# Buoyancy on the MCAT

Discussion in 'MCAT Discussions' started by ilovelucy, May 10, 2007.

1. ### ilovelucy This is Lucy.

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I am suffering with the practice problems with buoyancy. Had anyone seen this on the MCAT? If so was it a discrete or a whole passage. I am hoping for discrete!!!

Thanks!

3. ### HippocratesX Member

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ilovelucy,

Bouancy Force can show up in either form on the MCAT. It could be in a passage, or in a discrete form. I have seen it in at least few different discretes, so yes it IS tested on the MCAT.

What exactly are you having trouble with? Force of Buoancy = (density of fluid) x (volume of submerged object) x gravity.

When an object is submerged in fluid, it takes up space. When that object is removed, the volume that it occupied will fill back up with fluid. This volume of fluid must be supported by the pressure of surrounding liquid, since it cannot support itself. When there is no object present, the same volume of fluid must equal to its weight. Think of it like a water blob that has a weight and its pushing down on the surrounding fluid and its being supported by the same force it pushes down as (its weight). So when the object IS present...then the same force of surrounding fluid pushes up on the object...so that force is still the weight of the displaced fluid.

So if the problem asks you what is the net force on the object. It is the Force of Buoyancy minus the weight of the object. Since buoyance pushes up and the weight pushes down. The difference of those forces is the net force on the object in fluid. When the density of the object is less than the density of the fluid, the object will rise. (makes sense, right?) This is the same thing as the equation Fbuoyance - WeightObject = Net Force. Because...Fbuoyance = densityoffluid x volume-submerged x gravity; Wobject = densityofObject x VolumeofObject x gravity....so u can see why it rises when the object density is lower.

If the problem says that the object is "floating"....and this is not the same thing as being submerged...because once it is submerged completely in fluid, it will usually go up or down...So when the object is floating, the Net force on the object is = zero , which means the Force of buoyance = Weight Object. And again each of those equals (density x volume x gravity) from what i wrote above. aNd you just work the math to get what you want...

Its kind of hard to explain without a diagram, but if you read it while looking at a picture, it should make sense. good luck!

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