Page 63 - ebook fluid mechanics_finalize

FLUID MECHANICS

“Upthrust on body = weight of fluid displaced by the body”

Body floating in the fluid will upthrust upwards due to the fluid. The magnitude of the

thrust exerted by the fluid on the body is equal to the weight of the fluid displaced by the

body.

Upthrust force upwards , R1 =  1 gv 1 The force due to G1 at centroid v1,

(Immersed object)

Upthrust force downwards, R2 =  2 gv 2 The force due to G2 at centroid v2,

(Submerged Object)

Total Upthrust Force =  gv +  gv
1 1 2 2

Position of the center of gravity G1 and G2 not be in the same vertical lines and center

throughout the body does not float through the centroid of the whole body.

Example 3.5

A rectangular pontoon has a width B of 6 m, a length l of 12 m, and a draught D of 1.5 m in
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fresh water (density 1000 kg/m ). Calculate :

a. the weight of the pontoon

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b. its draught in sea water (density 1025 kg/m )
c. the load (in kilo Newton) that can be supported by the pontoon in fresh water if the

maximum draught permissible is 2 m.

Solution :

When the pontoon is floating in an unloaded condition,

Upthrust on immersed volume =weight of pontoon

Since the upthrust is equal to weight of the fluid displaced,

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