Archimedes principle

Who is Archimedes?

A little history…
The King must know: is his crown true gold?

Eureka, Eureka.

The law
Archimedes' Principle, law of physics that states that when an object is totally or partially immersed in a fluid, it experiences an upthrust equal to the weight of the fluid displaced.
The principle is most frequently applied to the behaviour of objects in water, and helps to explain floating and sinking, and why objects seem lighter in water. It also applies to balloons in the air.

The key word in the principle is “upthrust” (or buoyant force), which refers to the force acting upward to reduce the actual weight of the object when it is under water.
UPTHRUST AND BUOYANT FORCE

SINKING AND FLOATING OBJECTS
The reading of
spring balance is 2.7 N
The reading of
spring balance is 1.7 N

What is the reading of spring balance if the wood is attached to it ?

Density and Buoyancy
From Archimedes’s Principle :
Buoyant Force = Weight of fluid displaced
= mg (note : F = ma)
= ρVg (note : ρ = m )
V
Thus FB = ρ V g
Where ……
FB = Buoyant Force or Upthrust
ρ = Density of fluid
V = Volume of fluid displaced or
the volume of the object that immersed in the fluid.

Buoyant 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

The Law of Floatation
A floating object displaces its own weight of fluid in which it floats.

THINK!!!!!
1. Why the depth of ship immersed in the water different?

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

2. 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 balloon
1. 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)

Water Bridge in Germany

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.
PLIMSOLL LINE OF THE SHIP

If ballast tanks empty ⇒Upthrust > weight ⇒ submarine rises to surface
If ballast tanks full ⇒ Upthrust < weight ⇒ submarine sinks to bottom
SUBMARINE

Hydrometer
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 density.
lead shot to make it float upright

Wood in Water I
Two beakers are filled to the brim with water. A wooden block is placed in the beaker 2 so it floats. (Some of the water will overflow the beaker and run off). Both beakers are then weighed. Which scale reads a larger weight?
a
b
c
same for both

Wood in Water II
A block of wood floats in a container of water as shown on the right. On the Moon, how would the same block of wood float in the container of water?