Where does the rest of the weight go? Floating object, buoyant force...

[MCAT guy]

---

Members don't see this ad.

So I understand that the bouyant force is equal to the weight of the object (displaced weight!)... the question is, what supports the rest of this weight?

if mg = pVg , displaced, and lets say half of the object is submerged, then what is physically supporting the other half of the weight?

A different type of normal force on the water?

 

[fizzgig]

will see if i can make sense.

the bouyant force is equal to the weight of the water that an object displaces.

an object will continue into the water from the surface until it displaces its weight in water.

so if an object sits half submerged, then you know that the weight of the object is equal to the weight of water that takes up HALF the object's volume.

-a 10lb rock that gets totally submerged and has only displaced 1lb worth of water has an imbalance, yes. so it sinks.
-a 10lb object that displaces 10lbs of water before the object is submerged will float with some portion of it out of the water.
-a 10lb object that displaces exactly 10lb of water will bob just below the surface (i think).

 

[MCAT guy]

will see if i can make sense.

the bouyant force is equal to the weight of the water that an object displaces.

an object will continue into the water from the surface until it displaces its weight in water.

so if an object sits half submerged, then you know that the weight of the object is equal to the weight of water that takes up HALF the object's volume.

-a 10lb rock that gets totally submerged and has only displaced 1lb worth of water has an imbalance, yes. so it sinks.
-a 10lb object that displaces 10lbs of water before the object is submerged will float with some portion of it out of the water.
-a 10lb object that displaces exactly 10lb of water will bob just below the surface (i think).

This all makes sense, I understand the idea that mg = pvg (volume displace) OR the bouyant force is equal to the weight of the object that is displaced...

my question is, lets say I have an object that is 1000 kg, now it is displaced 50% in very dense liquid. 500 kg's are not beings supported by the buoyant force, what is then supporting it? is it the surface of the lake? Or is there a normal force inside of the object? Something is making that weight not sink (the weight above the submerged volume of the object)...

 

[loveoforganic]

If the object is floating, the weight of the object equals the buoyant force.

 

[MCAT guy]

If the object is floating, the weight of the object equals the buoyant force.

I think I understand now... the bouyant force is the entire weight as long as it floats... But the bouyant force moves up and down based upon how much of the volume is submerged.

In other words, the bouyant force is like static friction coefficient, it can be really small or really large depending on the stimulus?

The bouyant force could in effect overcome the weight, but it just doesn't physically?

 

[loveoforganic]

Well, we're going to get into a bunch of semantics here.

Weight, we'll define as downward force. Buoyant force we'll define as the force opposing weight.

You have a styrofoam ball that has a 10 g mass and occupies a volume of 1000 cm^3. Therefore, the density of the ball is 0.01 g/cm^3 (less than water). You place this styrofoam ball into a bath tub. It's going to float. The volume of water displaced by the ball will be enough to equal the mass of the ball. 10 g of water = 10 cm^3 of water. So 10 cm^3 of water are displaced by the floating ball. Here, buoyant force = weight.

If you were to press the ball so that it were fully submerged in the tub, the volume of the ball would be the volume of water displaced. 1000 cm^3 of water would be displaced. Buoyant force again = weight.

When you release that ball, the apparent weight of the ball shrinks drastically (back to its original 100 N). The same volume of water is still displaced though, so buoyant force is now much much greater than weight, and the ball will accelerate upward until buoyant force = weight and the ball is floating again.

 

[fizzgig]

I think I understand now... the bouyant force is the entire weight as long as it floats... But the bouyant force moves up and down based upon how much of the volume is submerged.

In other words, the bouyant force is like static friction coefficient, it can be really small or really large depending on the stimulus?

The bouyant force could in effect overcome the weight, but it just doesn't physically?

yes, if you dangle an object on a rope and slowly lower it into water, buoyant force goes up as the object displaces water. if Fbuoy = mg before the object is fully submerged, it'll float and as you continue to lower the rope'll just go slack..

 

[MCAT guy]

Well, we're going to get into a bunch of semantics here.

Weight, we'll define as downward force. Buoyant force we'll define as the force opposing weight.

You have a styrofoam ball that has a 10 g mass and occupies a volume of 1000 cm^3. Therefore, the density of the ball is 0.01 g/cm^3 (less than water). You place this styrofoam ball into a bath tub. It's going to float. The volume of water displaced by the ball will be enough to equal the mass of the ball. 10 g of water = 10 cm^3 of water. So 10 cm^3 of water are displaced by the floating ball. Here, buoyant force = weight.

Thanks, this helped.

If you were to press the ball so that it were fully submerged in the tub, the volume of the ball would be the volume of water displaced. 1000 cm^3 of water would be displaced. Buoyant force again = weight.

When you release that ball, the apparent weight of the ball shrinks drastically (back to its original 100 N). The same volume of water is still displaced though, so buoyant force is now much much greater than weight, and the ball will accelerate upward until buoyant force = weight and the ball is floating again.

Wouldn't we say that the Bouyant Force is greater than the weight, once you are holding it down underwater, the bouyant force would really want to push it up, so the bouyant force would exceed the weight of the actually object (100N), and be equal to 100N + whatever added force you put in?

Or, I think you are saying the weight of the object is 100N + force added to fully submerge (apparent weight > 100N).
___________

the object accelerates with a force of pVg which then would be equal to mass x acceleration, correct? the volume would then be 100% of the volume of the ball since it is submerged at this point...

THANKS guys, helped a bunch.

 

[MCAT guy]

yes, if you dangle an object on a rope and slowly lower it into water, buoyant force goes up as the object displaces water. if Fbuoy = mg before the object is fully submerged, it'll float and as you continue to lower the rope'll just go slack..

Thanks for this example, it helped me understand.