3.5 Archimedes Principle

3.5Archimedes PrincipleChapter 3, Forces and PressureExpected Learning OutcomesStudent should be able to:

Explain buoyant force.relate buoyant force to the weight of the liquid displaced.state Archimedes principle.describe applications of Archimedes principle.solve problem involving Archimedes principle.

Steel is denser than water.So we might expect heavy steel ships to sink.However, they can still float. Why?

If you try to lift a heavy object under water, You find it surprising light and much easier to lift than when it is out of the water.

Archidemes, was a Greek scientist who first discovered that an object submerged in a liquid is acted by an upwards buoyant force (or upthrust).THE PUZZLE OF KING HIERO'S CROWN

Buoyant Force & Floatation

An object immersed in water is subjected to two forces, the weight of the object which acts downwards and an upthrust or buoyant force which acts upwards.

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

3. An object which has a density less than the liquid in which it is submerged, will be subjected to buoyant force which causes the object to float on the surface of the liquid.4. Conversely, an object which has density greater than the liquid in which it is submerge, will sink to the bottom as the buoyant force is not sufficiently strong enough to support the weight of the object.When an object is immersed in a fluid, the object will displace a certain volume of the fluid.

Archimedes Principle

Figure shows an object attached to a spring balance. The weight of the object in air, W1 is greater than its weight when immersed in water, W2. The different in weight is due to an upward buoyant force acting on an object by the waterConsider the weight of an object in an air as its actual weight and the measured when the object is immersed in a fluid is its apparent weight.Hence,

Buoyant force = Actual weight, W1 Apparent weight, W2

(a)In air, a load of mass 6 kg has a actual weight, W1 = ____________(reading on spring balanced)(b)When the object is fully immersed in water, the reading on the spring balance, W2 = __________________ ( apparent weight)Upthrust ( buoyant force) is equal to the apparent _____________in weightBuoyant force / Upthrust = Apparent loss in weight = Actual weight apparent weight = _______________NArchimedes Principle states that for a body immersed completely or partially in a fluid, the upwards buoyant force acting on the body is equal to the weight of the fluid it displaces.

Displaced water

Activity: To investigate the relationship between buoyant force on an object and the weight of liquid displaced.Apparatus / Materials:Eureka can, a piece of stone, beaker, thread, spring balance and triple-beam balance.Arrangement of apparatus

Density and BuoyancyBuoyancy Force and Floatation

Buoyant force = weight the object floats and stationary

Buoyant force > weight the object moves up

Buoyant force < weight the object moves down

Think!

warm fresh watercold fresh waterwarm sea watercold sea water1. Why the depth of ship immersed in the water different?

Fresh water less dense than sea water and warm water less dense than cold water so warm fresh water need to be displaced more to keep the upthrust force equal with weight of the boat so it still can float.

If the plasticine is formed into a ball, it will sink.But when it is formed into a hull it will float. Why?

Because

APPLICATIONS

Hot air balloon1. rises upwards (Upthrust > Weight of hot air (helium gas) + weight of airship fabric + weight of gondola + weight of passengers.)( balloon expand)

2..descends(Upthrust < Weight of hot air (helium gas) + weight of airship fabric + weight of gondola + weight of passengers.)(balloon shrinks) 3. stationary (Upthrust = Weight of hot air (helium gas) + weight of airship fabric + weight of gondola + weight of passengers.)( balloon size uncanged)

PLIMSOLL LINE OF THE SHIP

The density of sea water varies with location and season. To ensure that a ship is loaded within safe limits , the Plimsoll line marked on the body of the ship acts as a guide.SUBMARINE

If ballast tanks empty Upthrust > weight submarine rises to surface

If ballast tanks full Upthrust < weight submarine sinks to bottomHydrometer

An hydrometer is an instrument used to measure the density of a liquid.

In a liquid of lesser density , the hydrometer is more submerged.The hydrometer floats higher in a liquid of higher densitylead shot to make it float upright