Fluid Forces & Buoyancy: Why is There a Force Pushing Up?

In summary, the question asks why there is a force pushing up on a cylinder of liquid partially immersed in a beaker of another fluid. The multiple choice answers suggest that the weight of the liquid cylinder is balanced by the forces from the surrounding fluid, specifically the atmospheric pressure and the buoyant force of the fluid on the cylinder. However, without further clarification, it is difficult to determine the correct answer as the question and answers do not provide enough information. It's possible that the intended answer is #3, as the weight of the cylinder is balanced by the forces from the surrounding fluid, but it's also possible that the distinction between the liquid in the cylinder and the surrounding fluid is significant and none of the answers are correct.
  • #1
happysmiles36
54
2

Homework Statement



There is a figure of a cylinder of liquid partially immersed in a beaker of a fluid.

Why is there a force that pushes up on the cylinder of liquid in the beaker?

Multiple choice:
1. The weight of the liquid cylinder and the atmospheric pressure acting on the cylinder is balanced by a force the rest of the liquid exerts back on the cylinder.

2. Normal force the glass exerts back on the liquid.

3. The weight of the liquid cylinder is balanced by a force the rest of the liquid exerts back on the cylinder.

4. Atmospheric pressure.

Homework Equations



None

The Attempt at a Solution



I think it is 3 because atmospheric pressure doesn't change I think because the surface area for the air to push down stays the same which would eliminate 1 and 4. 2 Doesn't seem like that is what happens.

I know the forces acting on the cylinder are:

Down: Weight of cylinder + atmospheric pressure
Up: Fluid on cylinder (I think it is buoyancy)

I really need help, been stuck thinking about this for a long time. My teacher also gave me a hint that is wasn't #1.
 
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  • #2
What does Archimede's principle say?
 
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  • #3
andrevdh said:
What does Archimede's principle say?

"Archimedes’ principle, physical law of buoyancy, stating that anybody completely or partially submerged in a fluid (gas or liquid) at rest is acted upon by an upward, or buoyant, force the magnitude of which is equal to the weight of the fluid displaced by the body."

I actually haven't even heard about this yet! So it would be #3 because the weight of the cylinder of liquid displaces the water and so the water creates the upward buoyant force. Thanks!
 
  • #4
Have you quoted the question exactly, word for word?
The first part discriminates between liquid in the cylinder and 'fluid' outside it. There is no further mention of 'fluid'. If the distinction matters then all answers are wrong. If it doesn't matter then it's hard to know what some of the answers intend.
The question asks why a certain force exists. All of the answers relate to forces balancing. The problem statement does not say anything is in balance.
 
  • #5
That's it, yes. The surrounding water previously supported the water before it was displaced and do now support the cylinder and its contents. This is the buoyant force. So a body will sink until the water it displaces is equal to its own weight, unless its weight is more than the weight of water with a volume equal to the object's total volume.
 
  • #6
andrevdh said:
That's it, yes. The surrounding water previously supported the water before it was displaced and do now support the cylinder and its contents. This is the buoyant force. So a body will sink until the water it displaces is equal to its own weight, unless its weight is more than the weight of water with a volume equal to the object's total volume.

That's all true, but the question and offered answers still don't make much sense.
If I reword it thus:
A cylinder containing a liquid, A, floats in a second liquid, B. What force enables it to float?​
... And reword the first answer as
The weight of the cylinder plus liquid A, plus the atmospheric pressure acting on the cylinder, is balanced by the forces on the cylinder from liquid B.​
Then that would be the answer.
 

What is fluid force?

Fluid force is a force that is exerted by a fluid, such as water or air, on an object that is submerged or partially submerged in it. This force is caused by the pressure of the fluid acting on the surface of the object.

How does buoyancy work?

Buoyancy is the upward force exerted by a fluid on an object that is partially or fully submerged in it. This force is caused by the difference in pressure between the top and bottom of the object. The greater the volume of the object, the greater the buoyancy force.

Why is there a force pushing up on objects in water?

This force is a result of the pressure of the water acting on the object. The deeper an object is submerged in water, the greater the pressure exerted on it, which results in a greater upward force.

How does the shape of an object affect fluid forces?

The shape of an object can greatly affect the fluid forces acting on it. For example, a streamlined shape will experience less fluid resistance and therefore less force compared to a flat or irregularly shaped object.

What is the relationship between fluid density and buoyancy?

The density of a fluid is directly related to the buoyancy force it exerts on an object. The denser the fluid, the greater the buoyancy force. This is why objects float more easily in less dense fluids, such as air, compared to denser fluids like water.

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