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Buoyancy Force
Shahd_6465
#1 Posted : Sunday, August 01, 2021 4:58:23 PM
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Hello, I have two question about the force of buoyancy.

1. When an object floats exactly at the surface of a liquid, is the buoyancy force 0? It is a bit confusing to me because according to the buoyancy force equation, no volume of the object is submerged in that case so Fb should be 0, but then if there is no force upwards (Fb), what is the force that is counteracting the force of gravity so that this object is not sinking?

2. I know that buoyancy force doesn’t change for a fully submerged object no longer where it moves as long as it is still fully submerged. When a fully submerged object moves from the middle of the water to resting at the bottom surface, how does the buoyancy force stay the same if there is an additional force now, which is the normal force and the force of gravity didn’t change?

Thank you!

INSTR_Calla_139
#2 Posted : Monday, August 02, 2021 5:28:20 PM
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Hello Shahd,

1. An object will not float exactly on the surface due to buoyancy force. some small amount of liquid will be displaced. If no fluid is displaced the forces causing the object to float would be the cohesive and adhesive forces of water.

2. I understand your thought process here, but I would think of it a little different. If the buoyancy force and the force of gravity on the object are equal then there will be no normal force because the object will just be floating along through the fluid with no force down on the bottom surface. However, if the force of gravity is larger than the buoyant force on the object then the normal force will be equal to the difference between the two forces.
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