
lighter. That’s why objects submerged in water appear to weigh less. The upward force
of water is called the buoyant force. When you enter a fluid you move the fluid out of the
area you occupy. When you climb into a bath tub the level rises because the bathtub has
you and the fluid in it. Any object placed in water displaces, or moves aside, a certain
amount of water. The amount of weight displaced has a definite weight. Because the
buoyant force was able to support this weight, this weight must be related to the size of
the buoyant force.
More than 2000 years ago, the Greek scientist Archimedes discovered the nature
of this relationship. He stated that the buoyant force of an object is equal to the weight of
the fluid displaced by the object. Te relationship between buoyancy and the weight of the
displaced fluid is called Archimedes Principle.
The size of the buoyant force determines whether an object will sink or float when
it is placed in a fluid. The buoyant force can be greater than, less than, or equal to the
weight of an object placed in a fluid. An object will float if the buoyant force is greater
than or equal to the weight. The object will sink if the buoyant force is less than the
weight.
The conditions for floating can be stated in terms of density.
Density is the ratio of mass to volume. The formula for density is Density = Mass
divided by volume (D=M/V). The density of water is 1 g/ml (density can also have the
unit g/cm3 when it is a solid)
You may wander how objects such as steel ships are able to float in water, since
the density of steel is 7.8 g/cm3. A ship is built of a shell of steel that is hollow inside.
The volume of the ship is made mostly of air. The ship and the air together have a
density that is less than that of water. They can displace a weight of water equal to or
greater than their weight.