another fluids question

[greenseeking]

2kg object submerged in the unknown fluid has an apparent loss of 0.5 kg. What is the specific gravity of the object?

given specific gravity of the fluid is 5.
Answer is 20.

I had a hard time understanding what the apparent loss of mass was. Would that make Fapparent=2kg-0.5?

Fapp=mg-Fb
1.5(10)=20-Fb
Fb=5
Fb= rho (fl) V g
plug in all the #'s to get V=1/10

rho object=2 kg/.1=20????

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[t0ny]

I always remember it as:

Fb - Fg = weight loss

Therefore: Fg - Fb = (0.5kg)(10m/s2)

You should be able to figure it out from there. Think about any loss of weight as due to the boyancy force pointing in the opposite direction from the gravitational force.

 

[BerkReviewTeach]

2kg object submerged in the unknown fluid has an apparent loss of 0.5 kg. What is the specific gravity of the object?

given specific gravity of the fluid is 5.

(1) Is the object a sinker or floater? Sinker, because it is submerged.

(2) For a sinker, density_object/density_medium = W/B

In this case, we need to modify it to fit the units given. Because g will cancel out, W/B = mass_object/apparent mass loss_object. We also know that density_object/density_medium = specific gravity_object/specific gravity_medium.

So we get:

mass_object/apparent mass loss_object = specific gravity_object/specific gravity_medium

2.0/0.5 = specific gravity_object/5

4 = specific gravity_object/5

4 x 5 = specific gravity_object = 20​

Best method for solving any type of relative density (specific gravity) problem.

 

[drillers]

stupid question here, but how would the method change if it was a floater instead of a sinker?

 

[Calpha male]

stupid question here, but how would the method change if it was a floater instead of a sinker?

You can think of the apparent weight of an object as the normal force on the object when it's sitting at the bottom of a container. If the object floats, Fb > Fg so there wouldn't be any normal force and an apparent weight question would not really be relevant. On the other hand, they may ask about the submerged volume of the floating object once it reaches the surface. In this case, Fb = Fg and you can relate the volume submerged to the densities of the object and the fluid.