Physics 101: Lecture 24, Pg 1

Exam 2 :

The average increased from 62 to 66 % !

Physics 101: Lecture 24, Pg 2

Physics 101: Physics 101: Lecture 24Lecture 24Fluids: Pascal and ArchimedesFluids: Pascal and Archimedes

Today’s lecture will cover Textbook Sections 11.5-11.6

Archimedes Principle & Buoyancy Pascal’s Principle

Note: Everything we do assumes fluid is non-viscous and incompressible.

Physics 101: Lecture 24, Pg 3

Pascal’s PrinciplePascal’s Principle

Any change in pressure applied to a completely enclosed

fluid is transmitted undiminished to all parts of the fluid and the enclosed walls.

Example: hydraulic lift Consider two pistons with area A1 and A2 at the same height at different ends of an enclosed fluid. Am externalForce F1 is applied to piston 1. Consequently the fluid

exerts a force on piston 2 which is related to F1 asfollows

F2 = F1 (A2 / A1)

Physics 101: Lecture 24, Pg 4

Archimedes PrincipleArchimedes Principle

An object fully or partially immersed in water experiences an upward force due to the difference in fluid pressure at different depths. This net force

exerted on the object by the fluid is called the buoyant force :

FB = P2 A – P1 A = (P2-P1) A = g h A

Using that h A=V is the volume of the fluid and thatV=m is the mass of the fluid one finds that FB = weight of fluid displaced by the object

Physics 101: Lecture 24, Pg 5

Archimedes Principle cont.Archimedes Principle cont.

When does an object float ?

An object floats when the upward acting buoyant force

is balanced by the downward acting weight of the object:

FB = weight of object

Also

FB= weight of displaced water

=> For a floating object: Wobject = Wdisplaced fluid

Physics 101: Lecture 24, Pg 6

Archimedes Principle (summary)Archimedes Principle (summary)

Buoyant Force (FB)Buoyant force = weight of fluid displacedFB = fluid x Vdispl g

W = Mg = object Vobject g

If object floats….FB=WTherefore fluid g Vdispl. = object g Vobject

Therefore Vdispl./Vobject = object / fluid

Physics 101: Lecture 24, Pg 7

Concept QuestionConcept Question

Which weighs more: 1. A large bathtub filled to the brim with water. 2. A large bathtub filled to the brim with water with a battle-ship

floating in it. 3. They will weigh the same.

Tub of water

Tub of water + ship

Overflowed water

CORRECT

Weight of ship = Buoyant force =

Weight of displaced water