Fluid

[npsa26]

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I am having trouble with fluids. Can anyone help me out with these problems.

1) 3 liters of a certain fluid weighs twice as much as 2 liters of water. What is the specific gravity of the fluid?

2) The specific gravity of a block is 5.6. When fully submerged, what is the buoyant force, in water, on this 2 kg block?

 

[pretender]

I am having trouble with fluids. Can anyone help me out with these problems.

1) 3 liters of a certain fluid weighs twice as much as 2 liters of water. What is the specific gravity of the fluid?

2) The specific gravity of a block is 5.6. When fully submerged, what is the buoyant force, in water, on this 2 kg block?

ya...i'd like to know how to do these as well. 😕

edit: is this correct for the first one? You know density = mass / volume and S.G = density fluid / density of water. You then say that m/2 = density of water and 2m/3 = density of the other fluid. soooo ...(m/2)/(2m/3) = 1.333???

 

[npsa26]

I figured them out. I was definitely overthinking the problems.

So for the first one, it says that the 3 L of the fluid is twice 2 L of water. Density of water is 1 kg/m^3 which is also 1 kg/L so, the water itself weighs 2 kg. The weight of the fluid is twice the water, so its 2 kg*2 = 4 kg. Density = mass/volume = 4 kg/3 L = 1.33
S.G = 1.33/1 = 1.33


for the second one, the specific gravity is 5.6. This is the density of the block as well.To find the volume submerged, Volume = mass/density. So the volume is 2 kg/5600 kg/m3.
So the Fb = (density of the fluid) (Volume submerged) (gravity) = 1000 kg/m3(2/5600)(10) = 3.5 N

With fluids, you have to be really careful with not confusing the density of the fluid and the object, which is what I often do.

 

[Quantum Chem]

1. Fluid forces can be found by using: Fb = gpV, where g is gravity, p is density of the fluid and V is volume. They are equal in this equation so set the two equal to each other. Since you want specific gravity just substitute p with the specific gravity. (10m/s2)*(3L)*(sp) = 2(2L)*(10m/s2)*(1sp). The first 2 in the right hand side is because this unknown liquid weights twice as much. Solve for sp and you get 4/3 as the specific gravity.

2. Fb = Volume of the object times gravity times the density of the fluid. Since you don't know the volume of the object you must find it using its specific gravity and the mass of the object. The volume is (2kg)/(5.6kg/L) which is 0.36 L. Now just plug that into the equation: Fb = (0.36L)*(10m/s2)*(1kg/L) = 3.6N

 

[LangLang]

I understand this. But isn't Fb = mg? so then density of water * g * V = mg. Solving for V you get 2/1000 = .002. I'm confused why this way doesn't work as well.