fluids

[rayden001]

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In EK, a question asks:

a 2kg object is submerged in an unknown fluid (with specific gravity of 5) has an apparent loss of .5kg. What is the specific gravity of the object?

EK's explanation reminded me of P-chem lectures 😕 Could some please help me explain how to get the answer?

 

[engineeredout]

La La La Ignore This Post

 

[engineeredout]

I've been thinking about this some more at work (I have NOTHING to do, but don't tell my boss that), and I really don't know how you're supposed to solve this without knowing the volume of the object, or the volume/mass of the liquid displaced.

What EK question is this?

 

[engineeredout]

JESUS I am dumb. This question has been bothering me for the past two hours until I realized what an idiot I am.

Bouyant force is the weight loss of the object. Volume of fluid displaced is the same as the volume of the object. Since only 1/4th of the object was compensated for, that means that the object is 4 times as dense as the fluid or SG = 20.

\Pretend my other posts don't exist.


YES!!!!


Is that any clearer?

 

[Soccerdoc11]

I don't quite understand the "only 1/4 of the object was compensated for" part.. Do you think you can explain it using the equations (pgv) for the buoyant force. Sorry but I really want to get a good hold on this subject for the test. Thanks

 

[minhaj]

JESUS I am dumb. This question has been bothering me for the past two hours until I realized what an idiot I am.

Bouyant force is the weight loss of the object. Volume of fluid displaced is the same as the volume of the object. Since only 1/4th of the object was compensated for, that means that the object is 4 times as dense as the fluid or SG = 20.

\Pretend my other posts don't exist.


YES!!!!


Is that any clearer?

If the object was four times denser than the fluid wouldnt the object be completely submerged in the fluid? OP, is the answer 5 x 10^-4?

 

[engineeredout]

Soccordoc: Well I'm still getting over my shame at not getting it last night despite having taken fluid mechanics but how about this:

The bouyant force is equal to the weight of the fluid displaced. We know that the bouyant force is 1/4th the weight of the object because .5kg out of 2kg was lost.


You can solve for the volume now if you want, so as you said, Fb = pVg.

p = 5 SG = 5000kg/m^3.
Fb = 5N
G = 10 m/s^2

Solve, you get .0001 m^3 as the volume of the liquid displaced which also equals the volume of the object.

The object is 2kg with a volume of .0001 m^3. Solving for density = m/v = 2/.0001 = 20,000kg/m^3 = SG of 20.

Longer way of getting the same answer, but it still works.

Did I help make it any clearer?

 

[engineeredout]

If the object was four times denser than the fluid wouldnt the object be completely submerged in the fluid? OP, is the answer 5 x 10^-4?

It is completely submerged in the fluid, as was said in the original question. The question is talking about the percieved weight of the object. Any object with a density greater than 1000 kg/m^3 will be completely submerged in water yes, but water will still be applying a force upward called the bouyant force.

I made this same mistake when looking at the problem late last night. If the question said that 25% of the volume of the object was submerged, you'd be correct in assuming that the density of the object was 1/4th of the density of the fluid.


Actually I have no idea where you came up with 5 X 10^-4

 

[Soccerdoc11]

Soccordoc: Well I'm still getting over my shame at not getting it last night despite having taken fluid mechanics but how about this:

The bouyant force is equal to the weight of the fluid displaced. We know that the bouyant force is 1/4th the weight of the object because .5kg out of 2kg was lost.


You can solve for the volume now if you want, so as you said, Fb = pVg.

p = 5 SG = 5000kg/m^3.
Fb = 5N
G = 10 m/s^2

Solve, you get .0001 m^3 as the volume of the liquid displaced which also equals the volume of the object.

The object is 2kg with a volume of .0001 m^3. Solving for density = m/v = 2/.0001 = 20,000kg/m^3 = SG of 20.

Longer way of getting the same answer, but it still works.

Did I help make it any clearer?

Yes, perfect thanks.. I had trouble seeing that the Bouyant Force would be that 5 N.

 

[minhaj]

makes sense engineeredout. I effed up.

 

[rayden001]

JESUS I am dumb. This question has been bothering me for the past two hours until I realized what an idiot I am.

Bouyant force is the weight loss of the object. Volume of fluid displaced is the same as the volume of the object. Since only 1/4th of the object was compensated for, that means that the object is 4 times as dense as the fluid or SG = 20.

\Pretend my other posts don't exist.


YES!!!!


Is that any clearer?

Hey engineered out, thanks for the clarification. It is actually 20. What a genius😎