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- Hi, I'm having difficulty understanding whether or not the buoyant force is the same or different in water and alcohol. Thanks for any help.

The buoyancy force on a floating object will be the same in either case. On the other hand the buoyancy force on an object submerged in water will be greater that the buoyancy force on the same object submerged in alcohol! Why is this so? because the density of water is greater than the density of alcohol.

The buoyancy force is equal to the weight of the displaced liquid and can be calculated from:

Fb=rho*g*V where rho is the density of the liquid, g is the acceleration of gravity and V is the volume of the submerged object.

When an object is submerged in a liquid the object displaces a volume of liquid equal to the object's volume. If the density of the liquid is greater the weight of the displaced liquid will likewise be greater because the object is practically "lifting" the displaced liquid. Due to Newton's 3rd law the force needed to lift the displaced liquid will be equal to the buoyancy force on the submerged object.

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I am teaching or have taught AP physics B and C [calculus based mechanics & electricity and magnetism] as well as Lab Physics for college bound students. I have a BS in Physics from the University of Pittsburgh and a Master of Arts in Teaching from same. I have been teaching physics for 34 years. I am constantly updating my skills and have a particular interest in modern physics topics.