How is Buoyance Used to Measure Specific Gravity
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Bouyancy An object either fully or partially submerged in a fluid is acted on by a downward force, W, equal to its weight and an upward force, B, equal to the weight of the fluid displaced by the object. That’s why concrete canoes (and steel ships) float; they are shells that use a relatively small amount of material to displace a much larger amount of water. A concrete canoe 20' long and 2.5' wide displaces approximately 2000 lb of water and yet weighs less than 200 lb! This phenomenon, called buoyancy, is the basis of the specific gravity test for aggregate. If we weigh an object in air, then weigh it suspended in water, the difference is just the weight of water displaced by the object: displaced water water in air in W W W If we divide the weight of the object in air by the weight of water displaced by the object, we get the specific gravity of the object: in air object object object s waterdi splaced water object water W V G W V where the symbol denotes the unit weight. Note that if we know the weight of water displaced by the object, we can also compute its volume: water displaced water object water displaced object water W V W V This is a convenient way to determine the volume of objects with irregular shapes. W B
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Buoyance
Transcript of How is Buoyance Used to Measure Specific Gravity
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BouyancyAnobjecteither fullyorpartially submergedinafluidisactedonbyadownwardforce,W,equal to itsweight and anupward force,B,equal to theweightof the fluiddisplacedbytheobject.Thatswhyconcretecanoes (andsteelships)float; they are shells that use a relativelysmallamountofmaterial todisplaceamuchlargeramountofwater.A concrete canoe 20' long and 2.5' widedisplacesapproximately2000lbofwaterandyetweighslessthan200lb!This phenomenon, called buoyancy, is thebasisofthespecificgravitytestforaggregate.
Ifweweighanobject inair,thenweigh itsuspended inwater,thedifference isjusttheweightofwaterdisplacedbytheobject:
displacedwaterwaterinairin WWW Ifwedividetheweightoftheobjectinairbytheweightofwaterdisplacedbytheobject,wegetthespecificgravityoftheobject:
inair object object object
swater displaced water object water
W VG
W V
wherethesymboldenotestheunitweight.Notethatifweknowtheweightofwaterdisplacedbytheobject,wecanalsocomputeitsvolume:
water displaced
water object water displaced objectwater
WV W V
Thisisaconvenientwaytodeterminethevolumeofobjectswithirregularshapes.
W
B
