Interplanetary Displacement

A small wooden block floats in a beaker of liquid on Earth, where gravitational acceleration is roughly 9.8 m / s 2 . \SI[per-mode=symbol]{9.8}{\meter\per\second\squared}. An identical wooden block floats in an identical beaker of liquid on planet Xylem, where gravitational acceleration is roughly 11.1 m / s 2 . \SI[per-mode=symbol]{11.1}{\meter\per\second\squared}.

How would the volume of displaced water vary in these two scenarios?

More water is displaced on Earth More water is displaced on Xylem The same amount is displaced in both scenarios Cannot be determined

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11 solutions

Archimedes' Principle: Any body immersed in a fluid is acted upon by an unbalanced upward force called the bouyant force, which is equal to the weight of the fluid displaced.

Recall that ρ = m v \rho = \dfrac{m}{v} , where ρ = d e n s i t y , m = m a s s , \rho= density, m=mass, and v = v o l u m e v=volume . We also know that w = m g w=mg where w = w e i g h t , m = m a s s w=weight,m=mass and g = g r a v i t a t i o n a l a c c e l e r a t i o n g=gravitational~acceleration . So we have

w = v × ρ × g w=v \times \rho \times g

Apply this to the fluid. Let v d v_d be the displaced volume of fluid, ρ d \rho _d be the density of the fluid displaced, m d m_d be the mass of the fluid displaced, w d w_d be the weight of the fluid displaced and g g be the acceleration due to gravity. We have

w d = b o u y a n t f o r c e = v d × ρ d × g w_d=bouyant~force=v_d \times \rho_d \times g

Rearranging the equation, we get

v d = w d ρ d × g v_d=\dfrac{w_d}{\rho_d \times g}

But we know that w d = m d × g w_d=m_d \times g , substitute

v d = m d × g ρ d × g v_d=\dfrac{m_d \times g}{\rho_d \times g}

v d = m d ρ d v_d=\dfrac{m_d}{\rho_d}

The gravitational acceleration has nothing to do with the volume displaced. It is because the volume displaced is equal to mass of the fluid displaced divided by density of the fluid displaced.

Note: It is assumed in the problem that the liquid is identical on both planets.

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I disagree with the answer here. My logic is as follows. Water, subject to more gravity (higher acceleration) would be more dense. "Wood" because it has an internal structure that holds a fixed amount of space would not be compressed by a higher gravity. Therefore, the wood would not displace as much water on (by volume) with higher gravity. This gets a little tricky and assumes equal air pressure on both planets. Without air pressure at all, the water would boil off completely and the volume of wood would be under the water vapor...thus being equal--the image, however, shows enough pressure to maintain liquid water.

PS. the question does not ask mass (weight) of water displaced, it asks volume.....The volume of the water would be less on earth due to the water being less compressed on Earth (all other factors being equal). Think of it like a swimming pool the deeper you go the more pressure increases. The effect would be exaggerated in higher gravity (water pressure building up faster). So the more displacement the wood block had, the more it would displace higher water pressure (same mass but smaller volume).

Also, either my math skills are old or vd = P G/W and not Vd = W/P G!

David Urry - 2 years, 10 months ago

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I think problem assumes water's density is same on both planets, since wood is floating on the surface buoyant force must balance gravitation pull on the wood. $ m * g = volume of displaced water * density * g,$ g cancels out on both side. if density of water is same on both planets, volume displaced would be same.

P.S. In case if density is higher on xylem, contrary to your statement when density is lower (on earth) water volume displaced would be higher.

Ashutosh Goswami - 2 years, 10 months ago

That's totally rubbish

You have not specified as to what (rto) indicates....density of liquid or object which is immersed.

If (rto) is density of object then that means w=weight of object But again you have written in the next line W=buoyant force............

Achyut Dhiman - 2 years, 10 months ago

Circular reasoning, put \rho {d} = m d / v d, you will get v d = v_d. Solution does not really solves the problem only somehow manages to give right answer!

Ashutosh Goswami - 2 years, 10 months ago

The best way is to do it as experiment on Earth- just go to a place with low gravity and high gravity. Where can I complain about the non-science included in Science Essentials?

Mark Smith - 2 years, 10 months ago

As Gravitation acceleration is more on Planet X, Water in beaker will be more dense in planet X as compared to Earth, hence less water will be displaced in case of Planet X.

Abhinav Kumar - 2 years, 10 months ago

The weight of the displaced liquid is going to equal the weight of the object for any value of gravity, right?

That was my first thought, then delighted to find Marvin had written it into mathematics and logic. That’s why I’ve joined and starting at simple stuff, to practise doing things in the logical language of mathematics. They say learn a language when you get old, this is mine.

Andrew Church - 2 years, 10 months ago

I answered that the difference in volume cannot be determined because we don’t know temperature and atmospheric pressure on Xylem. Shouldn’t that be considered?

Jan-Philipp Dombrowski - 2 years, 10 months ago

I have no problem on your great answer but a little doubt to the question. On earth, the wood block is declared "a small wood block" while on Xylem, it is "an identical wooden block".Does this affect the volume of displaced water? I thought that a small wood block sometimes could be bigger or smaller than an identical one.

星音 中梓 - 2 years, 10 months ago

So am I wrong in saying: unless you know the atmospheric pressure were equal this could not be determined?

Mr. Hyde - 2 years, 10 months ago

The block of wood has to absorb some tiny fraction of the water, right? Shouldn't that increase the mass of the wood thus skewing the displacement of the water?

Michael Stachowiak - 2 years, 10 months ago

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Ironically, wood is actually made of xylem which absorbs water and nutrients ... but that's independent of g.

https://en.wikipedia.org/wiki/Xylem

Tapani Lindgren - 2 years, 6 months ago
Marra DeGraff
Jul 13, 2018

When an object floats in a liquid — as the wooden block does — the liquid displaced is equal in weight to the weight of the object. Since weight = mass × gravity , \text{weight} = \text{mass}\times\text{gravity}, it might be tempting to say that more liquid will be displaced in the higher gravity scenario (on Xylem). Mass, however, is independent of gravity, so regardless of which planet we're on, the same mass of liquid will be displaced!

Another way to think about it is that a block of wood and a beaker of water are the same on any planet!

Amogha Pokkulandra - 2 years, 11 months ago

Helpful explanation, just keep not confusing gravity (force) with acceleration caused by gravity. Gravity itself is very dependent of mass :) But acceleration caused by a planet's gravity is same for the object and liquid, so you can eliminate it from the weight equation (weight of object = weight of liquid). Then, it's clear they must have the same mass regardless of planet.

K K - 2 years, 10 months ago

Note that this would change if the increased weight has an effect on the volume, i.e. the higher force might compress the water and the wood differently. Probably not that relevant on this scale, but if the gravity were to increase a thousandfold probably something different is going to happen...

Roland van Vliembergen - 2 years, 10 months ago

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This seems to make sense to me which is why I got the wrong answer Instead are used for thought experiment what would happen to the same block of wood in the test tube as it was traveling by rocket to the international space station and surmised that upon reaching zero gravity the water molecules affinity for each other would pull together and simply push the wooden block out of the blob of water in the test tube where simple surface tension Might keep the wooden block wetted Took the surface of the water but with nogravity there would be no intrusion of the block into the water in my mind This is the other extreme as noted above by Roland

Can anyone explain what happens in zero gravity as well as in 1/2 infinite gravity And then on the verge of infinite gravity

Charles Baumgartner - 2 years, 10 months ago

I once read a science fiction novel which got this wrong. It described the experience of swimming on the moon as "the problem was getting enough of your body under the water to make headway". I intuitively felt that this wasn't right and did some working out to arrive at the correct situation.

Paul Cockburn - 2 years, 10 months ago

So am I wrong in saying: unless you know the atmospheric pressure were equal this could not be determined?

Mr. Hyde - 2 years, 10 months ago

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UNLESS it was a very thick atmosphere that dramatically reduced the size of say a polystyrene object for much higher gravities, then no, as the difference in water density is not gonna change much, less than 5% change on earth from surface ocean to deepest.

PLUS I think you can assume in science questions about a dependant and independent variable, that all other variables are considered “controlled” unless otherwise stated.

Most experiements aim to reveal a general tendancy, general principles. That is, for things floating, adjust the gravity, no change. Such generalisations will always break down at some point.

Andrew Church - 2 years, 10 months ago

The block gets heavier but so does the water it displaces.

Brian Bohan - 2 years, 8 months ago

Looking at Marvin Kalngan's justification, this problem belongs under "Intermediate."

Dennis Rodman - 2 years, 8 months ago
Vinayak Joshi
Jul 15, 2018

It doesn't matter whether the gravity of "Xylem" has a greater pull force towards the center. Since both blocks are identical as well as the beaker of liquid, the same amount of liquid will be displaced no matter the gravity.

The question did not state that the liquid in the jar on planet Xylem was the same (Only the beaker), if it was more or less dense the displacement would be different, thus the displacement cannot be stated as the same with any certainty.

Geoff Anderson - 2 years, 10 months ago

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"An identical wooden block floats in an identical beaker of liquid on planet Xylem".

at the end it says "How would the volume of displaced water vary in these two scenarios?"

They are the exact same in both scenarios.

Vinayak Joshi - 2 years, 10 months ago

If you record a video of moving bodies and play it at a different speed the resultant video is still a physical movement. A physical movement of the same bodies, with the same mass, but different velocities and different accelerations. Therefore if you record a video of wood floating on water and you speed it up appropriately you effectively have a video of the movement of wood on water in Xylem. But both recordings look exactly the same, therefore the water level can'tt change.

P P - 2 years, 9 months ago

In the end, the liquid in Xylem would be attracted to Xylem at the same proportional rate as the wooden block, making gravitational attraction irrelevant

Simeon Jour
Jul 20, 2018

Intuitively; the floating effect depends on the difference between water and the wooden block density. Gravity doesn't affect their density, so it is the same on both planets.

Shanaya Khan
Jul 19, 2018

Isn't it goes simple like this:- The volume of the fluid displaced is equal to the volume of the part of the body (say block here) submerged under the fluid.

Qiang Wang
Jul 21, 2018

The argument that "The same amount is displaced under the same scenario" is under the assumption that the densities (and thus the volumes) of the wooden block and water are not affected by the gravity change.

Since we are not explicitly given this assumption, truly the correct answer should be "Cannot be determined".

But it is not an assumption that the densities of the wooden block and water are not affected; they do not change. Since density is mass/volume, and mass and volume are both unaffected by different gravitational fields, the density does not change when on planet Xylem.

Jessie Meanwell - 2 years, 9 months ago
Seth Gil
Jul 18, 2018
  • Using Archimedes' Principle, we get (ρ B)(V T)g=(ρ F)(V F)g, Where ρ B is the density of the block, V T is the total volume of the block, g is any generic gravity, ρ F the density of the fluid, and V F is the volume of fluid being displaced by the block. With some algebraic manipulation, we get (V_F)= ( ρ B ) ( V T ) ( ρ F ) \frac{(ρ_B)(V_T)}{(ρ_F)}

Assuming the liquid is ideal and doesn't compress under any change in gravity, the volume displaced will remain the same.

David Fairer
Jul 18, 2018

The weight of the wooden block is more on the planet Xylem, but so is the weight of the liquid that has to be displaced, and by exactly the same proportion. Regards, David

Gravity acts constant over the entire unit, and as the wooden block is not thrown into water it will comply only force due to its weight. And in that case the boyant force comes into picture which obeys Archimedes principle that water displaced by the wooden block is equal to weight of block. Now if we equate the two cases acceleration due to gravity is cut out from the equation. And not does necessary work on water displacement.

San Seng
Jul 17, 2018

Judging by the high standard of users, very simple logic would be the g won't affect the scenario. The densities of wood and water remain same, the downward wight mg increases per g increment but the buoyant force v(rho)g increases too, so it is still between m and (rho) the density.

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