Calculating Density from Weight: 341N Object Immersed in Water

[iamtrojan3]

Homework Statement

An object weighing 341 N in air is immersed in water after being tied to a string connected to a force scale. The scale now reads 307 N. Calculate the density of the object.

Homework Equations

Fb=(density of the surrounding liquid)(V)(g)

The Attempt at a Solution

Fb is difference in the forces, 341N-307N = 34N
v = ((initial weight)-(submerged weight))/9.8 = 3.47
mass = 341N/9.8

So Density = m / v

Density therefore = 10.3kg/m^3

Its wrong, i have no idea why, probably some unit conversion somewhere.

thanks!

 

[kuruman]

The 34N that you found is the buoyant force, i.e. the weight of the displaced water. Can you find the mass of the displaced water and then the volume of the displaced water? This volume is the object's volume.

 

[Feldoh]

The bouyancy force is B = (Density of liquid)(Volume of the object) * g.

V is not the difference in mass (which is what I think you were trying to do), it's the volume of the object.

So you know B and g, and the density of water is fairly constant so you can solve for the volume.

Density as you said is mass divided by volume, and you correctly found the mass, so you just need to find the volume.

 

[iamtrojan3]

The bouyancy force is B = (Density of liquid)(Volume of the object) * g.

V is not the difference in mass (which is what I think you were trying to do), it's the volume of the object.

So you know B and g, and the density of water is fairly constant so you can solve for the volume.

Density as you said is mass divided by volume, and you correctly found the mass, so you just need to find the volume.

Isn't that what i did? B = Density * V * g and like you said water is fairly constant, so V = B/g
which is 34N/9.8 = 3.47 which is the volume of the object, right?
So now i can do D = m/v for the density of the object given m of object is 341/9.8.

And i just confused myself, we're ignoring the density of water cause its "fairly constant" right?

 

[Kurdt]

You need to include the density of the water in your volume calculation.

 

[iamtrojan3]

Ok, now ill sound just plain stupid.
How do i find the density of water?

 

[Kurdt]

You can look it up. Its normally 1000kg per cubic metre.

 

[iamtrojan3]

Ahh i see what i did thank you all for the help