Bouyancy and Float at Ion

7/31/2019 Bouyancy and Float at Ion

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J. BERLIN P. JUANZON CE, MBA,MSCM


2/20

Net upward

force is

called the

buoyantforce!!!

Easier to

lift a rock

in water!!


3/20

The amount of

water displaced is

equal to thevolume of the

rock.


4/20

An immersed body is buoyed up by a force equal to theweight of the fluid it displaces.

(BF = VD) If the buoyant force on an object is greater than the

force of gravity acting on the object, the object will

float The apparent weight of an object in a liquid is

gravitational force (weight) minus the buoyantforce. (Win-liquid = Win-air BF)


5/20

A floating object displaces a weight of fluid equalto its own weight.


6/20


7/207

Forfloatingbodies, the weight of the entire body must be

equal to the buoyant force, which is the weight of the fluid

whose volume is equal to the volume of the submerged

portion of the floating body:

A solid body

dropped into afluid will sink,

float, or remain at

rest at any point

in the fluid,

depending on its

average density

relative to the

density of the

fluid.


8/20

An object surrounded by air is buoyed up by aforce equal to the weight of the air displace.

Exactly the same concept as buoyancy inwater. Just substitute air for water in thestatement

If the buoyant force is greater than theweight of the object, it will rise in the air


9/20

Since air gets less

dense with altitude,the buoyant force

decreases with

altitude. So heliumballoons dont rise

forever!!!


10/2010

The altitude of a hot

air balloon is

controlled by the

temperature difference

between the air inside

and outside the

balloon, since warmair is less dense than

cold air. When the

balloon is neither

rising nor falling, theupward buoyant force

exactly balances the

downward weight.


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A piece of metal weighs 350N in air andwhen it is submerged completely in water itweighs 240n.

A) Find the volume of metal

B) Find the specific weigh of the metal

C) Find the specific gravity of the metal


12/20

A wooden buoy of spgr 0.75 floats in aliquid with spgr of 0.85.

A) What is the percentage of the volume abovethe liquid surface to the total volume of the

buoy? B) If the volume above the liquid surface is 0.0145

cum, what is the weight of the wooden buoy?

C) What load that will cause the buoy to be fully

submerged?


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A baloon is filled with 3000cum of hydrogenhaving a specific weight of 1.1N/cum.

A) What lift does the baloon exert at the earth surface if

the baloon weighs 1,200N and air is 12.01 N/cum.


14/20

A piece of wood floats in water with 50mmprojecting above the water surface. When itis placed in glycerine of spgr= 1.35 , theblock projects 75mm above the liquid

surface. A) Find the height of the piece of wood.

B) Find the sp.gr of the wood.

C)Find the weight of the wood of it has a crosssectional area of 200mm x 200mm.


15/20

Forstability of submerged body, the

bodys center of gravity must lie

directly below the center of buoyancyof the displaced liquid. If the two

points coincide, the submerged body

is in neutral equilibrium for allpositions.


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16

Stability of Immersed and Floating Bodies

For floating bodies such

as ships, stability is an

important consideration

for safety.

Stability is

easily

understood byanalyzing a

ball on the

floor.


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17

An immersed neutrally buoyant body is (a) stable if the center of

gravity G is directly below the center of buoyancyB of the body,

(b) neutrally stable ifG andB are coincident, and (c) unstable if

G is directly aboveB.

A floating body possesses vertical stability, while an immersed

neutrally buoyant body is neutrally stable since it does not return to

its original position after a disturbance.


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18

When the center of gravity G of an immersed neutrally

buoyant body is not vertically aligned with the center of

buoyancyB of the body, it is not in an equilibrium state and

would rotate to its stable state, even without any disturbance.


19/20

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A floating body isstable if the body is bottom-heavy and thus

the center of gravity G is below the centroidB of the body, or if

the metacenterMis above point G. However, the body isunstable if pointMis below point G.

Metacentric height GM:The distance between the center of gravity G and the

metacenterMthe intersection point of the lines of action of the buoyant force through

the body before and after rotation.


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PROBLEM:

The center of gravity of a boat in the upright position is 3mabove the c.g. of part of water, the total displacement being 500tons

and the boat is tipped 30o causing the center of buoyancy to shift

sideways 2.4m. Compute the righting moment or upsetting moment.

3ft

Water levelG

B

500TONS

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Bouyancy and Float at Ion