More than meets the eye

People are often surprised to find just how much of an iceberg sits below the surface of the water. Suppose that the density of an iceberg is (uniformly) 0.92 g/mL 0.92 \text{ g/mL} , and that of the ocean is 1.02 g/mL 1.02 \text{ g/mL} . What fraction of the iceberg's volume is hidden below the ocean surface?


The answer is 0.902.

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8 solutions

Pranjal Jain
Dec 7, 2014

The ocean exerts a pressure on the iceberg equal to the force it would exert on an equal volume of water, in other words, the submerged volume of the iceberg. Therefore, we have:

ρ ocean water V submerged g = ρ ice V total g \rho_{\text{ocean water}}V_{\text{submerged}}g=\rho_{\text{ice}}V_{\text{total}}g V submerged V total = 0.92 1.02 0.9 \Rightarrow \dfrac{V_{\text{submerged}}}{V_{\text{total}}}=\dfrac{0.92}{1.02}≈0.9

This shows that about 90% of our iceberg will remain out of view deep below the water's surface.

I tried, but it didn't work. Missed 0.2%

sashi kiran - 3 years, 3 months ago
Gamal Sultan
Dec 11, 2014

The volume of the iceberg = X

The volume of the hidden part = Y

0.92 X = 1.02 Y

Y/X = 0.92 / 1.02 = 0.90196

But it was not volume, it was density

Pushan Paul - 6 years, 6 months ago

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Mass (g) = Density (gm/mL) * Volume (mL or cm^3)

Let m1= Mass (grams) of the entire iceberg

Let m2= Mass (grams) of the displaced water

Let X = Volume of the entire iceberg. (mL)

Let Y = Volume of the displaced water, which is equal to the volume of the submerged portion of the iceberg. (mL)

Since we can assume that acceleration due to gravity is the same for both the iceberg and fluid, we can ignore that when considering weight, because an object at rest in fluids must have the force of its weight and the buoyant forces in equilibrium.

Therefore, the weight of the whole iceberg must equal the weight of the water displaced.

If the weights must be equal, then we can divide out the gravitational constant and only deal with the masses.

m 1 g = m 2 g m1 * g = m2 * g //Here g represents gravitational field strength, not grams.

Therefore, m 1 = m 2 m1 =m2

. 92 g m L X m L = 1.02 g m L Y m L .92 \frac{g}{mL} * X mL = 1.02 \frac{g}{mL} * Y mL

Y X \frac{Y}{X} is the volume of submerged portion of the iceberg divided by the volume of the entire iceberg, this evaluates to mL/mL, so we can disregard the units mathematically.

. 92 1.02 \frac{.92}{1.02} is the density of the submerged portion of the iceberg divided by the volume of the entire iceberg, this evaluates to g/mL/(g/mL), so we can disregard the units again.

To say that Y X = . 92 1.02 \frac{Y}{X} = \frac{.92}{1.02} is to say that the fraction of the iceberg's volume that is submerged is equal to the ratio of the uniform density of the entire iceberg to that of the water it displaces.

Therefore, we can substitute the ratio of the given constants for Y X \frac{Y}{X} which is what we were prompted for initially.

Y X = . 92 1.02 . 901961 \frac{Y}{X} = \frac{.92}{1.02} \approx .901961

Chris M. - 6 years, 5 months ago
Fred Hayes
Dec 12, 2014

Iceberg displacement

Let Ρsea = 1.02 density of seawater

Let Ρice = 0.92 density of ice

Let Vbelow = volume in % of iceberg below water

Let Vabove = volume in % of iceberg above water.

By definition

V b e l o w + V a b o v e = 1.0 V_{below} + V_{above} = 1.0

The iceberg sinks until the volume of ice under water displaces the higher density seawater in a sufficient amount to equal the total weight of the iceberg

p i c e ( V b e l o w + V a b o v e ) = p s e a V b e l o w p_{ice}* (V_{below}+V_{above} )=p_{sea}* V_{below}

Solve for Vbelow; recognizing that

( V b e l o w + V a b o v e ) = 1.0 (V_{below}+V_{above} )=1.0

V b e l o w = p i c e p s e a V_{below}=\frac{p_{ice}}{p_{sea} }

V b e l o w = 0.902 V_{below}=0.902

Tony Sprinkle
Jan 4, 2015

At equilibrium, the weight of the iceberg equals the buoyant force from the water:

F B = m i g F_B = m_ig ρ w V s u b g = ρ i V t o t g \rho_wV_{sub}g = \rho_iV_{tot}g

where ρ w \rho_w and ρ i \rho_i are the densities of the water and the iceberg, respectively, and V s u b V_{sub} and V t o t V_{tot} are the volume of the submerged part of the iceberg and the total volume of the iceberg, respectively. Thus, the fraction of the iceberg's volume that is hidden below the ocean surface is:

V s u b V t o t = ρ i ρ w = 0.92 g/mL 1.02 g/mL = 0.90 \frac{V_{sub}}{V_{tot}} = \frac{\rho_i}{\rho_w} = \frac{0.92 \text{ g/mL}}{1.02 \text{ g/mL}} = \boxed{0.90}

Miguel Domingo
Dec 21, 2014

volume above water = A ;
Underwater portion's volume = U ;
Buoyant force = B = U * 1.02 (Archimedes principle) ;
Weight = W = (A+U) *0.92 ;
Upward and downward forces must be equal, so W = B ;
then U * 1.02 = (A+U) *0.92 ;
Hidden fraction = U / (A + U) = 0.92/1.02= 46/51~90.2%





Abhishek Roy
May 17, 2015

If the ice-berg is in equilibrium then its weight is exactly equal to buoyant force of the liquid. Suppose the volume of iceberg is equal to V. The weight of iceberg is then equal to 0.92 V. Buoyant force is equal to 1.02 V', V' being the volume inside water. now 0.92V = 1.02V'. From her V' can be found and hence is the answer.

Rahul Jain
Mar 6, 2016

Density is Mass Divided by Volume i.e, M v \frac{M}{v}

Let Volume Of Iceberg Out Side (Seen Part On The Surface ) = V1

And Volume Of Iceberg Hidden (Inside Ocean) = V2

.92 * V1 = 1.02 * V2

V2/ V1 = .92/1.02 = .90196 (Approx)

Stephen Bohrer
Apr 14, 2017

I'm curious about the degree of significance being used in the official answer and comments. The problem uses two degrees while the answers use three and more. Is that proper?

the problem asked for a fraction the answer given was in decimal. if you answer with a fraction it tells you you have the wrong answer. my answer of 901/1000 is correct but is not accepted.

kevin mages - 2 years, 5 months ago

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