# Buoyant force q

Discussion in 'MCAT Discussions' started by ssh18, 04.10.07.

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1. ### ssh18Member 5+ Year Member

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I'm a bit confused as to why weight of an object = buoyant force for a floating object. When I think about it intuitively, I'm always thinking that if an object is floating then it's buoyant force must be greater than the gravitational force pulling it down. I don't get it I need some help!

3. ### DiverDocKCUMB 2012 7+ Year Member

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Think about it in terms of density as well as the good ol formula. If the weight of the water displaced is greater than the weight of the object itself, then it will float. However, if the weight of the object is more than the weight of the fluid it displaces, its gonna sink.

Now take a look at the formula after reading and I think it might help. Hopefully! Fb= Mwaterg= density(Vdisplaced) g

4. ### AnEyeLikeMarsMember 7+ Year Member

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When it's floating it's not moving vertically. At that point, the buoyant force exactly cancels out the gravitational force (there's a net vertical force of 0)

5. ### Kraazy 7+ Year Member

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I think what confuses you is that if the object were completely submerged, the buoyant force upward would be greater than the force exerted by gravity. So you would have to exert an additional force to keep the object submerged (like holding an inflatable ball underwater).

One way to think about it is that floating objects are only partially submerged because they only need part of their potential buyant force in order to counteract gravity. Again, think of an inflatable floating in a pool

6. ### HippocratesXMember 7+ Year Member

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wait...somebody pls correct me if i'm wrong but weight of an object is not equal to its buoyant force?? The weight of the fluid it displaces is the buoyant force isn't it?...now you guys are confusing me lol

Buoyant force is the force that all fluids apply upwards when an object is immersed in it. When an object is immersed in a fluid, the fluid above the object exerts a downward pressure on the object. The fluid below the object exerts an upward pressure on the object. Pressure increases at lower depths, thus, the upward pressure exceeds the downward pressure. Thus, the fluid gives a total upward force, or buoyant force. The buoyant force equals the mass of the displaced fluid. The buoyant force equals mg. The M in Mg = Mass of fluid displaced and G = gravity. Note, Archimedes principle only works for a fully submerged object, not a partially submerged object. It can also apply to floating objects. Finally, a key point to remember is that: All solid objects will float in a fluid if the density of the object is less than or equal to the density of the liquid.

7. ### killinsound 10+ Year Member

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best explanation. know that Fb = pressure diff.

8. ### finality333fĭ-zĭsh'ən 2+ Year Member

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If an object is floating, then there is a cancellation of forces, it is at equilibrium (no net force). Meaning, the water is pushing up equal to what it is pushing down (its weight). So since there is no net force, those too must be equal and opposite in direction. Hence....Weight of the FLOATING object = Boyant force (pgV). Now if it is not floating because its density is greater then water (1000 kg/ m^3), which also means specific gravity is greater then one, then the weight of the object is greater then the boyant force and there is a net force downwards...which cause it to accelerate down. Meh sorry i was trying to review with this, lol.

9. ### poly800rockMember 2+ Year Member

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this was such a hard concept for me to grasp as well. One thing that made it all fall into place was the concept of apparent weight.

Apparent Weight = Fb - Weight(object in air).

for an object that isn't fully submerged: Fb = pVg (all liquid) = pVg (all object) which also translates to Fb= mg(liquid) = mg(object)

for fully submerged, Fb = p(liquid)V(object)g

10. ### Swiperfox 2+ Year Member

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Here is how I think of it, maybe it helps, maybe it doesn't.

First, you have to conceptualize the problem and see what is actually happening behind all those formulas and numbers. The best way to do this for any physics problem is in the form of either an energy, charge, or force balance. No sophisticated concepts here, you just need to account for everything interacting with the object of interest. For mechanics problems, like this one, you do this by drawing a free body diagram either on the paper or mentally.

Since the block is floating in the water, it has zero velocity and more importantly zero acceleration. Thus using newtons laws, the sum of forces F=ma= 0. Draw out the forces acting on the block and you will see that the buoyant force must be equal and opposite the weight of the block to prevent it from accelerating downward.

Buoyancy is really just a complicated term for something you already know how to deal with applied in a specific instance, force and pressure. You know that pressure in a fluid P is given by P = density*g*depth, and that the pressure on a surface is equivalent to a force F divided by the surface area A. Therefore, if you have a submerged object, the force acting upwards can be found by converting the fluid pressure acting on the bottom surface into an upwards force, or P*A = density*g*depth*A = Volume of water displaced*density* g.

11. ### sehnsucht 2+ Year Member

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the bouyant force needs to equal the weight to keep the object from completely sinking into the liquid. the way i think about it, the forces need to be equal to keep the object afloat.