# Net upward force-Fb

Discussion in 'MCAT Study Question Q&A' started by inaccensa, Aug 11, 2011.

1. ### inaccensa 7+ Year Member

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I need clarifications -

If an object is lets say at the bottom of a pool 10m deep, is there a net upward force?

If a cable holds the same object at a certain depth, lets say 5m--I know for sure that Fb exists and there is a net force upwards due to the pressure difference

If there is a net upward force on an object submerged in a fluid,why doesnt it accelerate up? or is there a net force upwards only when we look at objects held by a cable, string etc?
objects float since the Fb= weight of the object
objects sink since Fb < Weight of the object
I'm confused...argh

Thanks!

2. ### costales 2+ Year Member

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Fb is not the net force, it is the buoyant force which has the magnitude of the weight of the displaced fluid volume. If the object is stationary - or moving at constant v - the net force is zero.

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### inaccensa 7+ Year Member

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true, but i didnt meant that Fb was the net force, there was a q in kaplan about a mass held by a cable, which has a net force upwards that is equal to the difference between the weight and Fb, in this case the force is tension, but if there were no cable, then the object will accelerate up..that was my q regarding that point. But then again, isnt there always a net force upwards on an object submerged in a fluid

4. ### MT Headed snow, PBR, and bears Lifetime DonorVerified Account 2+ Year Member

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"Net force" has a very specific meaning in physics, and MCAT questions will test your knowledge of this. The net force is the sum of all forces on the object. Newton said that Fnet = m x a.

If the object is sinking faster and faster, the net force (weight + buoyancy) is down. If the object is rising faster and faster, the net force (weight + buoyancy) is up. If the object is tied to a cable at the bottom of the tank, the net force (weight + buoyancy + tension) is zero. If the object is resting on the bottom of the tank, the net force (weight + buoyancy + normal force) is zero.

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### inaccensa 7+ Year Member

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Great that clarifies a lot, now isnt that weight-buoyancy =fnet the direction that determines acceleration? Why do you have weight + buoyancy?
If the object is tied to the cable at the bottom, Tension+buoyancy = weight? then how will I determine the buoyancy if the tension is not know?

If an object is submerged in a liquid, not at the bottom, then its lets say resting somewhere in the middle? there is still an upward force? since the pressure at the bottom is greater--oh if there is no cable

6. ### MT Headed snow, PBR, and bears Lifetime DonorVerified Account 2+ Year Member

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These forces are vectors. You can add a 5N up vector and a 3N down vector and get 2N.

If the object is tied to the bottom to keep it from floating up and away, then the weight and tension point down, and the buoyancy point up.

If an object is submerged in the middle of a liquid (without a cable) and not accelerating in any direction, then the sum total of the forces acting on it, weight + buoyancy, must add to zero. The buoyancy would point up, and the weight would point down, and they would be equal in magnitude.

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7. OP

### inaccensa 7+ Year Member

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I'm sorry for a barrage of q's ,but i really want to understand this topic.
We say Fb= weight of the fluid displaced, this is when an object is submerged (but we never say that Fb=Weight of the object unless its floating) although the net force is zero and force due to buoyancy should equal weight of the object. Can you please clarify this.

If the object is tied to the bottom to keep it from floating up and away, then the weight and( tension point down????), and the buoyancy point up.

8. ### MD Odyssey 2+ Year Member

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Well, if an object is floating, that means that the mass of the water that it displaces (not the mass of the object) is equal to the mass of the object. Draw a free-body diagram, label the two forces, and then write out Newton's 2nd law for the system. Since it's floating, there is no acceleration, so set it equal to zero, and you should be able to see the relationship between the mass of the water and the mass of the object.

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