Archimedes Principle
The principle of Archimedes states that "The buoyant force acting on an object completely submerged in water equals the weight of the displaced water."
For surbmerged objects,
Buoyant force = Weight of the fluid displaced
For floated objects,
Buoyant force = Weight of the fluid displaced = Weight of the floated object
Application of Archimedes Principle (Plimsoll Line)
Plimsoll line is a mark on a vessel’s side indicating the maximum depth to which it may be safely immersed when loaded with cargo.
The maximum safe loading depth varies with ocean regions and seasons. In the tropics the water is warmer and therefore less dense than in temperate regions, so with the same load ships will float higher in cold regions than in the tropics. Summer and winter cause similar changes.
- The density of water is determined by the salinity of water at different locations
- A ship will submerge deeper in freshwater than that in sea water because the density of freshwater is smaller.
- The ship can be loaded with heavier load in sea water than in freshwater.
Application of Archimedes Principle (Hydrometer)
- Hydrometer is used to measure the density of liquid.
- The hydrometer works on the principle that a floating object displaces a volume of liquid whose weight is equal to the buoyant force.
- Hydrometer will sink deeper in the less dense liquid but float higher in the denser liquid.
Design of a hydrometer
Characteristic
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Explanation
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Lead shots are added to the base
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· To lower the centre of gravity of the hydrometer
· So that, hydrometer can float upright in a liquid.
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Thinner and longer glass tube is used
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· Will give a larger scale distance and can detect small changes in the densities.
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Big bulb at the base is used.
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· So that more liquid can be displaced
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For calibration, hydrometer is allowed to float in lower density and higher density liquid
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· Liquid level in lower density and higher density is marked. The space between the two marks is divided into a number of equal divisions that give density readings.
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The buoyant force
When an object is placed in a fluid, the fluid exerts an upward force we
call the buoyant force. The buoyant force comes from the pressure
exerted on the object by the fluid. Because the pressure increases as
the depth increases, the pressure on the bottom of an object is always
larger than the force on the top - hence the net upward force.
The buoyant force is present whether the object floats or sinks. Let's
consider a floating object, but the analysis is basically the same for a
submerged object.
We'll also consider a rectangular block, although a similar (more
complicated) analysis leads to the same result for funny-shaped
objects.
