-Tools:
-Bottle of water;
-Straw (Cannuccia);
-Play dough (Pongo).
-Procedure:
Pour to the brink of the bottle, some natural water and put in
a straw of medium size. On the extremity, we put some pieces
of play dough.
Let that one extremity, must get to the water level on the side
of the cap; and then we add play dough until it get to do it.
-Deductions:
Compressing the bottle at any point, there is the variation of
the air volume, it also changes the density of the air in the
straw which therefore rises and falls depending on the
pressure. For example, if the pressure rises, the air in the
straw remains compressed; in this way, the volume of air
decreases, and if instead we release the bottle, the straw is
pushed upward. The density of the straw is greater than the
density of water.
-Laws:
-Pascal’s law:
Pascal's law or the principle of transmission of fluidpressure is a principle in fluid mechanics that states that
pressure exerted anywhere in a confined incompressible fluid
is transmitted equally in all directions throughout the fluid
such that the pressure variations (initial differences) remain
the same. The law was established
by Frenchmathematician Blaise Pascal.
Pascal's principle is defined as:
“A change in pressure at any point in an enclosed fluid at rest
is transmitted undiminished to all points in the fluid.”
This principle is stated mathematically as:
is the hydrostatic pressure (given in Pascals in
the SI system), or the difference in pressure at two points
within a fluid column, due to the weight of the fluid;
ρ is the fluid density (in kilograms per cubic meter in the
SI system);
g is acceleration due to gravity (normally using the sea
level acceleration due to Earth's gravity, in SI in metres
per second squared);
is the height of fluid above the point of measurement,
or the difference in elevation between the two points
within the fluid column (in metres in SI).
The intuitive explanation of this formula is that
the change in pressure between two elevations is
due to theweight of the fluid between the
elevations. A more correct interpretation, though,
is that the pressure change is caused by the
change of potential energy per unit volume of the
liquid due to the existence of the gravitational
field. Note that the variation with height does not
depend on any additional pressures. Therefore
Pascal's law can be interpreted as saying that any
change in pressure applied at any given point of
the fluid is transmitted undiminished
throughout the fluid.
-Archimedes’ principle:
Archimedes' principle indicates that the upward buoyant
force that is exerted on a body immersed in a fluid, whether
fully or partially submerged, is equal to the weight of the fluid
that the body displaces. Archimedes' principle is a law of
physics fundamental to fluid mechanics. Archimedes of
Syracuse formulated this principle, which bears his name.
Consider a cube immersed in a fluid, with its sides parallel to
the direction of gravity. The fluid will exert a normal force on
each face, and therefore only the forces on the top and bottom
faces will contribute to buoyancy. The pressure difference
between the bottom and the top face is directly proportional to
the height (difference in depth). Multiplying the pressure
difference by the area of a face gives the net force on the cube the buoyancy, or the weight of the fluid displaced. By
extending this reasoning to irregular shapes, we can see that,
whatever the shape of the submerged body, the buoyant force
is equal to the weight of the fluid displaced.
The weight of the displaced fluid is directly proportional to the
volume of the displaced fluid (if the surrounding fluid is of
uniform density). The weight of the object in the fluid is
reduced, because of the force acting on it, which is called
upthrust. In simple terms, the principle states that the buoyant
force on an object is equal to the weight of the fluid displaced
by the object, or the density of the fluid multiplied by the
submerged volume times the gravitational constant, g. Thus,
among completely submerged objects with equal masses,
objects with greater volume have greater buoyancy.
Suppose a rock's weight is measured as 10 newtons when
suspended by a string in a vacuum with gravity acting on it.
Suppose that when the rock is lowered into water, it displaces
water of weight 3 newtons. The force it then exerts on the
string from which it hangs would be 10 newtons minus the 3
newtons of buoyant force: 10 − 3 = 7 newtons. Buoyancy
reduces the apparent weight of objects that have sunk
completely to the sea floor. It is generally easier to lift an
object up through the water than it is to pull it out of the
water.
For a fully submerged object, Archimedes' principle can be
reformulated as follows:
then inserted into the quotient of weights, which has been
expanded by the mutual volume
yields the formula below. The density of the immersed
object relative to the density of the fluid can easily be
calculated without measuring any volumes:
(This formula is used for example in describing the
measuring principle of a dasymeter and of hydrostatic
weighing.)
Example: If you drop wood into water, buoyancy will
keep it afloat.
Example: A helium balloon in a moving car. When
increasing speed or driving in a curve, the air moves in
the opposite direction to the car's acceleration.
However, due to buoyancy, the balloon is pushed "out
of the way" by the air, and will actually drift in the
same direction as the car's acceleration.
When an object is immersed in a liquid, the liquid
exerts an upward force, which is known as the buoyant
force, that is proportional to the weight of the
displaced liquid. The sum force acting on the object,
then, is equal to the difference between the weight of
the object ('down' force) and the weight of displaced
liquid ('up' force). Equilibrium, or neutral buoyancy, is
achieved when these two weights (and thus forces) are
equal.
Chiara Ferranti
Elena Ciculi
Ecaterina Popa
Jennifer Fabrizi
Chiara Battaglini