Buoyancy Force

[faith hopelove]

A quick question. How do you know when to use the density of the fluid vs the density of the object? Sometimes I find that I get answers wrong because I plugged in the density of the object rather than the density of the fluid. Also, gravity has no effect on mass/the buoyancy force?

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[labqi]

Buoyant force = p(fluid)v(submerged or displaced)g
Gravitational force= mg = p(object)v(object)g.

When the object is floating density,
Buoyant force = gravitational force

 

[Sammy1024]

Buoyancy always uses density if fluid.

 

[faith hopelove]

Oh ok so I was mixing up buoyancy force with the weight of the object?

 

[labqi]

Buoyant force = the weight of the displaced fluid OR the reduction in weight (not to be confused with apparent weight)

 

[NextStepTutor_1]

Also it might help to take a crack at deriving the formulas that you will use in hydro statics--namely gauge pressure and buoyant force. Gauge pressure is just the force per area on an object. So, you need to find the force a column of water exerts on an object--meaning you are dealing with density of liquid.

For buoyant force, it's actually derived from the difference in gauge pressure between the top of the object and the bottom, so again, you would use the density of the liquid.