Archimedes Principle & Specific Gravity

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SUMMARY

The discussion centers on the relationship between Archimedes' Principle and Specific Gravity (SG). Specific Gravity is defined mathematically as SG = pS / pW, where pS is the density of the substance and pW is the density of water. A common misconception is that SG is always 1 due to the belief that the weight of an object equals the buoyant force; however, Archimedes' Principle clarifies that the buoyant force equals the weight of the fluid displaced, not the object's weight. This distinction is crucial for understanding buoyancy and specific gravity in fluid mechanics.

PREREQUISITES
  • Understanding of fluid mechanics concepts
  • Familiarity with mathematical equations involving density and buoyancy
  • Knowledge of Archimedes' Principle
  • Basic physics principles related to weight and volume
NEXT STEPS
  • Study the derivation of Archimedes' Principle in detail
  • Explore applications of Specific Gravity in various materials
  • Learn about the relationship between buoyancy and density in different fluids
  • Investigate real-world experiments demonstrating Specific Gravity and buoyancy
USEFUL FOR

Students of physics, educators teaching fluid mechanics, and professionals in fields requiring knowledge of buoyancy and material properties will benefit from this discussion.

HelgaMan
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I was reading a lab online and I came across one that dealt with Specific Gravity and Buoyany and stuff.. and then they had these equations:

*Note: p = density, M = mass, V = Volume, g = gravitational acceleration, W = Weight
Subscripts: S = substance, W = water

SG = pS / sW = (MS / VS) / (MW / VW) = (MSg / VS) / (MWg / VW) = (WS / VS) / (WW / VW)

and

SG = (WS / V) / (WW / V) = WS / WW = WS / (buoyant force) = WS / (loss of weight in water)
= WS / (WS - weight of substance in water)

which all makes sense to me, mathmatically.. but then i thought about this part "WS / (buoyant force)" and that kinda throws me off because isn't Archimede's Principle that weight of an object is equal to the buoyant force.. and that makes SG always 1? but I am pretty sure that's not the case, though..


well, idk, I am sure all you smart people will see something that i dont, lol. :-p
 
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HelgaMan said:
which all makes sense to me, mathmatically.. but then i thought about this part "WS / (buoyant force)" and that kinda throws me off because isn't Archimede's Principle that weight of an object is equal to the buoyant force.. and that makes SG always 1? but I am pretty sure that's not the case, though..
No, it's not. Archimedes' principle states that the buoyant force equals the weight of the displaced fluid, not the weight of the object. Read all about it: http://hyperphysics.phy-astr.gsu.edu/Hbase/pbuoy.html#arch3"
 
Last edited by a moderator:
oh, okay, lol.

thanks :D
 

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