ARCHIMEDES’ PRINCIPLE

SITUATION THAT RELATE TO ARCHIMEDES’ PRINCIPLE

Have you ever

noticedthat objects

seem less heavy if you put them in

water?

When any object is in water, the water exerts an upward force on it

This force is called buoyant force

The buoyant force is the upward force resulting from an object being wholly or partially immersed in a fluid

When in water, the object experiences two forces:

(a) The actual weight which acts downwards

(b) The buoyant force which acts upwards.

BUOYANT FORCE

The buoyant force is equal to the apparent loss in weight.

Buoyant force = actual weight – apparent weight Buoyant force = actual weight – apparent weight

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Relate of buoyant force to the weight of the liquid

TIMES TO DO EXPERIMENT…..

ARCHIMEDES’ PRINCIPLE States that an object which is partially or wholly

immersed in a fluid (liquid or gas) is acted upon by an upwards buoyant force equal to the weight of the fluid displaces.

Buoyant force is created by the increase of pressure with depth

PRINCIPLE OF FLOATATION States that a floating body displaces its own weight of the liquid in

which it floats When a body is partially or totally submerged in a liquid, buoyant

force = weight of liquid displaced For an object floating in a liquid,

The weight of liquid displaced = the weight of the body (Buoyant force)

By applying the principle of floatation,(i) an object would float or remain stationary in a liquid when the buoyant force is equal to weight of the object(ii) an object would float or move upwards when buoyant force is more than the weight of the object(iii) an object would sink or move downwards in a liquid when the buoyant force is less than weight of the object

The buoyant force acting on an object depend on (i) the volume of the object

- larger object such as iron ship displace a larger volume of water thus experiencing a larger buoyant

(ii) the density of the liquid- the higher the density of liquid, the larger buoyant force.

Figure shows a block of density 0.9 g cm-3 submerged at depths in liquids of different densities

QUICK CHECK Two object A and B placed into liquid X and Y are

found to be submerged to position shown in the figure on the right. Compare (a) the density of liquid X to that liquid Y(b) the density of material A to the density of liquid X and liquid Y(c) the density of material B to the density of liquids X and Y

ANSWER:

(a) Density of liquid X is less than density of liquid Y(b) Material A is denser than liquid X and material A is

less dense than liquid Y(c) Material B is denser than liquid X and liquid Y

USE OF ARCHIMEDES’ PRINCIPLE

(a) Water transport (ship) Base on the principle of floating, ship can

stay afloat by displacing the weight of sea-water equal to its own weight

The ship would sink to a certain level until its weight is equal to the buoyant force

However, the ship may sink to deeply into water due to overloading or changes in water density: then there is danger of being swamped in rough seas.

To prevent this, a plimsoll line is marked on the hulls of all ships to show the depth which are safe for them to navigate

The depth for sea-water of normal density is at B while for freshwater is at F or TF

A ship will submerged deeper in fresh water because the density of fresh water is less than sea water – a ship must displace more water to obtain sufficient buoyant force to support its weight.

(b) Submarine

(b) Submarine

It has large ballast tanks to control its position and depth it can submerge below sea level

When afloat, water is driven out from the ballast tank by compressed air to reduce its weight and produce a larger buoyant force

When submerged, the ballast tank are filled with water again to increase the weight of the submarine

The submarine can submerged to a depth where the buoyant force = to its weight

(c) Hot-air balloons When the envelope of a balloon is filled with a

gas of lower density than air such as hydrogen ,helium or hot air, its weight decreases

The total weight of the balloon, WT

=Wf + Wg

The balloon experiences a buoyant force = Vρg

If the buoyant force = WT of the balloon….. it remains stationary or continues to move with constant velocity

If the buoyant force > total weight,…….balloon will rise upwards Balloons are often used in weather forecasting, sports, reaction activities and

for advertisements

(d) HYDROMETER Is an instrument for measuring densities of liquids Widely used in checking the condition of a car battery

by determining whether the relative density of acid in it has dropped below 1.20

Testing the content of natural rubber in rubber latex as in MARDEX rubber latex collection centres

Its consists of a wide bulb carrying a narrow glass stem The lower bulb is weighted with lead shots to keep it

floating upright The stem is graduated with a relative density scale

having unequal divisions and values decreasing upwards

By applying the principle of floatation, the immersed length of the hydrometer in a liquid decreased as the liquid density increase due to an increasing buoyant force

THE END