Forces acting on an object that is sunken at the bottom of a container with fluid?

[daftypatty]

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Hi, so if we looked at an object that is already sunken at the bottom of a container containing fluid, what are the forces acting on its free body diagram?

I know for sure the weight, normal force, and buoyant force should all be acting on it, but does the force/weight from the water above it also act on the object in a downward direction?

Thanks

 

[sazerac]

Hi, so if we looked at an object that is already sunken at the bottom of a container containing fluid, what are the forces acting on its free body diagram?

I know for sure the weight, normal force, and buoyant force should all be acting on it, but does the force/weight from the water above it also act on the object in a downward direction?

Thanks

Yes...

And on the sides of the object. And on the bottom of the object, pushing up. It contributes to the pressure felt by the object, but not to the net force acting on it.

 

[sazerac]

Unless you want to get really technical about what the buoyant force is... then technically the weight of the water is pushing down on the object's top, but the greater weight of the water is pushing up on the object's bottom. This is what creates the buoyant force in the first place.

 

[jd989898]

^ yep. It's called gauge pressure. Gauge pressure = Density of liquid * gravitational acceleration * distance below surface

 

[daftypatty]

Yes...

And on the sides of the object. And on the bottom of the object, pushing up. It contributes to the pressure felt by the object, but not to the net force acting on it.

How come the force from the water acting down on the object wouldn't contribute to the net fore acting on it?

Example:

In this question, they answer is D b/c Buoyant force + Normal Force = Weight of object.
But, how come we don't consider the weight of the water acting on the sunken objet?

 

[basophilic]

How come the force from the water acting down on the object wouldn't contribute to the net fore acting on it?

Example: View attachment 193199

In this question, they answer is D b/c Buoyant force + Normal Force = Weight of object.
But, how come we don't consider the weight of the water acting on the sunken objet?

I think the magnitude of the buoyant force is dependent only on the volume of the object that has submerged, and independent of the height beneath the water surface. Magnitude of Gauge pressure on the other hand is dependent on height beneath the water. Hence:
1. Buoyant force is 10 N at all heights beneath the surface (as long as entire ball is submerged in comparing those heights)
2. Gauge pressure increases directly with increasing height; hence, if ball was made of flexible material it will collapse or crumple up more and more the deeper it goes.

 

[daftypatty]

I think the magnitude of the buoyant force is dependent only on the volume of the object that has submerged, and independent of the height beneath the water surface. Magnitude of Gauge pressure on the other hand is dependent on height beneath the water. Hence:
1. Buoyant force is 10 N at all heights beneath the surface (as long as entire ball is submerged in comparing those heights)
2. Gauge pressure increases directly with increasing height; hence, if ball was made of flexible material it will collapse or crumple up more and more the deeper it goes.

Hi, I understand what you are saying but I'm wondering why we don't account for the gauge pressure/force that is acting on the object? I.e., if we were to draw a free-body diagram, the answer is indicating that we should draw a buoyant force going up, normal force going up, and a weight of object force going down, but how come you don't include the force caused by the water above it that is acting downwards? (i.e., the force that is causing the gauge pressure).

Because when I did this other example:

Here, the answer was D. The explanation was that there are 2 forces acting up & 2 forces acting downwards on this object.

Buoyant force + Normal force = Weight of object + Weight of water

So in this question they are accounting for the force by the water above it as acting on the object, but in my original question they do not account for this same force.... That is my confusion

 

[sazerac]

In your first example it was a bowling ball with its bottom exposed. There was a weight of water acting down on the top, and an even greater weight of water acting up on the underside of the object. The difference between these two forces is a net upward force, the buoyant force. Even if the bowling ball went deeper, you could add a million more newtons to the top and a million more newtons to the bottom of the ball pointing up, so the difference (and thus the buoyant force) would be the same as in shallower water.

In your second question, there is no water pushing up on the bottom of the circular plate, because it is a drain hole cover.

These are good questions. The first tests blindly if you know how to calculate a bouyant force. The second tests if you know where a bouyant force truly comes from, and what happens if some of the usual parameters (like being a fully submerged object) are changed.

 

[sazerac]

Remember, bouyant force is not a fundamental force. It is just a shorthand that takes into account the weight of water above the object, plus the even greater weight of water pushing up on the underside of the object.

 

[daftypatty]

In your first example it was a bowling ball with its bottom exposed. There was a weight of water acting down on the top, and an even greater weight of water acting up on the underside of the object. The difference between these two forces is a net upward force, the buoyant force. Even if the bowling ball went deeper, you could add a million more newtons to the top and a million more newtons to the bottom of the ball pointing up, so the difference (and thus the buoyant force) would be the same as in shallower water.

In your second question, there is no water pushing up on the bottom of the circular plate, because it is a drain hole cover.

These are good questions. The first tests blindly if you know how to calculate a bouyant force. The second tests if you know where a bouyant force truly comes from, and what happens if some of the usual parameters (like being a fully submerged object) are changed.

Ah.. Okay I see. I think I was confused/did not exactly know what a "buoyant force" was... Could you just further elaborate on how buoyant force works?

"It is just a shorthand that takes into account the weight of water above the object, plus the even greater weight of water pushing up on the underside of the object." -> when you say this, what do you mean by greater weight of water pushing up on the underside of the object? Like I'm confused as to where this greater weight would be coming from. Just having trouble visualizing it.

It's becoming more clear though, thanks for the help!

 

[sazerac]

At this point you would be better served by looking at the Wikipedia page for buoyant force. They have some cool diagrams with arrows and stuff.