Before we get into our main discussion, let's briefly discuss some parts of archimedes: density, and I am also going to show some animation videos i got on youtube that can help me explain everything.
So What is density? Did you guys
know?
The density or more precisely,
the volumetric mass density of a subtance is its mass per unit volume. The
symbol most often used for density is rho, although the latin letter d can also
be used. The density of a material varies with temperature and pressure. This
variation is typically small for solids and liquids but much greater for gases.
Increasing the pressure on an
object decreases the volume of the object and thus increases its density.
Increasing the temperature of a substance decreases its density by increasing
its volume. In most materials, heating the bottom of a fluid results in
convection of the heat from the bottom to the top, due to the decrease in the
density of the heated fluid. This causes it to rise relative to denser unheated
material. The reciprocal of the density of a substance is occasionally called
its specific volume, a term sometimes used in thermodynamics. Density is an intensive
property in that increasing the amount of a substance does not increase its
density; rather it increases its mass.
Example of density can be seen in
the way oil floats on water but balsamic vinegar sinks in oil. One substance is
denser than another causing the heavier liquid to sink and the lighter one to
rise to the top. Generally speaking solids are denser than liquid sand liquids
are denser than gases… but this is not always the case as there are many
exceptions to the rules. The density of an object can even be affected by
temperature and pressure.
That A little bit of density, we're moving into the bouyant force. In science, buoyancy or upthrust is an upward force exerted by a fluid that opposes the weight of an immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. thus, the pressure at the bottom of a column of fluid is greater than at the top of the column. Similarly, the pressure at the bottom of an object submerged in a fluid is greater than at the top of the object. This pressure difference results in a net upwards force on the object. The magnitude of that force exerted is proportional to that pressure difference, and is equivalent to the weight of the fluid that would otherwise occupy the volume of the object, that is the displaced fluid.
take a bucket full of water, try
to push an inverted glass bottle into the bucket, and then what do you see?
Yes, when we try to push an inverted glass bottle into a bucket full of water we have to apply downward force. It means bottle experiencing upward force. The upward force is due to force exerted by water on the bottles containing the enclosed air, opposing the downward motion of the bottle. Take an empty plastic bottle closed with an airtight stopper. Put it in the bucket full of water. What happens? when an empty plastic bottle closed with an airtight stopper is put in a bucket full of water, the floats in water. Now trying to push the bottle into the water. Is it possible to push it easily to the bottom of the bucket? Why? It is not easy to push the bottle to the bottom of the bucket. This is because an upward force exerted by the water on the bottle opposes the downward motion of the bottle. Now release the bottle. If the bottle is now released, it rises to the surface of the water and floats on it. In above examples the upward force experienced by an object immersed in any liquid is called the upthrust or buoyant force. This force acts opposite to force of gravity.
And then, what happens when an object is placed inside a fluid? Now because the pressure increases as we go deeper, the force exerted upward due to the pressure will be more than the pressure exerted downwards due to the pressure. So the force exerted on the bottom of the object due to pressure will be more than the force exerted on the top. So this net upward force exerted on objects submerged in fluids is called the buoyant force.
And now, we finally get into our main discussion of the principle of archimedes, physical law of bouyancy discovered by the ancient greek mathematician and inventor archimedes, stating that anybody completely or partially submerged in a fluid at rest is acted upon by an upward, or buoyant, force the magnitude of which is equal to the weight of the fluid displaced by the body. The volume of displaced fluid is equivalent to the volume of an object fully immersed in a fluid or to that fraction of the volume below the surface for an object partially submerged in a liquid. The weight of the displaced portion of the fluid is equivalent to the magnitude of the buoyant force. The Buoyant force on a body floating in a liquid or Gas is also equivalent in magnitude to the Weight of the floating object and is opposite in Direction; the object neither rises nor sinks.
We can take a simple example to
understand this. Take a 5 kg solid object and suspend it by a spring balance. The
balance will read 5 kilograms. Let this be the first case. For the second case,
take a beaker of water which has a small opening at the top. The water in the
beaker is filled right up to this point where the opening is. And we keep a
small container under the opening. Now slowly lower the solid object into the
water, and you’ll see that the reading will be a lower number as compared to
the previous case. Why is that? It’s because of the buoyant force acting on the
object. You’d have also noticed that some water was displaced. Now in this
context,we can say that the weight of this displaced water is equal to the
magnitude of the buoyant force acting on the object. That’s the Archimedes’
principle!
Let's look at some examples that used Archimedes to principle. First, submarine. A submarine has a large ballast tank, which is use to control its position and depth from the surface of the sea. A submarine submerges by letting water into the ballast tank so that its weight become greater than the bouyancy force. Conversely, it floats by reducing water in the ballast tank. Thus its weight is less than the bouyant force. Next, hot air ballon. The atmosphere is filled with air that exerts bouyant force on any object. A hot air ballon rises and floats due to the bouyant force. It descends when the ballon's weight is higher than the bouyant force. It becomes stationary when the weight equals the bouyant force. The weight of the hot - air ballon can be controlled by varying the quantity of hot air in the balloon. Next, ship. A ship floats on the surface of the sea because the volume of water displaced by the ship is enough to have a weight equal to the weight of the ship. And last, fishes. Certain group of fishes uses Archimedes’ principles to go up and down the water. To go up to the surface, the fishes will fill its swim bladder with gases. The gases diffuse from its own body to the bladder and thus making its body lighter. This enables the fishes to go up. To go down, the fishes will empty their bladder, this increases its density and therefore the fish will sink.
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