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What makes a boat float ?What makes a boat float ?
BouyancyBouyancy
StabilityStability
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Demotivation .com
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Stability of Floating ObjectStability of Floating Object stable ifstable if equilibrium after smallequilibrium after small
disturbancedisturbance
vertical & rotational stabilityvertical & rotational stability
(why no horizontal stability ?)(why no horizontal stability ?)
upward buoyant forceupward buoyant force centre of buoyancycentre of buoyancy
centroid of the displaced fluidcentroid of the displaced fluid
weight forceweight force centre of gravitycentre of gravity
rigid bodyrigid body centre of mass is fixedcentre of mass is fixed
(but not necessarily within object)(but not necessarily within object)
does not always coincide with geometric centredoes not always coincide with geometric centre
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Archimedes
Principle
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Archimedes PrincipleArchimedes Principle
The bouyancy force exerted on an objectThe bouyancy force exerted on an object
in a fluid is equal to the weight of fluid itin a fluid is equal to the weight of fluid it
displacesdisplaces
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For a FLOATING object:
FB = Wobj
= mobjg
= Vobj Vobj g
= WH2Odisp
= mH2Odispg
= VH2Odispl VH2Odispl g
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Mass(ship + cargo) = mass(displaced H2O)
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Stability of Floating ObjectStability of Floating Object
stable ifstable if
equilibrium restored after small disturbanceequilibrium restored after small disturbance
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Which is more stable ? Why ?
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upward buoyant forceupward buoyant force centre ofcentre of
buoyancybuoyancy centroid of thecentroid of the
displaced flu
iddisplaced flu
id
weight forceweight force centre of gravitycentre of gravity
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Which is more stable ? Why ?
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Calculating the metacentreCalculating the metacentre
centre of masscentre of mass
averageaverage of positions of particles making up aof positions of particles making up a
bodybody
(relative to datum)(relative to datum)
weightedweighted by theirby theirmassesmasses mmii
MM== 77mmii
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cylindrical
buoy - pine
(SG=0.5)
lead
(SG=11.3)
weight
depth ofimmersion in
seawater?(SG
=1.03)
centre mass? Centre
bouyancy?
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